Chapter 20: Ductwork, Airflow, and Ventilation: Moving Air Through Your Home

You have probably never given much thought to the sheet metal labyrinth hidden inside your walls, floor, and attic. The duct system is invisible, inaccessible, and almost never discussed — until something goes wrong. But if there is one system in your home with the biggest gap between "how it usually works" and "how it should work," it is the duct system.

Here is the number that should stop you: the U.S. Department of Energy estimates that the typical American home loses 20 to 30 percent of conditioned air through duct leakage. In some homes, it's higher. That means roughly one in four dollars you spend heating and cooling your home is being spent pushing air through cracks, gaps, and disconnected joints into your attic, crawlspace, or wall cavities — spaces that do you no good at all. You pay to heat or cool that air, run it through your expensive equipment, and then it escapes before it ever reaches a room.

This chapter covers the mechanics of duct systems — how they're designed, what they're made of, and why they leak — and then turns to the related topic of ventilation. Ventilation is the controlled movement of fresh air through your home, and it's a topic that has grown dramatically more important as homes have been built and renovated to be increasingly airtight. A tight home that isn't properly ventilated accumulates moisture, pollutants, and carbon dioxide to levels that harm both the building and its occupants. We'll cover bathroom fans (venting into the attic is still common and still wrong), kitchen exhaust, attic and crawlspace ventilation, and the heat recovery ventilators that allow tight modern homes to breathe without hemorrhaging energy.

Dave Kowalski's rural property gives us a case study in duct leakage: a system that's been leaking for decades and has never been tested. The Chen-Williams renovation gives us the opportunity to watch a system being designed and built from scratch — and to understand what a properly engineered system looks like.

20.1 Duct System Design: Supply, Return, and Why Balance Matters

A forced-air HVAC system moves air in a loop. The blower in the air handler draws air from the house through the return ducts, passes it through the filter and over the heating or cooling coil, and pushes the conditioned air back into the house through the supply ducts. For this to work as intended, the amount of air going out through supply registers must equal the amount of air coming back through return grilles. When it doesn't, you have pressure imbalances — and pressure imbalances cause a cascade of problems.

Supply ducts deliver conditioned air to each room or zone. Supply registers are typically in the floor, low on the wall, or in the ceiling, depending on the system type and regional climate conventions. For heating-dominated climates, floor or low-wall registers work well: warm air rises and distributes through the room. For cooling-dominated climates, ceiling or high-wall registers are often better: cool air falls and mixes with room air more effectively. In practice, most systems use one configuration for both heating and cooling, with good results as long as the distribution is even.

Return ducts are at least as important as supply ducts, though they're often underdesigned. Return grilles are typically larger than supply registers — they're moving the same volume of air but often through fewer, larger openings. The most common duct design failure in residential construction is inadequate return air capacity. When a room has a supply register but no direct return path, the room pressurizes when the system runs — air pushes in but has nowhere to go. This pressurization forces air out of the room through cracks, gaps, and under doors, which reduces comfort, creates noise (doors slamming when the system cycles), and can actually cause the system to work against itself.

Solutions to inadequate return air: - Transfer grilles: Simple grilles cut between adjacent rooms, near the ceiling, that allow air to flow from one room to the return path without requiring a full duct run. - Jump ducts: Short duct sections connecting a room to a hallway or plenum, providing a low-resistance return path. - Undercutting doors: Cutting 1–1.5 inches off the bottom of interior doors is an inexpensive and invisible way to allow air transfer. It's not as effective as a transfer grille but helps significantly. - Full return duct to each room: The best solution but the most expensive.

Static pressure. Ductwork designers work with a concept called static pressure — the resistance the duct system offers to airflow. Think of it like the electrical resistance in a circuit: more resistance means the same blower delivers less airflow. Undersized ducts, too many sharp bends, long flex duct runs, high-MERV filters, dirty coils — all increase static pressure. When static pressure exceeds the blower's design range, airflow drops below design, and the whole system underperforms.

Many HVAC contractors install systems without measuring static pressure and many installed systems have static pressure significantly above design. The result is reduced airflow, reduced comfort, coil icing in cooling mode, and accelerated equipment wear. An HVAC contractor who measures and balances static pressure is providing a level of service above the industry average.

📊 Common Causes of High Static Pressure | Cause | Typical Pressure Penalty | |-------|------------------------| | 1" filter (MERV 13 instead of MERV 8) | +0.15–0.25 in. w.c. | | Dirty evaporator coil | +0.10–0.30 in. w.c. | | Flex duct over 10 ft. | +0.08–0.15 in. w.c. per additional 10 ft. | | 90-degree flex duct bend | +0.03–0.08 in. w.c. | | Undersized return grille | +0.10–0.20 in. w.c. | Total design static pressure in residential systems should typically be 0.50 in. w.c. or less.

20.2 Duct Materials: Sheet Metal, Flex Duct, and Fiberboard

The duct system in most homes is built from one or more of three materials: galvanized sheet metal, flexible duct, or fiberboard. Each has legitimate applications and genuine failure modes.

Sheet metal (galvanized steel) is the gold standard for duct construction. It's rigid, durable, air-tight when properly sealed, and has very smooth interior surfaces that minimize airflow resistance. Sheet metal trunks and branches can last 30, 40, or 50+ years with minimal maintenance. The main drawbacks are cost (it requires skilled fabrication) and the need to install duct insulation separately (sheet metal itself has essentially no insulation value).

Properly installed sheet metal duct runs use slip-fit or drive-and-cleat connections at joints, sealed with UL 181-rated duct mastic (not duct tape — more on this below) or metal foil tape. Sheet metal should be insulated to at least R-6 wherever it passes through unconditioned space (attic, crawlspace, garage).

Flexible duct (flex duct) is the dominant product in residential construction because it's fast to install. A flex duct section is an inner liner of plastic film (polyester or equivalent), surrounded by a layer of fiberglass insulation, surrounded by an outer jacket. It comes in rolls and can be cut to length and connected to sheet metal fittings with clamps and tape. For runs that don't require fabrication, it saves significant labor.

The problem with flex duct is that it's very easy to install badly. The most common flex duct failures: - Sagging: Flex duct must be supported at maximum 4-foot intervals. Unsupported spans sag, creating low spots that trap condensation and restrict airflow dramatically. A sagging, kinked flex duct run can add the equivalent of 20 feet of straight run in pressure drop. - Compression: Flex duct loses most of its flow capacity when compressed. A flex duct pinched to 80% of its diameter loses about 60% of its airflow capacity. This happens constantly in real installations — under joists, through framing, around obstacles. - Not fully extended: Flex duct should be pulled tight and smooth. Installed loose and wavy, it dramatically increases pressure drop. - Poor connections: The connections between flex duct and sheet metal fittings are common leakage sites if not properly secured and sealed.

