Appendix E: Glossary

This glossary defines every key technical term used in How Your House Works. Entries are arranged alphabetically. Each definition is written in plain English, followed by the chapter where the term first appears in the main text. Where a term has both a common name and a technical name, both are listed.

A

AFCI (Arc Fault Circuit Interrupter): A circuit breaker or outlet device that detects the small electrical arcs that can ignite insulation or nearby materials and trips before a fire starts. Required by modern code in bedrooms and most living areas. First introduced: Chapter 16.

AFUE (Annual Fuel Utilization Efficiency): A percentage rating that expresses how much of the fuel burned by a furnace or boiler is actually converted to usable heat over the course of a heating season. An 80% AFUE furnace loses 20% of its fuel energy up the flue. First introduced: Chapter 23.

Air barrier: A material or assembly designed to resist the uncontrolled movement of air through the building envelope. Distinct from a vapor retarder; the two functions require different materials and are often confused. First introduced: Chapter 4.

Air handler: The indoor portion of a split HVAC system, containing the blower, the evaporator coil, and often the electric heat strips. Circulates conditioned air through the duct system. First introduced: Chapter 19.

Air sealing: The process of identifying and sealing penetrations, gaps, and cracks in the building envelope to reduce uncontrolled air infiltration and exfiltration. Often more cost-effective than adding insulation. First introduced: Chapter 4.

Ampere (amp, A): The unit of electrical current — the rate at which electrons flow through a conductor. A 20-amp circuit is rated to carry up to 20 amps of continuous load. First introduced: Chapter 12.

Anchor bolt: A bolt embedded in a concrete foundation that connects the mudsill (bottom plate) of the wood framing to the foundation, resisting lateral forces from wind and earthquakes. First introduced: Chapter 2.

Anode rod: A sacrificial magnesium or aluminum rod inside a tank water heater that corrodes preferentially, protecting the steel tank from rust. Typically needs replacement every 3–6 years. First introduced: Chapter 7.

Arc fault: An unintended electrical discharge that occurs when current jumps across a gap or through damaged insulation. Arc faults are a leading cause of residential electrical fires. First introduced: Chapter 16.

Asphalt shingles: The dominant residential roofing material in North America; made from a fiberglass or organic mat coated with asphalt and surfaced with mineral granules. Available in 3-tab and architectural (dimensional) styles. First introduced: Chapter 24.

ASHRAE: The American Society of Heating, Refrigerating and Air-Conditioning Engineers, a professional organization that publishes widely adopted standards for ventilation (Standard 62.2) and energy efficiency (Standard 90.1). First introduced: Chapter 20.

B

Backdrafting: The reversal of normal combustion appliance draft, where exhaust gases are pulled back into the living space rather than venting out. A serious carbon monoxide hazard. First introduced: Chapter 22.

Balance point: In heat pump systems, the outdoor temperature at which the heat pump's output exactly matches the building's heat loss. Below the balance point, supplemental electric resistance heat is needed. First introduced: Chapter 18.

Balloon framing: An older framing method (pre-1950s) in which wall studs run continuously from the foundation sill to the roof, with floor framing attached to the studs rather than bearing on a top plate. Creates continuous concealed cavities that require fire blocking. First introduced: Chapter 3.

Beam: A horizontal structural member that carries loads perpendicular to its length, transferring forces to columns or bearing walls. Larger and typically heavier than a joist. First introduced: Chapter 3.

Bearing wall: See load-bearing wall.

Blower door test: A diagnostic test in which a calibrated fan is temporarily mounted in an exterior door opening to pressurize or depressurize the house, allowing technicians to measure air leakage and locate its sources. Results are expressed in CFM50 or ACH50. First introduced: Chapter 4.

Bonding: The practice of connecting all metallic components of a system (pipes, ducts, appliance enclosures) to each other electrically, ensuring they are at the same voltage potential and preventing dangerous voltage differences. Distinct from grounding. First introduced: Chapter 12.

Boiler: A heating appliance that heats water (not steam, in most modern residential systems) and circulates it through radiators, baseboard units, or radiant floor tubing. First introduced: Chapter 18.

