The energy audit glossary

A US federal tax credit of 30% for efficiency upgrades and audits; it ended for property placed in service after December 31, 2025.

A 30% US tax credit for home solar, battery storage, geothermal heat pumps and similar; it ended for expenditures made after Dec 31, 2025.

Professional work to remove, seal or enclose a hazard such as asbestos, mold or lead paint, and to clean up safely afterward.

The US trade association for heating and cooling contractors that publishes the standard Manual J, S and D methods for sizing systems and ducts.

An airtightness score: how many times an hour the house's whole volume of air leaks out while a test fan pulls on it; lower means tighter.

The share of a furnace's or boiler's fuel that ends up as heat in your home over a year; a 95% furnace loses about 5% up the vent.

Proof that a specific indoor and outdoor heat pump or air conditioner combination was tested and certified, with its official ratings.

The continuous layer of materials in walls, ceilings and floors that stops air from leaking through the building envelope.

Uncontrolled air moving in and out of a house through cracks and gaps, carrying heat, moisture and outdoor pollutants with it.

Blocking the hidden gaps where air leaks in and out of a house, which cuts drafts, heating and cooling costs, and moisture problems.

A heat pump that takes heat from outdoor air in winter and releases heat outdoors in summer, using an outdoor unit with a fan.

The Census Bureau's ongoing yearly household survey, the main source for local figures like home age, heating fuel, income and home ownership.

The middle household income for your area; rebate programs set eligibility as a percentage of it, such as under 80% or 150% of AMI.

A mineral fiber once common in building products; low risk when left intact, but dangerous to breathe when materials are damaged or removed.

An international society of heating, cooling and ventilation engineers whose standards define things like commercial energy audit levels and climate zones.

A hidden opening from the living space into the attic, around chimneys, plumbing stacks, wiring, pot lights or wall tops, that leaks warm air.

A second heat source — electric elements or a furnace — that helps a heat pump in very cold weather or during defrost.

When exhaust from a furnace, boiler, water heater or fireplace comes back into the house instead of going up the chimney or vent.

The outdoor temperature where something changes: below the thermal balance point a heat pump needs help; below the economic one, fuel is cheaper.

The energy a home uses no matter the season — fridge, lights, electronics, water heating and standby power — apart from heating and cooling.

A test that uses a powerful, calibrated fan in an outside doorway to pull air out of the house and measure how leaky it is.

Loose fibers of cellulose, fiberglass or mineral wool blown into attics or wall cavities by machine; fast, and good at filling around obstacles.

Credentials from the Building Performance Institute showing a professional is trained to test and assess homes, such as Energy Auditor.

A unit of heat energy; BTU per hour (BTU/h) measures how fast a furnace, heat pump or air conditioner delivers heating or cooling.

Everything that separates inside from outside — foundation, walls, windows, doors and roof — and controls heat, air and moisture flow.

A colorless, odorless gas from incomplete burning of fuel; it reduces the blood's ability to carry oxygen and can kill at high levels.

Qualifying for an assistance or rebate program automatically because your household already receives certain benefits, such as SNAP, SSI or TANF.

Efficiency levels set by the Consortium for Energy Efficiency that utilities and programs use to decide which equipment qualifies.

The raw air-leakage result of a house's fan test: how many cubic feet of air per minute the fan must move to hold the house at test pressure.

Thirty-year averages of temperature, degree-days, precipitation and other weather at a station, used as the typical climate for a place.

An air-source heat pump built to keep heating efficiently in deep cold, usually verified by ENERGY STAR Cold Climate or the NEEP list.

The room or area holding a fuel-burning appliance such as a furnace, boiler or water heater; it's the focus of combustion safety testing.

A check that furnaces, boilers, water heaters and stoves burn cleanly and vent their exhaust outside, even with fans running.

Water that forms when humid air touches a surface colder than its dew point, on windows, cold pipes or hidden inside walls and attics.

A gas furnace or boiler that captures extra heat from its exhaust by condensing the water vapor in it, reaching 90% efficiency or more.

A layer of insulation running unbroken over the framing, usually rigid boards on the outside of walls, to stop heat escaping through studs.

A measure of how hot a place is: each day adds how far its average temperature rises above 65°F. More degree days means more air conditioning.

Heat delivered divided by electricity used at a given moment; a COP of 3 means three units of heat for each unit of electricity.

A flammable tar that builds up inside chimneys and flue pipes when wood smoke cools and condenses; too much of it can start a chimney fire.

A short, automatic cycle that melts frost off a heat pump's outdoor coil or an HRV's core in freezing weather; it's a normal part of winter operation.

An appliance that removes moisture from indoor air, most often used in basements and humid summers to keep humidity in a healthy range.