Flex duct has a practical lifespan of 15–25 years; the inner liner and insulation degrade over time, particularly in attic environments with temperature extremes. If you have flex duct in the attic that's over 20 years old and visibly degraded, replacement with new insulated flex (or better, sheet metal) is worth considering during a system replacement.

Fiberboard duct is made from compressed fiberglass and is typically used for residential trunk ducts. It has built-in insulation value, is easy to cut, and is relatively inexpensive. The interior surface, however, is significantly rougher than sheet metal — higher friction and more resistance to airflow. More importantly, fiberboard absorbs and retains moisture. In humid conditions, a fiberboard duct system can become a breeding ground for mold. Fiberboard duct is less common in new construction than it was in the 1980s and 90s.

⚠️ The Duct Tape Problem Despite its name, standard duct tape (the silver-gray fabric-backed adhesive tape in every hardware store) is not approved for sealing ductwork. Duct tape adhesive dries out and fails within a few years in the temperature extremes of an attic. Properly sealed ductwork uses either UL 181 mastic (a paste-like compound applied with a brush, that remains flexible for decades) or aluminum foil tape with UL 181 listing (rigid metal tape with long-lasting adhesive). If your ductwork is sealed with standard duct tape, assume those seals have failed and plan for re-sealing.

20.3 Duct Leakage: How Much Is Normal and What It Costs You

Let's return to that 20–30% figure. It's not an outlier from a few poorly-built homes. It's the average across the U.S. housing stock. Studies by Lawrence Berkeley National Laboratory and the Florida Solar Energy Center consistently find that duct leakage is among the largest single sources of energy waste in homes with forced-air systems.

How leakage happens. Ducts are assembled from many pieces: trunk sections, branch takeoffs, elbows, boots (the rectangular or round fittings where a duct terminates at a supply register). Every joint is a potential leak point. In typical construction, joints are connected mechanically but many are never properly sealed with mastic or appropriate tape. Over time, even connected joints shift and open slightly as the house settles and ductwork expands and contracts thermally. The cumulative effect of hundreds of slightly leaky joints is significant.

Supply-side leakage is the most costly: conditioned air that leaks out of supply ducts into unconditioned space is simply lost. Return-side leakage is equally bad in a different way: air leaking into the return from unconditioned spaces (attic, crawlspace) is dirty, may be dangerously humid, and must be conditioned before it reaches the house — you're paying to condition air you never wanted in the first place.

What leakage costs. If your heating and cooling bills total $2,400/year and 25% of conditioned air is being lost to duct leakage, you're spending $600/year on air conditioning your attic or crawlspace. Over 20 years, that's $12,000 — enough to pay for a complete duct system replacement with money left over.

📊 Cost of Duct Leakage: Sample Calculation - Home: 2,000 sq ft, mixed climate - Annual HVAC cost: $2,400 - Duct leakage (measured): 28% - Annual waste: $672 - Cost to seal ducts professionally: $1,500–2,500 - Simple payback: 2–4 years

Beyond operating cost, duct leakage has other effects: - Pressure imbalances between rooms (rooms with only supply registers pressurize and push air into the attic/walls) - Comfort problems — rooms that are always too hot or cold, drafts near register locations - Moisture problems — humid outdoor air drawn into the duct system through return leaks can cause condensation and mold inside the ducts - Combustion backdraft risk — return-side duct leakage can depressurize the house relative to the utility room, potentially pulling combustion gases from a water heater or furnace into the living space (this is a safety issue, covered further in Chapter 18)

Dave Kowalski's situation. Dave's home was built in 1968, and the sheet metal duct system in the basement has never been professionally sealed or tested. A visual inspection revealed joints connected with nothing but friction (no sealant or tape at all), and what tape had been applied decades ago had failed completely. He was essentially heating a large portion of his basement rather than his living areas. After professional sealing and testing (described in Section 20.4), his measured duct leakage dropped from 32% to 8%. His heating bill for the following winter dropped by $410.

💡 The Quick DIY Duct Audit Walk through your attic or crawlspace during HVAC operation and hold your hand near duct joints. If you feel moving air, those joints are leaking. Mark them with tape, then seal with duct mastic or foil tape on a dry day when the system isn't running. Accessible duct sealing is a genuine DIY project with real payback. Inaccessible ducts (inside walls or ceiling cavities) require aerosol-based sealing systems described in Section 20.4.

20.4 Blower Door Tests and Duct Blasters: How Pros Measure Airflow

Diagnostic testing of duct systems and building envelopes has evolved from guesswork to measurement. Two instruments — the blower door and the duct blaster — allow technicians to quantify exactly how leaky your house and ducts are. This transforms energy auditing from a visual walkthrough into science.

The blower door test measures the overall air leakage of the building envelope — the walls, windows, doors, ceiling, and floor. A technician installs a large calibrated fan and pressure gauge in an exterior door frame, seals the door opening with a fabric panel, and runs the fan to depressurize the house to a standard 50 Pascals below outdoor pressure (about 0.2 inches of water pressure — very slight). At that standard pressure, the fan flow rate is measured. This gives the leakage figure in cubic feet per minute at 50 Pascals (CFM50), which can be converted to air changes per hour (ACH50) by dividing by the house volume.

What the numbers mean: - Pre-1980 construction: 15–20+ ACH50 — very leaky - Average existing home: 8–12 ACH50 - Good new construction: 3–5 ACH50 - Energy Star certified new home: ≤ 3 ACH50 - Passive House standard: ≤ 0.6 ACH50

During the blower door test, a technician can walk through the house with a smoke pencil or a thermal camera to identify exactly where air is entering. Common leakage sites: electrical outlets on exterior walls, top plates (the framing connection between the wall and the attic floor), around plumbing penetrations in the basement, and the rim joist (the board sitting atop the foundation wall where the first floor framing begins). See Chapter 8 on insulation and air sealing for how to address these.

The duct blaster applies the same principle to the duct system. All supply registers and return grilles are covered with foam pads. A calibrated fan and pressure gauge connects to the duct system at the air handler location. The test pressurizes (or depressurizes) the duct system and measures airflow needed to maintain standard pressure. The result is duct leakage in CFM25 (cubic feet per minute at 25 Pascals differential).