BTU (British Thermal Unit): The unit of heat energy equal to the energy required to raise one pound of water by one degree Fahrenheit. Used to express the heating and cooling capacity of HVAC equipment. First introduced: Chapter 18.

Building code: A set of minimum standards adopted by a jurisdiction governing the construction, alteration, and maintenance of buildings. Most U.S. jurisdictions adopt model codes (such as the IRC) with local amendments. First introduced: Chapter 33.

Building envelope: The physical boundary between the conditioned interior of a building and the unconditioned exterior, including walls, roof, floor, windows, and doors. The integrity of the envelope determines heating and cooling loads. First introduced: Chapter 4.

Busbar: A solid metal bar, typically copper or aluminum, inside an electrical panel that distributes current from the service entrance to individual circuit breakers. First introduced: Chapter 13.

BX cable (armored cable, AC cable): Electrical cable with conductors enclosed in a flexible spiral metal armor. Often found in older installations and commercial applications; more resistant to physical damage than NM cable. First introduced: Chapter 14.

C

Carbon monoxide (CO): A colorless, odorless gas produced by incomplete combustion of carbon-containing fuels. Even low concentrations can cause serious health effects; high concentrations are rapidly fatal. First introduced: Chapter 22.

Casement window: A window hinged on one side that opens outward with a crank mechanism. Generally provides better air sealing than double-hung windows when closed. First introduced: Chapter 5.

Caulk: A flexible sealant used to fill gaps, joints, and seams in construction to prevent air, water, or pest infiltration. Available in silicone, latex, and hybrid formulations. First introduced: Chapter 31.

Certificate of occupancy (CO): A document issued by a local building department certifying that a newly constructed or substantially renovated building meets all applicable code requirements and may be legally occupied. First introduced: Chapter 33.

CFM (cubic feet per minute): The standard unit for measuring airflow volume. Used to rate ventilation fans, range hoods, HVAC blowers, and duct capacity. First introduced: Chapter 20.

Circuit breaker: An automatically operated electrical switch designed to protect a circuit from damage caused by excess current. Unlike a fuse, a tripped breaker can be reset. First introduced: Chapter 13.

Clean-out: A removable plug or fitting in a drain pipe that provides access for cleaning or inspection. Required by code at specific intervals and locations in the drain-waste-vent system. First introduced: Chapter 8.

CO detector (carbon monoxide detector): A device that measures airborne carbon monoxide concentrations and sounds an alarm before levels become dangerous. Required by code adjacent to sleeping areas in homes with combustion appliances or attached garages. First introduced: Chapter 22.

Compressor: In a refrigeration or heat pump system, the mechanical device that compresses the refrigerant vapor, raising its pressure and temperature and enabling heat transfer. The compressor is the heart (and usually the most expensive component) of the refrigeration cycle. First introduced: Chapter 19.

Condenser: The component of a refrigeration or heat pump system where the refrigerant releases heat to the surroundings — in a standard air conditioner, this is the outdoor unit. First introduced: Chapter 19.

Conduit: A rigid or flexible tube used to protect and route electrical wiring, required in many exposed locations and commercial applications. Made from metal (EMT, rigid steel) or plastic (PVC). First introduced: Chapter 14.

COP (Coefficient of Performance): A dimensionless efficiency ratio for heat pumps, expressing how many units of heat energy are delivered per unit of electrical energy consumed. A COP of 3 means 3 kWh of heat delivered per 1 kWh of electricity used. First introduced: Chapter 23.

CPVC (chlorinated polyvinyl chloride): A plastic pipe material rated for hot-water supply lines, typically cream or yellow in color. More heat-resistant than standard PVC. First introduced: Chapter 6.

Crawlspace: A below-grade or partially below-grade space beneath a house, typically 18 to 48 inches high, that provides access to floor framing, pipes, and wiring. Crawlspaces are highly prone to moisture problems. First introduced: Chapter 2.

D

Dead load: The permanent, static weight acting on a structure — the weight of the framing, sheathing, roofing, flooring, drywall, and other permanently installed materials. First introduced: Chapter 1.

