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ACH is an acronym for Air Changes per Hour and is a measurement of air infiltration. It is the total volume of air in a home that is turned over in one hour.


Air Barrier

The portion of the building enclosure which controls air flow. Comprised of low air permeance materials.


Air Barrier Component

Elements such as windows, doors and service entries inserted and sealed into the building enclosure.


Air Barrier Material

A material designed to be the primary air barrier in a building enclosure. Air Barrier materials should be ‘Air Impermeable’ defined as allowing less than 0.02 L/(s x m2) air permeance at 75 pascals pressure difference. See, we don’t just make this stuff up.


Air Infiltration

Air leakage into conditioned space. This is natural, unplanned (and generally unwanted) allowance of unconditioned external air into the conditioned internal space.


Air Leakage

Unwanted air flow through the conditioned space. This is natural, unplanned (and generally unwanted) air flowing in and out of the building enclosure. Air leaking out is called exfiltration and air leaking in is called infiltration.


Air Permeable Material

These are structural materials which can be part of the building enclosure but are not intended as part of the air barrier.



The American Society of Heating, Refrigeration and Air Conditioning Engineers. ASHRAE is an international technical society for all individuals and organizations interested in heating, ventilation, air-conditioning, and refrigeration.



Naturally occurring fibrous minerals that because of their extreme durability and fire resistance were used extensively in residential and industrial construction. The inhalation of asbestos fibers can cause serious illnesses such as asbestosis, lung cancer and mesothelioma.


Balloon Framing

A method of building framing where the walls are framed from long continuous wall studs that run from the sill plate to the roof eaves. Floors, windows and other fixtures are built around the continuous framing.


Band Joist

Located atop the foundation wall, the vertical lumber element to which the basement floor joists are attached forming the first floor. This can be a major source of air infiltration even in modern construction.


The below ground level foundation of a building, often used for storage and utility space.



One of the main structural elements in the building enclosure, traditionally wood though it can be a wide variety of building materials.


Below Grade

In building terms, grade is the ground level. Therefore, below grade…below ground level. All in the name.


Blower Door

A blower door is a vinyl sheathed aluminum frame which houses a powerful fan and pressure sensor. The frame is adjustable to different door frame sizes. Auditors use blower doors to test building tightness and identify air leaks.


Blower Door Test

Run by an energy auditor, the blower door depressurizes the house to measure air tightness and identify air leaks. A blower door test can also be run during weatherization work for contractors to find areas to be addressed.



The core of a water-based heating system. A boiler heats water which is carried through structures via a steam or a hot water distribution system. Steam systems have higher heating requirements (because of the higher operating temperature of steam vs. water). Almost any fuel sources are options but the most common in North America are oil and natural gas boilers.


Borate/Boric Acid

Borates are a class of chemicals added to cellulose as a fire retardant and anti-pest treatment. Borates are created from boron and are also used as treatments for wood and lumber.


BTL (Building Tightness Limit)

The same as Minimum CFM50. It is possible to increase the airtightness of a building to the point where natural air change rates (from air leakage) may not provide adequate ventilation to maintain acceptable indoor air quality. To help evaluate the need for mechanical ventilation in buildings, national ventilation guidelines have been established by ASHRAE.



British Thermal Unit. A measure of heat equal to the amount of energy to raise 1 pound of water by 1 degree Fahrenheit.


Building Envelope or Building Shell

The shell of a building that defines the division between the interior, conditioned space and exterior unconditioned space. Consists of an air, vapor and thermal barrier, a water control plane and the structural elements which include the foundation (or slab), the exterior walls and roof.

Building Tightness Limit

The limit as blower door tested that a building can be tightened before mechanical ventilation needs to be added to maintain sufficient fresh breathable air.


Cape Cod

A very popular style of house originating in 1700s New England. They are characterized by a steep pitched roof, a 2nd floor with half-height knee walls, full end gable walls and in older construction, a centrally located chimney. Modern variants tend to locate the chimney on one gable end. Cape Cod houses enjoyed a resurgence in popularity during the 20th century with rising demand for affordable housing options.


Capillary Action

The directional movement of liquids through a porous material, driven by molecular forces. Due to capillary action, liquids can move against gravity or other driving forces.