Two duct leakage figures are important: - Total duct leakage: All leakage including connections between the duct system and the conditioned space (register boots, for instance) - Duct leakage to outside (Qn,out): Leakage from supply ducts into unconditioned space — this is the figure that represents real energy waste

Energy codes in most states now require duct blaster testing for new construction. For existing homes, a duct blaster test as part of an energy audit gives you the data to make informed decisions about whether duct sealing is cost-effective.

🧪 Aeroseal: Sealing Inaccessible Ducts from the Inside Aeroseal is a technology that seals duct leaks from the inside out — useful when ducts are hidden inside walls and ceilings. A technician pressurizes the duct system with a mist of polymer particles in air. The particles circulate through the system and accumulate at leak points, eventually sealing gaps up to 5/8 inch diameter. Before and after duct blaster tests verify the result. Aeroseal costs $1,500–3,000 for a whole-house treatment but can reduce duct leakage by 70–90% in systems that can't be manually accessed. It's not magic — very large gaps or disconnected ducts require physical repair first — but for systems with distributed small leakage, it's remarkably effective.

The Chen-Williams renovation didn't need Aeroseal because they were installing entirely new ductwork. Priya and Marcus's contractor used sheet metal trunk lines with flex duct branches, all joints masted with duct mastic, all runs properly supported. A final duct blaster test before closing up the walls confirmed total duct leakage to outside at 4.2 CFM25 per 100 square feet of floor area — well within the Energy Star target of 6.0 CFM25 per 100 sq ft.

20.5 Bathroom and Kitchen Ventilation: Code Requirements and Common Failures

Ventilation of bathrooms and kitchens serves a specific, essential purpose: removing moisture, odors, and cooking byproducts from the home before they cause damage or degrade indoor air quality. This sounds obvious, but the number of homes with improperly installed or non-functional ventilation is staggering.

Bathroom exhaust fans: the basics. Every bathroom should have either a window or a mechanical exhaust fan. Most building codes require mechanical exhaust — an openable window is considered insufficient by current standards because windows are rarely opened in cold weather when moisture from showers is most problematic.

Fan capacity is measured in CFM (cubic feet per minute). The minimum for most bathrooms is 50 CFM; larger bathrooms need more (roughly 1 CFM per square foot of floor area for rooms over 100 square feet). A fan rated at 50 CFM should exchange the air in a 5×8-foot bathroom in about 4 minutes. In practice, many installed fans don't come close to their rated CFM because of duct restrictions — we'll address this below.

Fans are also rated for noise in sones. Most older bath fans are rated 3–4 sones — very audible. Modern quiet fans run at 0.3–0.8 sones and are barely perceptible. If your bathroom fan sounds like a small aircraft, it's not just annoying — people are less likely to use a loud fan, meaning moisture accumulates. A quiet fan is a meaningful upgrade.

The attic duct problem. This is one of the most common code violations in residential construction, found in older homes and in new construction alike: the bathroom exhaust duct terminates in the attic rather than exiting through the roof or exterior wall.

🔴 Bathroom Fans Must Terminate Outside Every building code and every manufacturer installation instruction requires bathroom exhaust to be ducted to an exterior termination point — a roof cap or a sidewall cap. Terminating in the attic is a code violation. In cold weather, the warm, humid air from your shower travels through the duct, hits the cold attic, and condenses. Over time, this deposits gallons of moisture in the attic. The result: soaking wet insulation (which loses its insulation value), wood rot in the sheathing and rafters, and eventually mold. This is one of the most common causes of attic mold, and the remediation (mold treatment, insulation replacement, sheathing repair) costs $3,000–10,000 or more.

Fixing an improperly terminated bathroom fan duct requires running a flexible insulated duct from the fan housing to a roof cap or exterior wall cap. This is a straightforward job that most mechanically-inclined homeowners can accomplish with basic tools. The key requirements: - Use insulated flex duct (prevents condensation inside the duct itself in cold climates) - Keep the run as short and straight as possible - Use a proper exterior termination cap with a damper (prevents cold/hot outdoor air from flowing back into the bathroom when the fan is off) - Seal all connections with mastic or foil tape

How long to run the bath fan? A bath fan needs to run long enough to exhaust the moisture from a shower. The old rule was 20 minutes; modern guidance suggests running it for the duration of the shower plus 20 minutes. Installing a timer switch (which runs the fan for a set period after you leave the bathroom) is one of the best low-cost investments you can make in bathroom ventilation. A humidity-sensing fan switch is even better — it runs the fan until the humidity drops to a setpoint.

Kitchen exhaust. Kitchen exhaust fans (range hoods) deal with a different mix: cooking vapors, grease aerosols, combustion products from gas ranges, and moisture. The stakes for not exhausting kitchen air properly include grease accumulation in cabinets and above the range (fire hazard), moisture damage, and exposure to nitrogen dioxide from gas combustion.

Range hoods should be sized to the cooking appliance. Residential range hoods range from 100 CFM (small, over low-output cooktops) to 1,200 CFM or more (for professional-style ranges). A general rule: allow 100 CFM per 10,000 BTU of range output for gas ranges; for electric ranges, 150 CFM is a common minimum.

Recirculating range hoods (which filter the air and blow it back into the kitchen) are better than nothing but significantly inferior to externally-vented hoods. They remove grease and odors through filters but return moisture and combustion products to the kitchen air. If you have a recirculating hood and venting to outside is possible, the upgrade is worthwhile.

⚖️ DIY vs. Professional: Bathroom Fan and Range Hood Installation - DIY appropriate: Replacing a bathroom fan with a new fan of the same size and using the existing duct run, provided the duct terminates correctly. Installing a new timer or humidity-sensing switch. Cleaning a range hood grease filter (or replacing charcoal filters on recirculating hoods). - Professional recommended: Installing new ductwork through the attic to a roof cap (involves working on the roof and in the attic; not inherently dangerous but benefits from experience). Running new ductwork for a range hood through cabinetry or walls. Venting a gas range hood in an island (requires routing ductwork down through the floor). - DIY with caution: Relocating existing ductwork that terminates in the attic to a proper exterior termination — this involves working in the attic in conditions that range from uncomfortable to hazardous (summer heat, fiberglass insulation). Use appropriate PPE (respirator, long sleeves, eye protection).

20.6 Attic and Crawlspace Ventilation: Why It Matters and What Can Go Wrong

Attic and crawlspace ventilation is one of those topics where the "obvious" answer (more is better) collides with the nuanced reality. Let's look at each space.