Decking (roof): The structural sheathing layer fastened to roof rafters or trusses, upon which underlayment and finished roofing materials are installed. Typically OSB or plywood. First introduced: Chapter 24.

Deflection: The bending or sagging of a structural member under load. Building codes limit allowable deflection to prevent damage to finishes, tile cracking, and perceptible floor bounce. First introduced: Chapter 3.

Dehumidifier: An appliance that removes moisture from air by cooling the air below its dew point and collecting the condensate. Used to control humidity in basements, crawlspaces, and humid climates. First introduced: Chapter 22.

Differential settlement: Uneven sinking of a foundation in which different portions settle at different rates or amounts, causing cracking, racking, and structural distortion. More damaging than uniform settlement. First introduced: Chapter 2.

Drain field (leach field): The underground network of perforated pipes through which septic effluent disperses into the surrounding soil for final treatment. Also called a leach field. First introduced: Chapter 11.

Drain-waste-vent system (DWV): The complete system of pipes that carries wastewater and solid waste out of a building (the drain and waste portion) and admits air to maintain proper drain flow and prevent siphoning of traps (the vent portion). First introduced: Chapter 8.

Drip edge: A metal flashing installed at the edges of a roof to direct water away from the fascia and into the gutters, preventing water infiltration behind and beneath the roofing materials. First introduced: Chapter 24.

Drywall (gypsum board, sheetrock): A wall and ceiling panel material consisting of a gypsum plaster core faced on both sides with paper. The dominant interior finish material in modern construction. First introduced: Chapter 28.

Ductwork: The network of supply and return air channels, typically made of sheet metal, rigid fiberglass board, or flexible insulated tubing, that distributes conditioned air from the air handler or furnace to the rooms of the house and returns it for reconditioning. First introduced: Chapter 20.

E

Efflorescence: White, chalky mineral deposits that appear on masonry and concrete surfaces when water migrates through the material and evaporates, leaving dissolved salts behind. An indicator of water infiltration. First introduced: Chapter 27.

Egress: A means of exit from a building or room. Building codes specify minimum egress requirements — including minimum window opening sizes — for sleeping rooms to allow occupant escape and firefighter entry. First introduced: Chapter 32.

Engineered lumber: Structural wood products manufactured by bonding wood fibers, veneers, or strands under heat and pressure, including LVL, I-joists, glulam, and PSL. Generally stronger, more consistent, and more dimensionally stable than sawn lumber. First introduced: Chapter 3.

ERV (Energy Recovery Ventilator): A mechanical ventilation device that exhausts stale indoor air and draws in fresh outdoor air while transferring both heat and moisture between the two streams, preserving humidity levels in the house. First introduced: Chapter 20.

Evaporator: The component of a refrigeration or heat pump system where the refrigerant absorbs heat from the surroundings — in a standard air conditioner, the indoor coil where cooling occurs. First introduced: Chapter 19.

Expansion tank: A small pressure vessel containing a rubber diaphragm and a pre-charged air chamber, used to absorb the pressure increase that occurs when water is heated in a closed system. Required on water heaters in systems with backflow preventers or check valves. First introduced: Chapter 6.

F

Fascia: The horizontal board running along the edge of the roof at the eaves, to which gutters are typically attached. Part of the trim assembly that closes the rafter tails. First introduced: Chapter 24.

Fiberglass insulation: Insulation made from spun glass fibers, available as batts/blankets or as loose-fill. The most widely used insulation material in residential construction. First introduced: Chapter 4.

Flashing: Sheet metal (typically galvanized steel, aluminum, or copper) used to direct water away from vulnerable joints, transitions, and penetrations in the building envelope — around chimneys, skylights, windows, doors, and roof edges. The most common source of water intrusion when improperly installed. First introduced: Chapter 24.

Frost depth: The depth below grade at which soil freezes in a given climate. Foundation footings must be placed below the local frost depth to prevent frost heave. First introduced: Chapter 2.