Carbon Footprint

Total carbon dioxide (some measures include all greenhouse gases) emissions produced by an individual, group or event. This can be by transportation, food consumption and most any economic activity. Emissions for all GHG are usually measured in carbon dioxide equivalents.


Cathedral Attic

A vaulted ceiling where the insulation has been installed along the roof deck. This makes the attic conditioned space, allows for its use as storage and improves the performance of any hot water piping or ductwork



One of several types of joint sealant. Typically refers to silicon based caulking today but the term dates back to mixtures of hemp fibers and pine tar used to water seal boats.


Combustible Area Zone (CAZ)

The CAZ is the area which houses the appliances that heat the home through combustion, such as a boiler or a furnace. Typically located in the basement, they can also be located in other areas such as garages or portions of the main floors. The CAZ needs to maintain a minimum pressurization to avoid spillage, back drafts, or other hazards.


CAZ Worst Case Depressurization Test

A combustion appliance test evaluating the potential for backdrafting flue gases. An auditor first creates the ‘worst case depressurization’ (activating exhaust fans and closing door to produce the highest negative pressure in the house). He then visually assesses the flue gas spillage and tests the flue gas pressure with a pressure guage.


Cellulose Insulation

 Cellulose is one of the more common and least expensive forms of insulation. Cellulose is shredded paper typically with boric acid as a flame retardant and anti-pest treatment. Cellulose can be blown loosely over flats or densepacked in wall cavities to achieve approximately R-3 to 3.5 per inch. Cellulose retards but does not stop air movement.


Cubic Feet per Minute. The volume measure for air flow in building science.



This is the airflow (in Cubic Feet per Minute) needed to create a change in building pressure of 50 Pascals. CFM50 is the most commonly used measure of building airtightness and represents a fictional wind blowing on all sides of the structure at approx. 20 mph.



When water moves from a vaporous state to liquid. Occurs when surface temperatures drop below the dew point.


Conditioned Space

The space within the building enclosure that is being environmentally controlled.


Deep Energy Retrofit

A turbo charged version of a typical retrofit. A deep energy retrofit is one where the goal is achieving 50-75% reduction in energy use. This usually involves very high levels of insulation, enormous reduction of air leakage and window replacement (yes, I said it).



To dry out air in a conditioned space, usually using a mechanical system to remove water.


Dew Point

The temperature at which the water vapor in humid air will transition to liquid.. Solids below the dew point will cause vapor to condense on its surface.



Domestic Hot Water



The movement from greater concentration toward lesser concentration of molecules through a medium.



Sometimes also shown as SEER. Stands for Energy Efficiency Rating, used to measure the efficiency of air conditioners. More efficient ACs have a higher SEER/EER number.



In building science, a material’s ability to emit radiation (usually referring to heat and light). Emissivity runs from e = 0 (perfect reflection of radiations) to e = 1 (no emissions at all). The darker and duller the surface the closer emissivity will be to 1 and conversely the more reflective a surface the closer to 0.


The ability of one system to exert force over a distance on another system.


Energy Audit

Energy audits are assessments of the insulation, air leakage, moisture, air quality and heating systems in your home. Done responsibly, an audit should account for each factor and how they interact.


Energy Recovery Ventilator

Also abbreviated as ERV, this is a mechanical system that recovers heat and moisture while exchanging internal air for fresh external air.


Energy Star

Standard for energy efficiency in many consumer products. First developed by the US Dept of Energy, it has since evolved into an international standard for efficiency.


Energy Star Homes

A residential program implemented by the EPA and Dept of Energy that maintain an energy efficiency standard 15% higher than that of the International Residential Code.


Energy Star Windows

Similar to Energy Star homes, Energy Star provides a minimum performance standard for windows. These standards vary by climate zone.



Otherwise called the building shell or building envelope, this is the separating barrier between conditioned and unconditioned space. Consists of a structural element and a vapor, thermal and air barriers and usually a drainage plain.



U.S. Environmental Protection Agency. Another group we like.