Attic ventilation. The goal of attic ventilation in most climates is to keep the attic temperature and humidity close to outdoor conditions. In summer, this prevents heat buildup in the attic from radiating down through the ceiling (or, if the attic is sealed and conditioned, this doesn't apply). In winter, it prevents warm, humid air that leaks from the living space into the attic from condensing on cold roof sheathing.

The standard approach — still the most common in U.S. construction — is vented attic design: soffit vents at the eaves allow outdoor air in, ridge vents at the peak let air out, and natural convection (or wind) drives continuous airflow through the attic. Code requires a minimum net free ventilation area: typically 1 square foot of vent area per 150 square feet of attic floor (or 1:300 if a vapor retarder is present).

Common attic ventilation problems: - Blocked soffit vents: Insulation installed in the attic floor that falls over the soffit vents blocks intake air. Proper installation uses rafter baffles (cardboard or plastic channels stapled to the roof sheathing) that maintain an airway over the insulation from soffit to ridge. - Mismatched vent types: Mixing ridge vents with high gable-end vents creates short-circuit airflow — air enters the ridge and exits the gable vents (or vice versa), bypassing the soffit entirely. The lower half of the attic gets no ventilation. - Insufficient vent area: Vents installed that are too small, or vents with solid screens that have been painted over (eliminating the net free area). - Bathroom fans terminating in attic: As discussed in Section 20.5, a major moisture source that no amount of ventilation can fully offset.

An alternative approach gaining traction in hot climates is the unvented (conditioned) attic: spray foam insulation is applied to the underside of the roof deck, making the attic part of the conditioned envelope. HVAC equipment in the attic stays within the thermal boundary. This eliminates duct leakage to outside (all ducts are now inside the conditioned space), improves HVAC efficiency dramatically, and eliminates ice dams in cold climates. However, it requires proper implementation — an improperly detailed unvented attic can trap moisture and cause major damage. This is not a DIY project.

Crawlspace ventilation. Crawlspace design philosophy has changed dramatically in the past 20 years. The traditional approach — a vented crawlspace with foundation vents around the perimeter — was intended to prevent moisture buildup. In humid climates, it often makes things worse: warm, humid summer air enters the vents, contacts the cooler crawlspace floor and framing, and condenses. The result is chronically wet framing, rot, and mold.

The modern approach in most climates is the encapsulated crawlspace: seal the foundation vents, install a continuous vapor barrier (at least 6-mil poly, ideally 20-mil reinforced liner) over the entire ground surface and lapped up the walls, insulate the walls, and condition or ventilate the crawlspace space with the house. This approach maintains the crawlspace at similar conditions to the rest of the house — warm enough in summer that condensation doesn't form, dry enough in winter that wood moisture content stays safe.

Encapsulating a crawlspace costs $3,000–8,000 professionally. It pays back through: - Reduced heating and cooling costs (the floor is now within the thermal boundary) - Elimination of moisture damage and pest attraction - Cleaner air in the living space (the house draws air up from the crawlspace through floorboards, bypasses, and mechanical gaps — you're breathing what's down there)

📊 Vented vs. Encapsulated Crawlspace | Factor | Vented Crawlspace | Encapsulated Crawlspace | |--------|------------------|------------------------| | Moisture in humid climates | Often problematic | Controlled | | Moisture in dry climates | Usually fine | Usually fine | | HVAC efficiency with ducts | Lower (ducts in unconditioned space) | Higher | | Pest attraction | Higher (moisture/organic material) | Lower | | Initial cost | Low | $3,000–8,000 | | Required maintenance | Monitor for moisture, pests | Inspect liner annually |

20.7 Heat Recovery Ventilators and Energy Recovery Ventilators: Fresh Air Without Heat Loss

Modern homes are intentionally airtight. The drive toward energy efficiency — spray foam insulation, triple-pane windows, comprehensive air sealing — has produced houses that lose dramatically less heat through air leakage. This is good for energy bills. But it creates a problem: where does fresh air come from?

In a leaky older house, fresh air infiltration was constant and involuntary — cracks, gaps, and loose windows continuously exchanged stale indoor air with outdoor air. You were comfortable and the air was fresh, but you were paying an enormous energy penalty. As we tighten homes, we reduce this uncontrolled infiltration. At some point — roughly below 0.35 ACH natural (about 3 ACH50) — the house no longer breathes enough to maintain acceptable indoor air quality through infiltration alone. We need to intentionally introduce fresh air.

The naive solution — simply cut a hole in the wall and push or pull outdoor air — works but wastes energy. In winter, you're bringing in 20°F outdoor air and heating it to 70°F. In summer, you're bringing in 90°F air and cooling it to 75°F. This energy cost partially offsets the efficiency gains from tight construction.

Heat Recovery Ventilators (HRVs) solve this problem elegantly. An HRV is a mechanical ventilation device with two airstreams running through a heat exchanger core: - One stream pulls stale exhaust air from the house (typically from bathrooms, utility rooms, and kitchen) - The other stream pulls fresh outdoor air in

In the heat exchanger core, the two airstreams pass in close proximity through thin plates or channels without mixing. In winter, the warm outgoing exhaust air transfers heat to the incoming cold fresh air — typically recovering 70–80% of the heat that would otherwise be lost. The fresh air entering the house is still cool (it doesn't reach full indoor temperature) but far warmer than outdoor temperature. In summer, the core works in reverse: outgoing cool exhaust air cools incoming hot outdoor air.

Energy Recovery Ventilators (ERVs) work on the same principle but use a core that transfers both heat and moisture. In winter, the outgoing warm, humid exhaust air transfers both heat and some humidity to the dry incoming cold air. In summer, the outgoing cool, dry air removes both heat and humidity from the incoming hot, humid air. ERVs are generally preferred in very dry climates (where maintaining some interior humidity in winter is desirable) or very hot, humid climates (where managing summer humidity is critical). In mixed climates, HRVs and ERVs perform comparably.

💡 Do You Need an HRV/ERV? If your home's blower door test shows below 3 ACH50, mechanical ventilation is strongly recommended. ASHRAE Standard 62.2 (the indoor air quality standard for residential buildings) specifies required ventilation rates based on floor area and number of bedrooms — for a 2,000 square foot, 3-bedroom home, roughly 60–75 CFM of continuous ventilation. This can be supplied by an HRV or ERV, or by a simpler system that periodically opens a fresh air damper connected to the return duct.