French drain: An underground drainage system consisting of a gravel-filled trench with a perforated pipe that collects and redirects groundwater away from a structure. First introduced: Chapter 25.

Frost heave: The upward movement of soil or a foundation caused by the expansion of water as it freezes. Foundations not extending below the frost line are vulnerable. First introduced: Chapter 2.

Furnace: A forced-air heating appliance that burns fuel (gas, oil, or propane) or uses electric resistance to heat air, which is then distributed through ductwork. First introduced: Chapter 18.

Fuse: An overcurrent protection device that interrupts a circuit when excess current melts a metal element. Once blown, a fuse must be replaced (unlike a circuit breaker, which resets). Found in older electrical panels. First introduced: Chapter 13.

G

Galvanized steel pipe: Steel pipe coated with zinc to resist corrosion, widely used in water supply and drain systems installed before the 1970s. Subject to internal corrosion and scaling that reduces flow over time. First introduced: Chapter 6.

GFCI (Ground Fault Circuit Interrupter): A device that monitors current imbalance between hot and neutral conductors and trips in milliseconds when it detects a path of current through ground — protecting against electric shock. Required in kitchens, bathrooms, garages, outdoors, and near water. First introduced: Chapter 16.

Girder: A large, primary horizontal structural beam that supports the ends of floor joists and transfers their loads to posts or foundation walls. First introduced: Chapter 3.

Grout: A cementitious or epoxy compound used to fill the joints between ceramic, porcelain, or stone tiles. Grout in wet areas requires proper maintenance and resealing to prevent water infiltration. First introduced: Chapter 29.

Grounding: The connection of electrical equipment and the neutral conductor to the earth, providing a safe return path for fault currents and protecting against electric shock and equipment damage. First introduced: Chapter 12.

H

Header: A horizontal structural member spanning the top of a door or window opening in a framed wall, transferring loads from above around the opening to the trimmer and king studs on either side. First introduced: Chapter 3.

Heat exchanger: In a furnace, the metal component that separates combustion gases from the air being heated. Cracks in the heat exchanger allow combustion gases including carbon monoxide to enter the living space. First introduced: Chapter 18.

Heat pump: An HVAC system that moves heat rather than generating it, using the refrigeration cycle to transfer heat from a cooler space to a warmer one. In heating mode, it extracts heat from outdoor air (or ground) and delivers it indoors; in cooling mode, it reverses to function as an air conditioner. First introduced: Chapter 18.

Heat pump water heater (HPWH): A water heater that uses heat pump technology to move heat from surrounding air into the water tank, consuming roughly two to three times less electricity than a conventional electric resistance water heater. First introduced: Chapter 7.

House wrap: A water-resistive barrier (WRB) material, typically a synthetic fabric (Tyvek, Typar, and others), installed over sheathing and behind cladding to resist liquid water while allowing water vapor to escape from the wall cavity. First introduced: Chapter 26.

HSPF (Heating Seasonal Performance Factor): A seasonal efficiency rating for heat pumps in heating mode, expressing the total heat delivered (BTU) divided by total electricity consumed (watt-hours) over the season. Higher HSPF indicates greater heating efficiency. First introduced: Chapter 23.

Hydrostatic pressure: The pressure exerted by a body of water against a containing surface — in residential construction, the pressure that groundwater exerts against basement and crawlspace walls and floors. The primary driver of water infiltration in below-grade spaces. First introduced: Chapter 2.

I

Ice dam: A ridge of ice that forms at the edge of a roof when heat escaping from the living space melts snow on the upper roof, which refreezes when it reaches the cold eaves. Ice dams force water back under shingles, causing leaks. First introduced: Chapter 24.

I-joist (engineered I-joist): A manufactured structural member with a wide-flange cross-section resembling the letter I, consisting of an OSB or plywood web bonded between solid or LVL flanges. Offers a high strength-to-weight ratio and comes in precise, consistent dimensions. First introduced: Chapter 3.

IRC (International Residential Code): The model building code for one- and two-family dwellings, published by the International Code Council (ICC) and adopted (with amendments) by most U.S. jurisdictions. First introduced: Chapter 33.