Loose-fill insulation, usually cellulose, blown tightly into closed wall cavities through small holes so it fills them, resists settling and slows air movement.

When more air is being pushed out of a house than is coming in, so the inside sits at slightly lower pressure than outdoors.

The cold (or hot) outdoor temperature a heating or cooling system is sized for; colder than most winter days, but not the record low.

The temperature at which air becomes saturated and water starts to condense on surfaces, like water beading on a cold glass.

The US federal energy department, which sets appliance efficiency standards and funds home energy rebates and weatherization through the states.

The upward pull in a chimney or vent that carries exhaust out of the house; weak or reversed draft lets combustion gases spill indoors.

A copper section of the main drain that uses warm shower water going down the drain to preheat the cold water coming in.

A heat pump paired with a gas, propane or oil furnace or boiler; controls switch to the fuel when it's colder or cheaper to do so.

A test that pressurizes your heating and cooling ducts with a calibrated fan to measure how much air escapes through leaks.

The part of duct leakage that escapes outside the heated and cooled space, into attics, crawlspaces, garages or outdoors.

A heat pump that heats and cools through ductwork, like central air conditioning, either on its own or paired with a furnace.

An air-source heat pump with wall- or ceiling-mounted indoor heads connected by refrigerant lines, needing no ductwork.

A high-efficiency, variable-speed fan motor for furnaces, air handlers and HRVs that uses far less electricity than older motors, especially at low speed.

Cooling efficiency measured at one hot test condition rather than averaged over a season; useful for judging performance on the hottest days.

The US government's energy statistics agency, the usual source for state electricity, natural gas, heating oil and propane prices.

Heating with electric elements that turn electricity directly into heat, such as baseboards, electric furnaces and heat pump backup strips.

The government-backed blue label for products and some homes that beat minimum efficiency standards by meeting stricter criteria.

A yearly ENERGY STAR designation for the top-performing certified products in a category, a step above regular ENERGY STAR.

Yearly energy use divided by floor area, such as kBtu per square foot, for comparing homes of different sizes.

EPA's 1993 county map of radon potential, from Zone 1 (highest) to Zone 3 (lowest); EPA says not to use it to decide whether to test.

A wood stove or insert tested to meet EPA's smoke emission limits; it burns cleaner and more efficiently.

Like an HRV, but its core also transfers some moisture between the outgoing and incoming air, which helps manage humidity.

Yearly income figures, by household size, that the US government publishes and assistance programs use to set income limits, such as 200% of poverty.

The number of hours a heating or cooling system would have to run flat out to deliver a whole season's heating or cooling.

A heat pump that exchanges heat with the ground through buried pipes or well water, giving steady efficiency even in deep cold.

An electric system that moves heat instead of making it: out of the house in summer and into it in winter, using a refrigerant loop.

An electric water heater with a small heat pump on top that pulls heat from the surrounding air, using far less electricity than a standard tank.

A calculation of how fast a home loses heat on the coldest days it's designed for, used to size heating equipment correctly.

A measure of how cold a place is: each day adds how far its average temperature falls below 65°F. More degree days means more heating.

Whether a place needs more energy for heating or for cooling over a year, which changes which upgrades and equipment matter most.

A very fine high-efficiency particulate air filter used in portable air cleaners and vacuums to trap smoke, dust and other small particles.

A US home energy score where 100 equals a typical home built to 2006 standards and 0 is net zero; each point is about 1% of energy use.

An Inflation Reduction Act rebate run by states for efficient electric equipment and envelope upgrades in lower- and moderate-income homes.

A professional check-up of how your home uses and loses energy, with tests and a written, ranked list of upgrades.

A US Department of Energy 1-to-10 rating of an existing home's expected energy use, a bit like a miles-per-gallon rating for houses.

A federal rebate, run by states, that pays $2,000 to $8,000 based on the modeled energy savings of whole-home upgrades.

A climate with long, hot, muggy summers, like the US Gulf Coast and Florida, where controlling humidity matters as much as temperature.

A ventilation system that swaps stale indoor air for fresh outdoor air while transferring heat from the outgoing air to the incoming air.

A heat pump's heating efficiency averaged over a whole heating season; higher is better, and HSPF2 numbers can't be compared directly with older HSPF.

Short for heating, ventilation and air conditioning: the equipment and ducts that heat, cool and move fresh air through a home.

An inexpensive gauge, often combined with a thermometer, that shows a room's relative humidity so you can keep it in a healthy range.

A ridge of ice at the roof edge that traps melting snow behind it, which can back up under shingles and leak into the house.

The model US energy code for new homes and additions, updated about every three years; states and cities adopt an edition, often with changes.

The US energy code's climate map: zones 0 (hottest) to 8 (coldest), plus a moisture letter — A for moist, B for dry, C for marine.