HRV/ERV installation and controls. An HRV or ERV connects to the house's existing duct system or runs its own dedicated duct network. In a home with a forced-air system, the most common approach is to connect the HRV to the return air duct — fresh air from the HRV enters the return stream, gets filtered by the HVAC system filter, and is distributed through the supply ducts. The exhaust ductwork runs to bathrooms and utility rooms.

Controls range from simple on/off timers to sophisticated touchscreen controllers that manage ventilation rate, boost mode (for showers), and seasonal adjustments. At minimum, set your HRV to continuous low-level ventilation (typically 50–70 CFM for most homes) with a boost function for bathrooms.

Annual maintenance is straightforward: - Clean or replace the filter (typically twice a year) - Clean the heat exchanger core (remove and rinse with water annually) - Check exterior hoods for debris and pest intrusion - Check condensate drain (HRVs produce condensation in heating season)

📊 HRV vs. ERV: When to Choose Which | Climate/Condition | Better Choice | |------------------|--------------| | Cold, dry winters (northern U.S., Canada) | ERV (retains some humidity) | | Cold, moderate humidity winters | HRV | | Hot, humid summers | ERV (removes humidity in summer) | | Mixed climate, tight house | Either; ERV slightly more common | | Occupants with respiratory sensitivities | HRV (no moisture transfer means better mold resistance in core) |

The Chen-Williams renovation presents the ideal case for HRV installation. Priya and Marcus are building a tight, well-insulated house from the ground up. Their energy audit target is below 1.5 ACH50 — far below the threshold for adequate natural ventilation. Their HVAC designer specified a 120 CFM HRV, ducted to all bathrooms for exhaust and connected to the main return duct for supply. The cost was approximately $1,800 for equipment and $600 for installation (combined with the duct system work). Over 20 years, the HRV will recover roughly $6,000 in heating and cooling energy that would otherwise be spent on unrecovered ventilation air — while maintaining excellent indoor air quality that a leaky house cannot guarantee.

The broader picture. The logic of tight construction plus controlled mechanical ventilation is: when you control the air exchanges, you control the energy penalty and the air quality. A well-designed tight house with an HRV will have better air quality than a leaky house, because the HRV continuously introduces filtered outdoor air at a controlled rate, while the leaky house introduces random amounts of air from random locations (crawlspace, attic, wall cavities) with no filtration at all.

🔗 The Ventilation-Combustion Interaction One caution for homes with combustion appliances (gas furnaces, water heaters, fireplaces): tight construction combined with powerful exhaust fans can depressurize the house to a degree that backdrafts combustion equipment — pulling combustion gases back down the flue into the house. This is why combustion safety testing (a blower door test with combustion appliances operating) is a critical part of any major air sealing project. If you significantly tighten your house — adding spray foam, new windows, comprehensive air sealing — have a combustion safety test done afterward. This is not optional.

20.8 Pulling It Together: The Duct System You Have vs. the System You Need

If your home has a forced-air HVAC system, the duct system is either a source of silent energy waste and comfort problems, or it's doing its job well. You probably don't know which, because most homeowners have never had it tested.

A practical action plan:

Step 1: Start with what you can see. Walk through the attic or crawlspace where ducts are accessible. Look for disconnected sections, collapsed flex duct, joints that are obviously unsealed. This free inspection often reveals low-hanging fruit.

Step 2: Check your registers. Hold a piece of tissue at each supply register while the system runs. Is airflow strong, moderate, or barely perceptible? Compare room to room. Rooms with weak airflow may have duct restrictions, disconnections, or severe leakage before they reach the register.

Step 3: Consider a professional energy audit. A home energy audit with blower door and duct blaster testing ($300–600, often subsidized by your utility) gives you measured data: how leaky is your house, how leaky are your ducts, and where the leaks are. With this data, you can prioritize repairs by return on investment.

Step 4: Seal accessible leaks. Armed with your audit results, seal what you can reach: return grille connections to framing, accessible flex duct joints, the connection between supply boots and the ceiling/floor. Mastic and foil tape. This is a satisfying DIY project with direct payback.

Step 5: Consider aeroseal for inaccessible ducts. If your audit reveals significant leakage in ducts that can't be accessed, get a quote for Aeroseal from a certified contractor.

Step 6: Evaluate ventilation. If your house is tight (under 5 ACH50 from a blower door test), evaluate your ventilation strategy. Exhaust-only ventilation (bath fans running on timers) is the simplest and cheapest. A balanced HRV or ERV is better for houses under 3 ACH50.

Dave Kowalski's final takeaway: he spent $1,800 on professional duct sealing, replacing the failed duct tape with mastic and adding proper supports to sagging flex runs. The next season's heating bill was $410 lower. The project paid for itself in 4.4 years. He now runs the system each fall knowing that nearly every dollar he spends on heating actually goes into his living spaces — not up into his unfinished basement ceiling.

⚠️ Don't Ignore the Signs Persistent cold spots in specific rooms, humidity problems in winter, excessive dust from registers, ice dams on the roof despite adequate insulation (which can indicate attic air leakage), high heating and cooling bills relative to neighbors with similar homes — these are all signals that your duct system and ventilation deserve attention. The technology to diagnose these problems precisely exists and is accessible. The cost of measurement is low; the cost of continued waste is not.

20.9 Duct Cleaning: When It's Genuinely Needed and When It's a Scam

Few HVAC services generate more confusion and predatory marketing than duct cleaning. You'll see ads for whole-house duct cleaning at prices ranging from $99 to $1,500, often with alarming language about the pounds of dust and debris inside your ducts. Some of this marketing is legitimate; much of it is not. Here is how to tell the difference.

What duct cleaning actually does — and what it doesn't

A legitimate duct cleaning uses a powerful truck-mounted or portable vacuum attached to the main return trunk, combined with agitation tools (rotating brushes and compressed air whips) inserted into individual supply branches. The vacuum holds the system at negative pressure while the agitation tools dislodge dust, debris, and any accumulated organic material from the duct walls, pulling everything toward the vacuum. Done thoroughly by a qualified contractor, this physically removes material that has settled inside the duct system over years.

What it does not do: it does not address mold growth inside ductwork (a separate issue requiring antimicrobial treatment), it does not seal leaks, it does not improve airflow if the system is undersized or improperly designed, and it does not eliminate the source of dust if the source is a leaky duct system pulling in dusty attic or crawlspace air.