J

Joist: A horizontal structural member, typically repetitively spaced, that spans between beams or bearing walls to support a floor or ceiling. First introduced: Chapter 3.

Junction box: An enclosed metal or plastic box that houses electrical wire connections. Required by code to remain accessible (not buried in walls) and to protect connections from physical damage. First introduced: Chapter 14.

K

Key (plaster): The mechanical bond between plaster and its support, formed when plaster is pressed through gaps in wood lath or metal lath and hardens into a mushroom-shaped anchor on the back side. When keys fail, plaster separates and sags. First introduced: Chapter 28.

Kilowatt-hour (kWh): The practical unit of electrical energy consumption, equal to the energy consumed by a 1,000-watt device running for one hour. The unit on which your electric bill is based. First introduced: Chapter 12.

Knob-and-tube (K&T) wiring: An early residential wiring method (pre-1950) using individual conductors insulated with rubber and cloth, supported by ceramic knobs and routed through ceramic tubes where passing through framing. Has no ground conductor. First introduced: Chapter 14.

L

Lath: Thin wood strips (wood lath) or metal mesh (metal lath) nailed to wall framing as a base to which plaster adheres. Found in homes built before drywall became standard, roughly pre-1950. First introduced: Chapter 28.

Leach field: See drain field.

Lead paint: Paint containing lead compounds, used widely in residential construction before 1978. When disturbed by sanding, scraping, or demolition, lead paint creates hazardous dust. First introduced: Chapter 34.

Live load: Variable, non-permanent loads on a structure — the weight of occupants, furniture, snow on a roof, and stored goods. Design standards specify minimum live load assumptions for different occupancy types. First introduced: Chapter 1.

Load-bearing wall: A wall that carries structural loads from above (roof, floors, other walls) and transfers them to the foundation. Removing a load-bearing wall without proper structural support causes potentially catastrophic consequences. First introduced: Chapter 3.

Load path: The route by which structural forces travel through a building from their point of application (roof loads, floor loads) down through walls, beams, columns, and foundation into the earth. First introduced: Chapter 1.

LVL (Laminated Veneer Lumber): Engineered lumber made by bonding multiple thin wood veneers with their grains parallel, creating a strong, dimensionally stable member used for headers, beams, and rim boards. First introduced: Chapter 3.

M

Main disconnect: The switch or breaker that controls power to the entire electrical system of a building. Required to be accessible and clearly identified; used to shut off all power in an emergency. First introduced: Chapter 12.

Manual J: The standard ACCA calculation method for determining residential heating and cooling loads, taking into account climate, building envelope, occupancy, and equipment. A properly sized HVAC system requires a Manual J calculation; oversizing or undersizing has significant performance consequences. First introduced: Chapter 23.

MERV rating (Minimum Efficiency Reporting Value): A standardized scale (1–16 for residential use) measuring an air filter's ability to capture particles of various sizes. Higher MERV captures smaller particles but restricts airflow more, potentially reducing system performance. First introduced: Chapter 22.

Mini-split (ductless mini-split): A split-system heat pump or air conditioner that connects a small outdoor compressor to one or more indoor air-handling units (heads) via refrigerant lines, without requiring ductwork. First introduced: Chapter 19.

Mitigation system (radon): An active sub-slab depressurization (ASD) system that draws radon gas from beneath a foundation slab and exhausts it above the roofline, reducing indoor radon concentrations. The standard and highly effective remediation approach. First introduced: Chapter 34.

Mudding: The process of applying joint compound (mud) over drywall tape and fastener heads to create smooth, level wall surfaces before painting. Typically done in multiple coats. First introduced: Chapter 28.

N

NEC (National Electrical Code, NFPA 70): The model standard for electrical installation in the United States, published by the National Fire Protection Association and adopted by most jurisdictions. First introduced: Chapter 16.

Neutral conductor: In an electrical circuit, the conductor that completes the path back to the service panel and is connected to earth ground at the main panel. In residential wiring, typically identified by white or gray insulation. First introduced: Chapter 12.