How clean and healthy the air inside a home is, affected by ventilation, humidity, combustion gases, particles, chemicals and radon.

The 2022 US law that created state-run home energy rebates and expanded home energy tax credits, which a 2025 law later cut short.

Using a heat-sensing camera to show warm and cold spots on walls and ceilings, revealing missing insulation and air leaks.

Shorthand for large amounts of heat energy: kBtu is a thousand BTU and MMBtu is a million BTU, used to compare fuels and yearly energy use.

A rate of using or producing energy equal to 1,000 watts; running 1 kW for an hour uses 1 kilowatt-hour (kWh), the unit on your electricity bill.

The unit on your electricity bill: one kilowatt-hour is 1,000 watts used for one hour, such as a 100-watt bulb left on for 10 hours.

A short wall, often in a 1½-story house, that separates a finished upstairs room from the unheated attic space behind it.

An early type of house wiring that runs separate wires on porcelain knobs and through porcelain tubes, with no ground wire; insulation must not cover it.

Paint containing lead, common in older homes; sanding, scraping or removing it creates toxic dust that's especially harmful to young children.

The total cost of owning an option over its life — purchase, installation, energy, maintenance and replacement — minus rebates.

A federal program, run by states, tribes and territories, that helps lower-income households pay heating and cooling bills and avoid shutoffs.

An engineering calculation of how much heating and cooling a house needs, room by room, used to size equipment correctly.

Window glass with a thin, invisible metal coating that reflects heat, keeping more warmth in during winter and out in summer.

Outdoor air deliberately brought into a house to replace air removed by big exhaust fans, range hoods, dryers or fireplaces.

A precise pressure gauge auditors use to read small pressure differences, in pascals, during blower door, duct leakage and combustion safety tests.

The US industry standard for designing and sizing a home's ducts so each room gets the airflow the heating and cooling equipment is meant to deliver.

The US industry standard for calculating how much heating and cooling a house needs, room by room, so equipment can be sized correctly.

The US industry standard for choosing heating and cooling equipment whose real capacity, from the maker's data, fits the home's calculated loads.

A mild coastal climate with cool, wet winters and dry summers, like the US Pacific Northwest and coastal British Columbia.

A 1-to-16 rating of how well a furnace or air-handler filter catches airborne particles; higher numbers trap smaller particles.

The US government's metro area definition: counties tied by commuting to an urban core of at least 50,000 people (an MSA) or 10,000 (a micro area).

A non-condensing gas or oil furnace, around 80% efficient, that sends its exhaust up a chimney; 80% AFUE is still the US minimum for most gas furnaces.

Fungi that grow on damp surfaces indoors; they can damage materials and affect health, and moisture is always the root cause.

A furnace, boiler or water heater that takes combustion air from the room and relies on hot exhaust rising up a chimney, so house pressure can reverse it.

A widely used list of cold-climate air-source heat pumps, kept by Northeast Energy Efficiency Partnerships, with tested performance down to 5°F (−15°C).

A net-zero home produces as much energy as it uses in a year, usually with solar; a net-zero ready home is built so it could, once panels go on.

A chemical from fertilizer, manure and sewage that can seep into wells; high levels are dangerous for babies, so test well water for it.

The US weather and climate agency; its National Centers for Environmental Information publish the climate normals and degree-days used for US cities.

A way to pay for upgrades through a monthly charge on your utility bill, ideally offset by the energy savings.

The July 4, 2025 US tax law that ended the 25C and 25D home energy credits after 2025 and the 45L new-home credit after June 30, 2026.

Installing heating or cooling equipment with much more capacity than the house needs, usually from rules of thumb, not a load calculation.

Financing for energy upgrades that you repay through a charge on your property tax bill, tied to the property rather than to you.

A small unit of air pressure; airtightness tests hold a house at 50 pascals, about 1/2,000 of normal air pressure, so results can be compared.

Fine, inhalable airborne particles 2.5 microns or smaller, from smoke, cooking, candles and outdoor pollution.

A US Department of Energy national laboratory whose building research includes the Building America Solution Center and county climate zone data.

A way to express tiny concentrations: 1 ppm is one part of a gas in a million parts of air, used for carbon monoxide readings.

How well a material resists heat flow; higher is better, and insulation is rated with it, such as R-20 for walls or R-50 for attics.

A shiny foil layer, usually in attics, that reflects the sun's radiant heat; it helps most in hot, sunny climates.

A natural radioactive gas from uranium in soil and rock that can build up in homes; long-term exposure raises the risk of lung cancer.

EPA's radon action level: fix your home if testing shows 4 pCi/L or more, and consider fixing it between 2 and 4 pCi/L.