The EPA's position

The U.S. Environmental Protection Agency's guidance on duct cleaning is notably restrained. The EPA states that duct cleaning "has not been shown to actually prevent health problems" and that the evidence it benefits air quality is "inconclusive." The EPA recommends duct cleaning in specific circumstances but does not recommend it as a routine maintenance practice.

The specific circumstances where duct cleaning is genuinely warranted:

• Substantial mold growth is visible inside the ductwork or on components of the HVAC system (verify this with a qualified inspector — mold-like discoloration can be other things, and a positive determination is important before paying for remediation)
• Infestation by rodents or insects — if mice or squirrels have been living in your ductwork, the debris and contamination they leave behind warrants cleaning
• Excessive dust and debris that is visibly being blown from registers — not just "looks dusty in there" but actively blowing particles into the room
• After major renovation that generated significant dust inside the house — drywall dust and similar fine particulates settle inside ducts and can be worth removing before resuming normal operation
• After a flood or fire that sent contaminated material into the duct system

📊 Duct Cleaning: Legitimate vs. Predatory | Indicator | Legitimate Contractor | Predatory Service | |-----------|----------------------|-------------------| | Price | $300–$700 for a whole house | $99 "whole house" deal | | Method | Truck-mount vacuum + mechanical agitation | Blower-only or compressed air only | | Scope | Every supply and return branch | Quick pass through main trunk | | Upcharges | Mold treatment (if mold is confirmed) | Mandatory chemical treatment, filter replacement, "sanitizing" | | Timeline | 3–5 hours for a typical home | Under 1 hour | | NADCA membership | Yes (or equivalent credential) | No mention |

NADCA — the National Air Duct Cleaners Association — has published source removal standards for the industry. A contractor who follows NADCA standards will use negative pressure throughout the cleaning process and will mechanically agitate every branch. If a contractor shows up with a shop vacuum and some compressed air cans, they're not doing source removal cleaning.

The most common scam: the bait-and-switch

A company advertises a $99 whole-house duct cleaning. A technician arrives, does a quick walk-through, and tells you your ducts are heavily contaminated with mold, require chemical sanitizing treatment, and the total will be $800. The "mold" may be ordinary dust, and the sanitizing treatment (often an antimicrobial spray) is applied whether or not there is any microbial growth. The chemicals used may introduce their own indoor air quality concerns.

If a technician tells you that you need additional treatments, ask to see the mold before you agree to any treatment. If they can't show you visually confirmed mold growth (not just "this looks black"), decline the upsell.

When duct cleaning makes sense as a practical matter

Even outside the EPA's specific circumstances, there are situations where homeowners reasonably choose duct cleaning:

• Moving into an older home where you don't know the history and the system looks heavily loaded with dust — peace of mind has real value
• After the first season following a renovation that included drywall work
• If you or a household member has significant respiratory sensitivities and you want to start fresh

The key: hire a NADCA-certified contractor, understand that the benefit is primarily removing accumulated debris rather than improving health outcomes, and don't pay for chemical treatments unless you have visually confirmed mold.

💡 The Better Investment

If your concern is indoor air quality rather than duct appearance, the highest-return investment is usually not duct cleaning but duct sealing (stopping the infiltration of attic or crawlspace air into the return system in the first place) combined with a higher-quality filter and regular filter changes. A well-sealed system with a MERV 11 filter changed every 90 days will deliver better air quality than a cleaned system with a MERV 6 filter that isn't maintained.

20.10 Static Pressure in Depth: Diagnosing What the Numbers Are Telling You

The concept of static pressure was introduced in Section 20.1, but it deserves deeper treatment because it is the single most useful diagnostic metric for a forced-air duct system — and one that almost no residential HVAC system has ever been properly tested for.

What static pressure is, physically

Imagine you're trying to blow air through a straw. When the straw is short and wide, you barely have to work. When the straw is long, narrow, or kinked, you have to work much harder. The "hardness" of the blowing effort corresponds to static pressure: the pressure differential the blower must create and maintain to push air through the duct system at the design flow rate.

Every blower in an HVAC system has a fan curve — a chart showing how much air it can deliver (in CFM) at different static pressures. As static pressure increases, airflow decreases. When static pressure gets high enough, the blower is working at the edge of its capacity and may deliver significantly less than its rated airflow. The cooling coil doesn't get the airflow it needs, the heat exchanger runs hotter than designed, and the system underperforms in every measurable way.

How static pressure is measured

A technician uses a digital manometer — a pressure gauge that measures in inches of water column (in. w.c.) — with pressure taps inserted into the duct system at two points: one in the supply plenum (just downstream of the air handler) and one in the return plenum (just upstream of the air handler, after the filter). The difference between these two readings is total external static pressure (TESP).

Total external static pressure should be compared to the air handler's maximum rated static pressure, which is on the equipment nameplate or in the manufacturer's documentation. Common residential air handlers are rated for 0.50 in. w.c. TESP; premium equipment may be rated to 0.80 in. w.c.

Reading static pressure findings

If TESP is significantly above the rated maximum, the system is under-delivering airflow. The questions become: where is the excess resistance coming from?

To diagnose this, a technician takes additional readings at specific points:

• Filter pressure drop: Measure before and after the filter. A clean MERV 8 filter typically adds 0.10–0.15 in. w.c. A MERV 13 filter can add 0.20–0.30 in. w.c. even when clean, and significantly more as it loads with debris.
• Coil pressure drop: Measure before and after the evaporator coil. A clean coil might add 0.15–0.25 in. w.c. A coil coated with dust and debris (common in systems that have been run without filters, or with filters that have slipped out of their tracks) can add 0.40 in. w.c. or more.
• Supply-side drop: The pressure reading in the supply plenum minus the reading at the end of a long duct run. Large drop indicates undersized or restricted supply ductwork.
• Return-side drop: The pressure at the return plenum minus the pressure at various return grille locations. Large drop indicates inadequate return capacity.

📊 Interpreting a Static Pressure Diagnosis | Finding | Likely Cause | Action | |---------|-------------|--------| | High filter drop even with new filter | Filter MERV rating too high for system | Downgrade to MERV 8–10 or install media cabinet | | High filter drop with "clean" filter | Filter clogged (change monthly if needed) | More frequent changes or bigger filter housing | | High coil drop | Dirty evaporator coil | Professional coil cleaning | | High supply-side drop | Undersized trunk or excessive flex duct | Upsize trunk or reduce flex duct restrictions | | High return-side drop | Inadequate return grille area or undersized return duct | Add return grilles or jumper ducts |

The MERV filter trap

One of the most common causes of high static pressure in residential systems is well-intentioned homeowners installing MERV 13 or MERV 16 filters in systems designed for MERV 8. The thinking is reasonable: better filter = cleaner air. The problem is that most residential air handlers were designed with a 1-inch filter slot and a rated static pressure that assumes a MERV 8 filter. A thick, high-MERV filter can add 0.25–0.35 in. w.c. of pressure drop that the system wasn't designed to handle.