Expanded Polystyrene or EPS

This is the white beaded ‘styrofoam’ most people are familiar with (think Dunkin Donuts coffee cups). It is typically formed into rigid foam boards and has a higher vapor permeability than either XPS or Polyisocyanurate (foil faced). EPS would need to be paired with a vapor barrier in building shell applications.



It is air flowing out of a building, driven by pressure and temperature. In residential audits,



The aesthetic and protective exterior of a building.


Faucet Aerator

A mesh screen found at the tip of modern faucets. Rather than coming out in a stream, the aerator spreads the flow into droplets. This improves flow, reduces splashing and saves water.


Fiberglass Insulation

Fiberglass consists of extremely fine glass fibers, used in making various products, such as yarns, fabrics, insulators, and structural objects or parts. As an insulation, it is cost effective, temperature resistant and equally effective in most climates. The main drawback is that it does not stop air flow in any way. Unless paired with an effective air barrier, fiberglass insulation won’t function at the advertised R-value.



A building material for directing exterior water away from the building. Occasionally used in other capacities such air sealing chimney enclosures.


Fossil Fuels

Non-renewable energy source created through geologic forces from ancient biomass. A primary source of pollution, examples include petroleum, coal and natural gas.


French Drain

An exterior perimeter drain around a building. These drains help reduce basement moisture problems by moving rain water away from the foundation.



Ground level in relation to the building.


Habitable Space

The space within the building enclosure where folks sleep, dine, cook etc. on a continual basis.



Top framing members over all building egresses (primarily windows and doors).


Heat Exchanger

A system which transfers heat usually for the retention of a greater portion of domestic heating. Air to air heat exchangers capture a portion of outgoing heat by transferring it to the incoming air flow. Water heat recovery systems do the same for a water medium, capturing some of the potentially wasted outgoing heated water and preheating incoming water.


Heat Gain

The amount of heat gained within a conditioned residential space from non-heating system sources. This includes sunlight, body heat, incandescent lights and the heat radiating from an active boiler/furnace.


Heat Recovery System

A system which extracts heat from outgoing hot water lines to retain a greater portion of domestic heat. The captured heat can preheat incoming cold water.


Heating Load

Heating load is the maximum number of BTUs the heating system will need to provide for a comfortable interior temperature. The load is calculated on a worst case scenario (in other words, the coldest it is likely to ever get in your area).


Home Energy Rating System. HERS is a residential energy efficiency scoring system. Existing homes are scored on a scale where 0 indicates a net zero energy use home and 100 represents an ‘American Standard Building’. The lower the score the more efficient the building. In 2009, the Department of Energy created a new scale, the EnergySmart Home Scale which subtracts HERS Index scores from 100 (making 100 a net zero energy home and higher scores better).


Home Performance With Energy Star

 One of the national weatherization programs in the U.S. The program concerns itself with energy efficiency contractors trained in proper assessment of home performance. The program is overseen by the EPA and Department of Energy.



 U.S. Department of Housing and Urban Development



 An acronym that stands for Heating, Ventilation, and Air Conditioning. This refers to all of your building’s major mechanical systems.


Hydronic System

 Sometimes called Forced Hot Water. This is a mechanical conditioning system which uses water for transporting heat through the building.



 A tool for measuring and recording air humidity levels.



 Indoor air quality. In auditing, IAQ figures prominently in ensuring sufficient fresh breathable air, reasonable humidity levels and freedom from mold and chemicals.


Ice Dam

 A fossil fuel powered way to remove snow from your roof. Ice dams are a lip of ice that forms on the outside edge of a roof. When large enough, water can pool behind the lip, possibly causing roof leaks and water damage. When an upper portion of the roof is above the freezing point (melting snow) and a lower portion is below freezing, ice dams can form.



 International Energy Conservation Code. The international code standard for energy use in construction. In 2012, Maryland other states adopted new energy efficiency code for new construction and retrofits. More testing, including blower door and duct blaster testing is required on new and existing construction. 


Infrared Thermometer

 A thermometer which uses the infrared spectrum to detect surface temperatures at a distance. This is a useful tool for homeowners to do their own home energy investigations without the much more substantial investment of an infrared camera.


