Chapter 20 Key Takeaways: Ductwork, Airflow, and Ventilation

The Leakage Problem

The average American home loses 20–30% of conditioned air through duct leakage. This is not a fringe problem — it's the norm. One in four dollars you spend on heating and cooling may be going into an attic, crawlspace, or wall cavity rather than a room.

Duct leakage is invisible and silent. The only way to know for certain is to test. Utility-sponsored energy audits with duct blaster testing are often free or subsidized — take advantage of them.

Duct System Basics

• Supply ducts deliver conditioned air to rooms. Return ducts bring house air back to the equipment.
• Supply and return must be balanced. Rooms without return air paths pressurize when the system runs, pushing conditioned air into the building envelope.
• Static pressure is the resistance your blower works against. Too-high static pressure (over ~0.50 in.w.c.) reduces airflow, reduces efficiency, causes coil icing, and wears out the blower motor.

Duct Materials

Never use standard duct tape (gray, fabric-backed) to seal ducts. It fails within a few years. Use UL 181 duct mastic or UL 181 aluminum foil tape.

Sealing: What You Can Do

Accessible duct sealing — any duct joint you can reach in a basement, crawlspace, or accessible attic — is an excellent DIY project: - Buy UL 181 duct mastic at an HVAC supply house (~$28/gallon) - Apply with a brush or gloved hand to all joints - Pay special attention to: air handler connections, branch takeoff collars, and supply boot perimeters - Inaccessible ducts can be sealed from inside with Aeroseal (professional service; evaluate by payback)

Bathroom and Kitchen Ventilation

🔴 Bathroom fans MUST terminate outside — through a roof cap or sidewall cap with damper. Termination in the attic deposits moisture directly onto roof sheathing, causing rot and mold. This is a code violation found in enormous numbers of homes.

• Fans should be quiet (under 1.5 sones) so people actually use them
• Run bath fans for the shower duration plus 20 minutes, or use a humidity-sensing switch
• Range hoods should vent to exterior when possible; recirculating hoods return moisture and combustion products to the kitchen

Attic Ventilation

Standard vented attic design uses soffit + ridge vents. Common failures: - Insulation blocking soffit vents (fix: install rafter baffles) - Mismatched vent types creating airflow short-circuits

Alternative: spray foam unvented (conditioned) attic — eliminates duct losses to outside entirely and eliminates ice dams, but requires professional installation.

Crawlspace

In humid climates, traditional vented crawlspaces often create more moisture problems than they solve. An encapsulated crawlspace (sealed vents, complete ground vapor barrier, insulated walls, conditioned with house) is the modern best practice and pays back through: - Reduced HVAC costs (floor is now inside the thermal boundary) - Reduced moisture damage and pest attraction - Better indoor air quality (house draws air from crawlspace through gaps)

HRV and ERV — Mechanical Ventilation

Tight modern homes (below 3–5 ACH50) require mechanical ventilation to maintain indoor air quality. Options: - Exhaust-only: Bath fans on timers; simple, cheap, but unbalanced (slightly depressurizes house) - HRV: Balanced exhaust + supply with heat recovery (~70–82% efficiency). Transfers heat only. - ERV: Like HRV but also transfers moisture. Better for very dry winters or very humid summers.

HRV/ERV annual maintenance: clean filters every 6 months, rinse heat exchanger core annually. Neglecting this causes mold in the core.

The Action Priority List

1. Test your ducts (energy audit with duct blaster — often free from utility)
2. Seal accessible duct leaks with mastic ($78 in materials, weeks-to-months payback)
3. Verify bathroom fans terminate outside — if not, fix it
4. Know your home's airtightness — if below 5 ACH50, evaluate mechanical ventilation
5. Consider crawlspace encapsulation if you have humidity, pest, or energy problems