If you want the air quality benefit of a high-efficiency filter without sacrificing airflow, the answer is a media cabinet: a wider filter housing (4–5 inches deep) that holds a thicker media filter at the same or lower pressure drop than a 1-inch MERV 13 filter. Retrofitting a media cabinet is a standard HVAC service — cost is typically $300–$600 installed — and it's the right answer for anyone who wants both filtration efficiency and system performance.

⚠️ Signs Your System Is Fighting High Static Pressure

• Furnace or air handler cycles on and off more frequently than normal
• You can hear the blower "strain" or notice airflow at registers is weaker than it used to be
• Cooling system struggles to maintain setpoint on moderately hot days despite a properly charged refrigerant system
• Heating system lockout or limit switch trips (a safety mechanism that shuts down a furnace when heat exchanger temperatures exceed safe limits due to insufficient airflow)
• Ice formation on the evaporator coil during cooling season (inadequate airflow causes the coil to drop below freezing)

Any of these symptoms warrants a static pressure measurement. The test takes about 30 minutes and any competent HVAC technician should be able to perform it.

20.11 Zoning Dampers: How Duct Zoning Works Mechanically

A single-zone forced-air system treats the entire house as one thermal space. When the thermostat calls for heating or cooling, conditioned air flows to every room simultaneously. This is simple and relatively inexpensive, but it's inefficient in homes where different areas have very different heating and cooling needs — and it means that rooms that don't need conditioning right now are getting it anyway.

Duct zoning solves this with a system of motorized dampers inside the ductwork that can open or close independently, routing conditioned air to only the zones that need it at any given time.

The components of a zoning system

Zone dampers: Motorized dampers installed inside the duct branches serving each zone. They're typically circular or rectangular metal blades that rotate to open or close the duct. Most use 24-volt motors controlled by the zone panel. They come in normally-open (fail-open, which means the damper opens if power is lost — generally preferred for safety) and normally-closed configurations.

Zone thermostats: Each zone has its own thermostat, which independently calls for heating or cooling based on conditions in that zone.

Zone control panel: The central controller that receives signals from each zone thermostat and opens or closes the appropriate dampers while calling for heating or cooling from the main HVAC equipment. When zone 2's thermostat calls for cooling, the zone panel opens zone 2's damper, closes the dampers for zones that are satisfied, and starts the air handler and compressor.

Bypass duct or variable-speed equipment: This is the critical piece that many homeowners and even some contractors don't understand. When dampers close for satisfied zones, the remaining open ductwork serves a smaller portion of the house. The blower is now pushing the same (or nearly the same) volume of air through a smaller duct network, which dramatically increases static pressure. Normally, this would stall the system or cause the equipment to operate in a damaging condition.

There are two solutions:

1. Bypass damper: A duct running directly from the supply plenum back to the return plenum, with a pressure-activated or motorized bypass damper. When supply pressure rises because zone dampers have closed, the bypass damper opens and short-circuits some air back to the return. This bleeds off the excess pressure. It's a simple, reliable, and relatively inexpensive solution, but it has a downside: the bypassed air is recirculated rather than delivered to a zone, which means the system runs longer to satisfy the calling zone, reducing efficiency.

2. Variable-speed (ECM) air handler: A more sophisticated and energy-efficient solution. The variable-speed blower reads static pressure and adjusts its motor speed to maintain design pressure regardless of how many zones are open. When three of four zones are satisfied and only one zone damper is open, the blower slows down rather than pushing full airflow through a restricted system. This is the correct long-term solution for a multi-zone system and is standard in higher-end installations.

Where zoning makes the most economic sense

Not every home benefits equally from zoning. The homes where zoning delivers clear value:

• Multi-story homes where upper floors are always hotter in summer and the ground floor always needs more heat in winter
• Homes with large areas of south- or west-facing glass that create extreme solar gain in specific rooms or wings
• Homes with finished basements that are used differently from the main living space — often cooler in summer (benefiting from the earth's thermal mass) and needing less heating
• Homes where different members of the household have significantly different temperature preferences and occupy different parts of the house on different schedules

💡 Zoning Is Not a Substitute for Load Calculation

A common misconception: zoning can compensate for an HVAC system that is oversized or undersized for the house. It can't. Zoning manages distribution, not capacity. If your system is oversized for your house, it will short-cycle in each zone. If it's undersized, it will run continuously in every zone without meeting setpoint. The first step is always properly sizing the equipment to the load — zoning is a distribution refinement added on top.

Adding zoning to an existing system

Retrofitting zone dampers into an existing duct system is possible but ranges from straightforward to complex depending on the duct layout. Systems with accessible main trunk lines and distinct branch runs to different parts of the house are reasonable retrofit candidates. Systems with complex flex duct networks that serve multiple areas from single branches are much harder to zone effectively without significant duct modification.

Before investing in a zoning retrofit, have an HVAC contractor evaluate your duct layout and confirm that the branch structure actually allows meaningful zone isolation. A poorly designed zoning retrofit — one where the dampers don't truly isolate the target zones — provides little benefit while adding system complexity and potential failure points.

20.12 Register and Grille Placement: The Room-by-Room Design Logic

The location of supply registers and return grilles in each room is not arbitrary. Good register placement follows from the physics of air distribution — how air moves when it enters a space, how temperature stratification works, and how to get the conditioned air where the occupants actually are.

Supply register placement principles

Perimeter placement: The traditional (and still preferred) approach for residential systems is to locate supply registers on the exterior perimeter of the room — under windows, along exterior walls, near exterior doors. The reasoning: these are the locations of maximum thermal load. In winter, the coldest air in the room is adjacent to the exterior wall and window surfaces. By placing supply registers here, the warm supply air rises along the cold surface, creating a curtain of warm air that counters the cold infiltration and reduces drafts. In summer, placing registers high on exterior walls or in the ceiling creates a wash of cool air that flows down the wall, countering heat gain through the envelope.