LED bulbs

Light emitting diode. LEDs are extraordinarily long lasting and very efficient relative to either incandescents or CFLs. LEDs produce light with a semiconductor ‘filament’ instead of the standard metal one.



Standing for Leadership in Energy and Environmental Design. This program from the United States Green Building Council sets standards for green building design and construction.


Low-E Coating

Low E coatings are designed to allow sunlight in but block/reflect a higher degree of solar heat. Low E or low emissivity coatings are a thin, silvery metal layer on the window pane. Very useful in sunny climates to reduce cooling loads by limiting solar heat gain.


Low Emissivity

See Low E Coating directly above. We’re basically talking about the same thing.


Low Flow Showerhead

Showerhead with less than the 2.5 gpm water flow.


Net Zero Home

A home or building which produces as much energy as it consumes. This is usually achieved by high levels of thermal resistance, limited air leakage and renewable energy sources. 


Off Gassing

Off gassing are the chemical vapors released from paints, finishes and other materials into the home. This is an indoor air quality issue and must be considered as the building envelope is tightened.


Off Peak Electricity

Off peak electricity is the period of time, usually night, when there is less demand for electricity. Utilities often charge reduced rates for off-peak usage.


On Demand Hot Water

A mechanical system which delivers hot water ‘on demand’. Clever name, no? This reduces heat loss when there is no hot water use since the system does not maintain a reservoir of pre-heated water.



Oriented strand board. Made by orienting strands of wood slivers and compressing them, end of Terminator-style, into a board. OSB use has exploded in the last 30 years, largely supplanting plywood and planed lumber in building shell construction.



A unit of pressure equivalent to one newton per square meter.


Passive Heating

Heating systems for a house that lack a active mechanical component. Usually some variant of solar heating (solar hot air, thermal masses or solar heat gain).


Passive Solar

Utilizing solar in passive manner such as with thermal masses or solar light tubes instead of actively with photovoltaic panels or solar hot water systems.



This is the amount of time it will take to recoup the costs vs. savings. (Costs/Savings)


Peak Oil

The point in time when maximum global oil production is reached and then begins to decline. This is based on the observed production of individual oil fields and witnessing the same peak and decline of production in larger geographical regions.



Also abbreviated as PV. Usually references photochemical cells which generate electric current when under direct sunlight.



Polyisocyanurate foam insulation. This is a closed cell solid foam insulation. It usually comes with foil backing. Polyiso can act as an air barrier and has the highest R-value (R-7 per inch) of any common insulating material. However, it does not hold up under direct sunlight and does degrade to approximately R-6 after 5-10 years.


Power Vented Exhaust

A combustion system’s gas vent that is coupled with a fan system to expedite the removal of flue gases.


Programmable Thermostat

A thermostat which allows for setting different room temperatures over periods of time. One of the most cost effective ways to reduce your heating bill.



A measure of the resistance of an insulating or building material to heat flow, expressed as R-11, R-20, and so on; the higher the number, the greater the resistance to heat flow.


Renewable Energy

Energy which comes from natural sources such as wind, sun and hydro. These sources naturally replenish. Compare to non-renewables like coal, natural gas and oil.



Increasing the energy efficiency and structure of an older building.


Return on Investment

This is the return on the dollar for money spent. (Savings/Costs).


Savings to Investment Ratio

This is a metric showing what the lifetime savings versus cost of a proposed piece of efficiency work. It is the Return on Investment multiplied by the number of years the repair will be in effect, usually standardized to 30 years. In efficiency programs, this is often one factor considered by banks to determine the feasibility of a repair before offering loan money.


Sealed Combustion

A means of providing air for heating and hot water appliances where the combustion air and post-combustion flue gases are both piped outside. Usually done through a concentric vent where both intake and exhaust run in the same pipe.


Seasonal Energy Efficiency Ratio or SEER

A measure for the energy efficiency of cooling units. The SEER rating is determined by dividing the cooling BTUs produced by the energy used in watt-hours. The higher the SEER, the more efficient the cooling unit.



The exterior layer of the building envelope installed over the stud walls. Historically this has been planed lumber boards but in the 20th century this has been supplanted by plywood and later oriented strand board (OSB).



The outer covering of a residence. Provides improved aesthetics and acts as weather protection.