NM cable (non-metallic sheathed cable, Romex): The most common type of residential wiring; two or more insulated conductors plus a bare ground wire enclosed in a plastic sheath. Not suitable for exposure or damp locations. First introduced: Chapter 14.

O

Ohm: The unit of electrical resistance, measuring opposition to current flow. Ohm's Law states that Voltage = Current x Resistance (V = IR). First introduced: Chapter 12.

OSB (oriented strand board): An engineered wood panel made from compressed, adhesive-bonded wood strands oriented in alternating perpendicular layers. Widely used for wall and roof sheathing and subfloors. First introduced: Chapter 3.

P

pCi/L (picocuries per liter): The unit for measuring radon gas concentrations in air. The EPA recommends mitigation when indoor radon levels reach 4 pCi/L or above. First introduced: Chapter 34.

P-trap: A drain fitting shaped like the letter P (when viewed from the side), used beneath every fixture drain. The curved portion retains a water seal that blocks sewer gases from entering the living space. First introduced: Chapter 8.

Permit: Official authorization from a local building department to proceed with specific construction, alteration, or repair work. Permits trigger inspections that verify code compliance. First introduced: Chapter 33.

PEX pipe (cross-linked polyethylene): A flexible plastic tubing widely used for both supply and radiant heating applications. More resistant to freezing, easier to install around obstacles, and less prone to corrosion than metal pipe. First introduced: Chapter 6.

Platform framing: The dominant modern residential framing method, in which each floor level is built as a complete platform before the walls of the next level are raised. Wall studs are typically one story tall. First introduced: Chapter 3.

Plenum: A chamber or space within the HVAC duct system that acts as a collection or distribution point for air — the supply plenum collects heated/cooled air from the equipment and distributes it to supply ducts; the return plenum collects return air. First introduced: Chapter 20.

Plywood: A structural wood panel made from an odd number of thin wood veneers (plies) bonded with the grain of adjacent layers perpendicular, providing dimensional stability and resistance to splitting. First introduced: Chapter 3.

Polybutylene pipe: A gray plastic pipe used in residential water supply systems from the 1970s through the mid-1990s, found to be prone to failure and the subject of major class-action settlements. First introduced: Chapter 6.

Pressure regulator (pressure-reducing valve, PRV): A device installed on the main water supply line that reduces incoming municipal water pressure (often 80–150 psi) to a safe range (typically 60–80 psi) to protect fixtures and appliances. First introduced: Chapter 6.

R

Radiant heat: A heating system that delivers heat via radiation from warm surfaces — typically electric resistance cables or hot water tubing embedded in floors, walls, or ceilings — rather than by convection through air. First introduced: Chapter 18.

Radon: A naturally occurring radioactive gas that forms from the decay of uranium in soil and rock. Radon can accumulate to dangerous concentrations in below-grade spaces and is the second leading cause of lung cancer in the U.S. First introduced: Chapter 34.

Refrigerant cycle (vapor-compression cycle): The thermodynamic process used by air conditioners and heat pumps in which a refrigerant alternately evaporates (absorbing heat) and condenses (releasing heat) as it is compressed and expanded. First introduced: Chapter 19.

Return air: Air drawn from the conditioned space back to the air handler or furnace for reconditioning and recirculation. First introduced: Chapter 20.

Rigid board insulation (foam board): Insulation in rigid panel form, typically EPS, XPS, or polyisocyanurate, installed on exterior walls, roofs, and under slabs. Provides continuous insulation without thermal bridging through framing. First introduced: Chapter 4.

RRP rule (Renovation, Repair, and Painting rule): An EPA regulation requiring contractors who disturb lead paint during work in pre-1978 housing to use lead-safe work practices and be certified in those practices. First introduced: Chapter 34.

R-value: A measure of a material's resistance to heat flow. The higher the R-value, the better the insulation performance. R-values of multiple layers are additive. First introduced: Chapter 4.

S

SEER (Seasonal Energy Efficiency Ratio): An efficiency rating for air conditioning systems expressing the total cooling output (BTU) divided by total electrical energy consumed (watt-hours) over a cooling season under standardized conditions. Higher SEER means lower operating cost. First introduced: Chapter 23.