Throw distance: A supply register's "throw" is how far it can project air across a room before the airflow velocity drops below about 50 feet per minute (the threshold below which you no longer feel air movement). Register throw depends on the register's design and the airflow velocity through it. A ceiling register with adequate throw will project air across an 18-foot room; a floor register in the middle of the room will dump air straight up and provide poor coverage of the perimeter.

Avoiding stratification: In rooms with high ceilings (9 feet or taller), ceiling-mounted supply registers must be carefully sized. If airflow velocity is too low, the cool supply air in summer doesn't reach the occupied zone — it pools at the ceiling. Registers with adjustable deflection can angle the air stream down toward the occupied zone.

Return grille placement principles

Return grilles pull air out of the room and back to the air handler. Their placement matters less for comfort (they're not delivering conditioned air) but matters for balance.

High-wall returns: In cooling-dominated climates or rooms with cooling priority, high-wall return grilles pull the warmest air (which rises) back to the system. This is more efficient cooling.

Low-wall or floor returns: In heating-dominated climates, low-wall returns pull cooler floor air back to the system — more efficient heating.

Central vs. room-by-room returns: The most economical approach (and the most common in residential construction) is a single large central return, often in a hallway. Air returns from all rooms by flowing under doors and through the hallway. This works adequately with interior doors open, but creates the room pressurization problems described in Section 20.1 when doors are closed. Dedicated returns in each bedroom or room are the better solution, though more expensive to install.

Return grille sizing: Return grilles must be sized for low velocity — if return air velocity is too high, the grille becomes noisy and the pressure drop across it becomes excessive. The general rule is to size return grilles for a maximum face velocity of 400–500 feet per minute. A 14×24-inch return grille at 400 FPM handles approximately 700–800 CFM — which may be the entire airflow of a small system. If your return grille hisses or makes noise when the system runs, it is undersized for the airflow.

🔵 A Simple Register Audit

To check your own system: with the system running, move a piece of tissue at each supply register. Observe which direction the air flows (is it blowing toward the room's center, toward a window, toward the floor — wherever the register is designed to direct it?). Then check your return grilles: hold the tissue near each one and confirm air is being pulled in. A return grille with no suction means either the grille is blocked or the return duct behind it is disconnected. Both are worth investigating.

20.13 Insulating Ductwork in Unconditioned Spaces: Why It Matters and How to Do It

If you have ductwork running through your attic, crawlspace, or unattached garage, insulation quality is directly tied to system efficiency and performance. An uninsulated duct in a 130°F attic will lose a significant fraction of its cooling capacity before the air even reaches a register. Properly insulated ductwork in unconditioned spaces is one of the highest-return efficiency investments you can make.

Why unconditioned spaces are so hostile to ductwork

Attics in summer are brutal: surface temperatures on the roof deck can exceed 150°F, and attic air temperature is typically 20–40°F above outdoor ambient. A cooling system working to bring air to 55°F at the air handler may be delivering 68°F air at a register 40 feet away through an uninsulated duct in such an attic. The physics is simple: the greater the temperature difference between the duct interior and the surrounding environment, the faster the heat transfer.

Crawlspaces in humid climates present a different problem: warm, humid summer air surrounding cold supply ducts causes condensation on the duct exterior. This condensation saturates any fiberglass insulation, promotes mold growth on the duct exterior, and drips onto the crawlspace floor — contributing to the moisture problems that make crawlspace encapsulation (Section 20.6) so valuable.

Current code requirements

Energy codes in most states — following IECC 2018 or later — require a minimum of R-6 insulation on ductwork in unconditioned spaces. Some code editions require R-8 in the most extreme climate zones. R-8 is now the recommended minimum by energy efficiency practitioners regardless of code minimums, and R-12 is increasingly specified for attic ductwork in hot climates.

Most flex duct sold today has R-6 or R-8 insulation built in (the insulation layer is integral to the flex duct product). For sheet metal ductwork in unconditioned spaces, insulation must be added separately.

Adding insulation to existing sheet metal ductwork

If you have uninsulated or under-insulated sheet metal ducts in an attic or crawlspace, the improvement process is:

1. Seal first, insulate second. Adding insulation over leaky joints traps the leakage — the duct still leaks, but you can no longer see or access the joints. Always seal with mastic or foil tape before adding insulation.

2. Select the insulation product. Options include: - Duct wrap: Fiberglass batt or mineral wool in a foil-faced blanket designed to wrap around duct sections. Applied with the vapor barrier facing outward. Comes in R-4, R-6, and R-8 varieties. - Rigid foam insulation boards: Cut to shape and adhered or taped around duct sections. More durable in crawlspace environments where duct wrap can be damaged by moisture or pests. - Pre-insulated flexible duct sleeve: For round sheet metal branch ducts, pre-made insulated flex sleeves can be slipped over the metal duct. Easiest to install.

3. Cover all surfaces. It's not enough to wrap three sides of a rectangular duct; the full perimeter must be covered without gaps. Particular attention to joints, takeoffs, and transitions where the insulation is harder to seal.

4. Protect the vapor barrier. In humid environments, the foil facing on duct wrap serves as a vapor retarder, keeping exterior humidity from condensing inside the insulation layer. Tears and gaps in the facing compromise this function. Seal tears with foil tape.

The return-on-investment case

Insulating previously uninsulated attic ductwork in a hot climate can reduce cooling-season duct losses by 10–20% of total airflow. For a system moving 1,200 CFM of air and losing 15% of its cooling capacity through attic heat gain, proper insulation can recover the equivalent of roughly half a ton of cooling capacity — at a fraction of the cost of upgrading the cooling equipment.

For a home with $1,800/year in cooling costs and 15% duct heat gain, the annual savings from proper duct insulation in the attic might be $200–$270. A professional installation of duct insulation for a typical attic duct system costs $500–$1,500. Payback: 3–6 years — before accounting for equipment longevity benefits from reduced operating stress.

✅ Check This Yourself

Go into your attic or crawlspace while your cooling system is running and feel the supply ducts. If the duct surfaces feel warm (in summer), heat is transferring through the duct wall and you're losing cooling capacity. If visible flex duct has insulation that looks compressed, torn, or sagging away from the inner liner, the insulation's effective R-value is a fraction of its rated value. Both situations warrant remediation.

Cross-references: Chapter 8 (building envelope, air sealing, and insulation — the context in which ductwork operates), Chapter 18 (heating systems — furnaces and boilers that supply conditioned air to or through the duct system), Chapter 19 (air conditioning — cooling systems that use the same duct system), Chapter 10 (basements and crawlspaces — spaces where ducts run and where encapsulation decisions are made).