Fixing high radon, usually with a pipe through the basement floor and a fan that pulls soil gas from under the slab and vents it outside.

Measuring radon with a detector left in the home; long-term tests, over 90 days, give the most reliable yearly average.

The US measures radon in picocuries per liter (pCi/L) and Canada in becquerels per cubic meter (Bq/m³); 1 pCi/L equals 37 Bq/m³.

A small air gap behind siding or brick that lets rainwater that gets past the cladding drain away and lets the wall dry.

The fluid a heat pump or air conditioner pumps around to move heat; newer systems use R-454B or R-32 instead of the older R-410A.

The two climates heat pump heating ratings are usually given for: Region IV, like the Midwest, and the colder Region V, like the northern tier of states.

How much moisture the air holds compared with the most it could hold at that temperature, shown as a percentage.

A professional certified through RESNET, the Residential Energy Services Network, to inspect, test and model homes and issue HERS Index scores.

The yearly savings from an upgrade as a percentage of its cost; a 4-year payback is roughly a 25% annual return.

Stiff insulation boards made of foam plastic, used on foundations, under slabs and as a continuous layer over walls and roofs.

The band of framing on top of the foundation wall around the edge of the floor; it's prone to air leaks and often poorly insulated.

A furnace, boiler, water heater or fireplace that brings in combustion air from outdoors through its own pipe and vents exhaust straight outside.

A heat pump's average heat output per unit of electricity over a whole heating season, including defrost and backup heat, instead of at one temperature.

An air conditioner's or heat pump's cooling efficiency averaged over a whole cooling season; higher is better.

When heating or cooling equipment turns on and off frequently in short bursts instead of running in steady, longer cycles.

How many years an upgrade takes to pay for itself: the cost after rebates divided by the yearly savings.

How many output levels heating or cooling equipment has: single-stage runs fully on or off, two-stage adds a low setting, and variable-speed adjusts smoothly.

A Wi-Fi-connected thermostat that schedules, learns or adjusts temperatures automatically and can be controlled from your phone.

Soffit vents let outside air into the attic at the eaves, and baffles keep insulation from blocking that airflow.

The share of the sun's heat a window lets into the house, from 0 to 1; lower keeps homes cooler, higher adds free winter heat.

Liquid foam sprayed in place that expands to insulate and air seal in one step; closed-cell is denser and higher R, open-cell lighter.

Warm indoor air rises and escapes near the top of a house, pulling cold outside air in near the bottom; it's strongest on cold days.

The middle household income for a whole state; LIHEAP and some weatherization and rebate programs set income limits as a share of it, such as 60% of SMI.

A water heater with no storage tank that heats water only as it flows, avoiding the heat a tank loses while it sits waiting.

The unit many US gas bills use: one therm is 100,000 BTU of heat energy, roughly the energy in 100 cubic feet of natural gas.

Heat taking a shortcut through materials that insulate poorly, like wood studs, steel or concrete, running from the warm side to the cold side.

Lowering the heat (or raising the cooling setpoint) while you sleep or are away, then returning to normal before you need it.

Electricity prices that change with the time of day, usually cheapest overnight and highest when demand peaks.

A unit of heating and cooling capacity: one ton equals 12,000 BTU per hour (3.5 kW), based on the cooling from melting a ton of ice in a day.

Bacteria used as warning signs in well water: total coliforms point to surface contamination, and E. coli to contamination from sewage or manure.

How fast heat passes through a window, door or wall assembly; lower is better, the opposite of R-value.

A water heater's overall efficiency rating from a standard daily hot-water test; higher is better, and the best heat pump models score above 3.

Money back from your electric or gas utility for buying efficient equipment or making upgrades, often stackable with government programs.

A layer that slows water vapor moving through walls and ceilings, to keep it from condensing inside the structure.

A compressor that ramps its speed up and down to match the heating or cooling needed, instead of switching fully on and off.

A pebbly, brownish or silvery loose-fill attic insulation that may contain asbestos from a Montana mine, so it shouldn't be disturbed.

Chemicals that evaporate into the air from paints, glues, cleaning products, new furniture, flooring and some building materials.

Adjusting energy use for how cold or hot a period was, using degree-days, so bills from different years or homes can be compared fairly.

A US Department of Energy program, delivered by state and local agencies, that makes low-income households' homes more energy efficient.

Lab testing of water from a private well for bacteria and chemicals, done on a regular schedule and after floods or repairs.

A combustion safety check run with exhaust fans on and doors set to create the lowest pressure the furnace room is likely to see.

A handheld X-ray fluorescence instrument that measures lead in paint through the layers, without scraping off samples, during a lead inspection.

Dividing a home into areas with separate thermostats so each can be heated or cooled only when and as much as needed.