SEER2: An updated version of the SEER rating that uses a more realistic external static pressure in testing, producing ratings that are slightly lower than SEER but more representative of actual installed performance. Became the regulatory standard in 2023. First introduced: Chapter 23.

Septic tank: A watertight buried tank in which household wastewater collects and undergoes primary treatment through settling and anaerobic bacterial digestion. Solids settle to the bottom as sludge; liquids flow to the drain field. First introduced: Chapter 11.

Service entrance: The electrical conductors and equipment connecting the utility's distribution system to the interior wiring of a building, including the service drop or lateral, meter socket, and main panel. First introduced: Chapter 12.

Sewer lateral: The privately owned sewer pipe that connects a building's drain system to the municipal sewer main in the street. The property owner is typically responsible for the entire lateral from the building to the street connection. First introduced: Chapter 11.

Sheathing: Structural panels (OSB or plywood) fastened to wall studs, roof rafters, or floor joists, providing rigidity, a nailing surface for finishes, and structural diaphragm action. First introduced: Chapter 26.

Slab-on-grade: A foundation type in which a concrete slab is poured directly on prepared ground, serving as both the foundation and the floor. Common in frost-free climates and on level sites. First introduced: Chapter 2.

Smoke detector: A device that detects combustion products and sounds an alarm. Ionization type detects fast-flaming fires; photoelectric type detects slow-smoldering fires. Modern code and best practice recommend dual-sensor or combination units. First introduced: Chapter 35.

Soffit: The horizontal underside of an overhanging roof eave. Often vented to allow air flow into the attic as part of the attic ventilation system. First introduced: Chapter 24.

Spray foam insulation: A two-component expanding foam insulation that is sprayed in place and expands to fill cavities, air seal gaps, and adhere to surfaces. Available in open-cell (lower density, vapor-permeable) and closed-cell (higher R-value, vapor-resistant) formulations. First introduced: Chapter 4.

Stack (plumbing): The large-diameter vertical drain pipe running through the building that collects wastewater from branch drains at each floor and carries it to the building drain. The upper portion, above the highest branch connection, serves as the main vent (stack vent). First introduced: Chapter 8.

Stem wall: A foundation wall that rises from a footing to support the floor framing, creating a crawlspace beneath the floor. First introduced: Chapter 2.

Step flashing: Individual pieces of metal flashing woven with roofing courses at the junction of a roof surface and a vertical wall or chimney, shedding water away from the joint. First introduced: Chapter 24.

STC rating (Sound Transmission Class): A single-number rating indicating how effectively a wall, floor, or ceiling assembly attenuates sound transmission. Higher STC indicates better sound isolation. First introduced: Chapter 28.

Stud: A vertical framing member in a wall, typically 2x4 or 2x6 lumber spaced 16 or 24 inches on center. First introduced: Chapter 3.

Sub-slab depressurization: See mitigation system (radon).

Subfloor: The structural panel layer (OSB or plywood) fastened to floor joists, serving as the base for finish flooring materials. First introduced: Chapter 29.

Sump pump: An electric pump installed in a pit (sump pit) in the lowest area of a basement or crawlspace that automatically removes accumulated groundwater to the exterior. First introduced: Chapter 27.

Supply air: Conditioned air delivered from the HVAC system through supply registers into the rooms of a building. First introduced: Chapter 20.

Supply line (plumbing): The pipes carrying potable water from the water main or well to fixtures and appliances throughout the building. First introduced: Chapter 6.

T

Tankless water heater (on-demand water heater): A water heater with no storage tank that heats water only as it is drawn, using high-capacity gas burners or electric elements. Provides a continuous supply of hot water but requires adequate flow rate and gas/electrical capacity. First introduced: Chapter 7.

Taping (drywall): The application of paper or fiberglass mesh joint tape over drywall joints before applying joint compound, reinforcing the joint against cracking. First introduced: Chapter 28.