Sill Plate

Located atop the foundation wall, the bottom horizontal lumber to which the basement floor joists are attached. This forms the first floor. The sill plate can be a major source of air infiltration even in modern construction.


Slab On Grade

More commonly known as a slab foundation. A very simple, cost effective alternative to a full basement, slab concrete is poured directly into an excavated mold.


Solar Gain

The energy gained (usually we’re talking about heat) from sunlight.


Solar Heat Gain Coefficient or SHGC

SHGC is an abbreviation seen on window performance labels. It shows the percent of solar heat allowed through the window.


Spray Foam

A polyurethane-based insulation that is applied via spraying on the insulating surface. The two main flavors are a denser impermeable closed cell foams and a less dense semi-permeable open cell foam.


Stack Effect

One of the main drivers of air transported heat loss in homes. Warm air naturally drifts upward within the building envelope. This updraft grows stronger as the indoor-outdoor temperature difference grows (the stack draft will be much stronger if outdoor temp is 20 F compared to 50 F). This thermal updraft finds every crack and seam to escape in the building’s peak. The updraft also creates a corresponding negative pressure, pulling in cold air at the building’s base.



A wood beam used in residential construction.

Sun Tunnel or Solar Tube

A form of passive solar lighting. Solar tubes or sun tunnels are tubes which run from the roof and terminate in a globe which diffuses the light through an interior room.



A resource, energy or material which can be extracted and used in a manner which is viable over an indefinite period. Compare with finite resources like petroleum.


Tankless Coil

A tankless coil is a means of producing hot water with boilers where the water heating coil is immersed directly in the combustion water heating chamber. This is a very inefficient way to produce hot water as it requires the entire boiler to fire year-round.


Temperature Controls

Temperature controls monitor the heating distribution system’s return and supply lines. By examining the difference in temperature between the departing and returning supply, it closely controls the boiler/furnace output.



100,000 BTUs. Often used to standardize the comparison of different energy sources. When used for home heating and hot water, natural gas is delivered in therms.


Thermal Barrier

The temperature control layer of the building envelope. AKA Insulation.


Thermal Bridging

The movement of heat across a building assembly through a poorly insulated section, often compromising thermal performance of the wall assembly. The most common example is vertical wall studs in a building wall.


Thermal Envelope

Same as thermal barrier. The temperature control layer of the building envelope. AKA Insulation.


Thermal Imaging Camera

 Infrared camera. A camera which uses the infrared spectrum. Thermal cameras are a mainstay of the auditing industry as they can detect gaps in insulation, air infiltration and water leaks.


Top Plate

The stud board that forms the top of a wall in platform framed construction.



The inverse of R-value. Another measure of the thermal resistance (actually thermal conductance but it’s so often conflated with R-value, we might as well call it resistance) of a building assembly or insulating product, most often used with windows.



Unvented roofs are those which do not have an active ventilation system. More and more modern construction use unvented roof designs. In cold climates, this requires that the interior air barrier (or other condensing surfaces) stay warmer than the dew point so as to avoid condensation. This makes venting which is intended to transport moisture out of the building unnecessary but requires controlling interior moisture sources.


Vapor Barrier

Building shell layer which controls/prevents the diffusion of water vapor through it.



Most modern residential construction ventilates the roof & occasionally the basement. The main goals with ventilated roofs are controlling roof temperature, extending the life of roof sheathing and, during the winter, slowing the melting of snow. A desired benefit is controlling moisture. A roof ventilation system usually consists of continual venting along the roof edge (known as soffit or edge vents), channel vents running along the roof deck (known as channel or proper vents) leading to either a ridge vent (running the length of the roof peak) or gable end vents.



More acronym fun, standing for Volatile Organic Compound. VOCs are carbon-based organic (I know that’s redundant) compounds which can evaporate at room temperature. Found in many home products, they can contribute to air quality issues.



Material additions to the seams of doors and windows to reduce the leakage of interior conditioned air.



Extruded Polystyrene foam insulation. This foam board insulation has advantages in that it is water resistant, can act as an air barrier and has higher insulating values (R-5.5 to R-6.5 per inch) than other common insulating materials.