T&P valve (temperature-pressure relief valve): A safety device on a water heater that opens automatically to release water or steam if the tank reaches a dangerously high temperature or pressure, preventing a potentially explosive failure. First introduced: Chapter 7.

Thermal bridge: A component or assembly that conducts heat more readily than surrounding materials, bypassing the insulation and reducing the effective thermal performance of the assembly. Wood studs in an insulated wall are a common thermal bridge. First introduced: Chapter 4.

Thinset: A cementitious adhesive mortar used to bond tile to a substrate. Proper thinset installation requires full coverage behind the tile. First introduced: Chapter 29.

Tile backer (cement board, Hardiebacker, Durock): A rigid substrate panel made from cement, aggregate, or fiberglass-reinforced material, used behind tile in wet areas such as showers, tub surrounds, and floors. First introduced: Chapter 31.

Truss (roof truss): A prefabricated structural assembly of lumber members arranged in triangulated patterns, used to span the full width of a building without interior bearing walls. Factory-engineered and increasingly dominant over site-built rafter framing. First introduced: Chapter 3.

U

U-factor (U-value): A measure of the rate of heat transfer through a window, door, or other assembly. The inverse of R-value; lower U-factor means better insulation performance. First introduced: Chapter 5.

Underlayment (roofing): A water-resistant or waterproof membrane installed between the roof deck and the finished roofing material. Types include traditional asphalt felt and modern synthetic underlayments. First introduced: Chapter 24.

V

Vapor barrier: A material with very low water vapor permeance, used to prevent moisture migration into walls, floors, or ceilings. Proper placement depends on climate and assembly; incorrect placement can cause condensation within assemblies. First introduced: Chapter 4.

Vapor retarder: A more general term for materials that limit but do not entirely stop water vapor transmission. Classified into three classes based on permeance; Class I is essentially a vapor barrier; Class III materials (like latex paint) are relatively permeable. First introduced: Chapter 4.

Volt (V): The unit of electrical potential difference, analogous to water pressure in a pipe. Standard residential circuits are 120V; large appliances use 240V. First introduced: Chapter 12.

W

Water hammer: A banging or thudding sound in plumbing pipes caused by the sudden stoppage of fast-moving water, creating a pressure shock wave. Often occurs when solenoid valves (dishwashers, washing machines) close abruptly. First introduced: Chapter 10.

Water main: The public utility pipe, buried in the street, that carries pressurized potable water to individual service connections. First introduced: Chapter 6.

Water meter: A utility-owned device that measures the volume of water flowing into a building's supply system. Located at or near the property line; houses the main shutoff valve that must be known to every homeowner. First introduced: Chapter 6.

Watt: The unit of electrical power, equal to voltage multiplied by current (W = V x A). A 1,500-watt space heater draws 1,500 watts. First introduced: Chapter 12.

Weep holes: Small openings left in the mortar joints of masonry veneer walls, or in window frame assemblies, that allow water that infiltrates behind the cladding to drain out rather than accumulating and causing damage. First introduced: Chapter 26.

Wildfire-resistant construction: Building materials and techniques designed to resist ignition from radiant heat, flame contact, and ember intrusion, including Class A roofing, enclosed eaves, non-combustible cladding, and multi-pane windows. First introduced: Chapter 35.

X

XPS (extruded polystyrene): A rigid foam insulation board (commonly pink or blue) with a consistent cell structure providing R-5 per inch and relatively low moisture absorption. Often used under slabs and for exterior continuous insulation. First introduced: Chapter 4.

Z

Zone valve: An automatic valve used in hydronic heating or cooling systems to open or close the flow of water to a particular zone, controlled by a thermostat for that zone. First introduced: Chapter 18.

Zoning (HVAC): A system configuration that divides a building into independently controlled temperature zones, each with its own thermostat and zone dampers or valves, allowing different temperatures in different areas and reducing energy waste in unoccupied spaces. First introduced: Chapter 21.

Note: Terms that appear in multiple chapters are cross-referenced to the chapter of first introduction. For code citations, always consult the edition adopted by your local jurisdiction, as adoptions vary.