Pressure difference across portions of the building enclosure or across air barriers.


T.D., Temperature Difference measured between 2 objects or environments


Process whereby a porous material extracts on or more substances from an atmosphere, a mixture of gases, or a mixture of liquids. (from Gatley, “ Understanding Psychrometrics”


Air Conditioning Contractors of America is a group of over 4,000 air conditioning contractors who work together to improve out industry, promote good practices, and keep homes and building safe, clean ad comfortable.


Air changes per hour. A common unit of measure of ventilation rate for a space, or air leakage rate for a building, defined as the volumetric flow rate divided by the volume of the space considered.


A substance or compound used for bonding surfaces together, usually applied in the form of a liquid or paste. An adhesive and sealant or base coat may be the same material.


(1) Process in which fluid molecules are concentrated on a surface by chemical or physical forces, or both; (2) surface adherence of a material in extracting one or ore of the substances present in an atmosphere or mixture of gases and liquids, unaccompanied by physical or chemical change. (from Gatley, “Understanding Psychrometrics”)


(Annual Fuel Utilization Efficiency) - A rating that denotes the efficiency of gas heating equipment. It is the amount of heating your equipment delivers for every dollar spent on fuel. A higher rating indicates more efficient equipment. This rating is calculated in accordance with the Department of Energy test procedures.

Air Barrier

A term often abbreviated from an Air Barrier System or Air Barrier Material. The complete air barrier system is comprised of materials and assemblies, each with their own performance requirements.

Air Barrier Material

A material that has sufficiently low air permeance and adequate strength that it can be part of an air barrier system. Recommended maximum air permeance for a material is 0.02 l/(s-m2)@ 75 Pa (0.004 cfm/sf @ 0.3” WC) when tested according to ASTM E 2178 or E 283.

Air Barrier System

Air barriers are three-dimensional systems of materials designed, constructed, and/or acting to control air flow across a building enclosure, or between a conditioned space and an unconditioned space. In multi-unit/townhouse/apartment construction an air barrier system should also separate the conditioned air from any given unit and adjacent units. The pressure boundary of the enclosure should, by definition, be coincident with the plane of a functional air barrier system. In multi-unit/townhouse/apartment construction the air barrier system may also be the fire barrier and smoke barrier between units. In such assemblies the air barrier system must also meet the specific fire resistance rating requirement for the given separation.

Air Change

Replacing the volume of a space in one hour; space, volume in cubic feet divided by 60 minutes yields 1.0 Air Change in CFM.

Air densities

The density of dry air at sea level is 0.0745 lb/ft2 at 70F or about 1/800th the density of water. But as altitude and temperature (or both) increases, the density drops dramatically. This is because the density of air is proportional to the pressure and inversely proportional to temperature. And the higher you go into the atmosphere, the lower the pressure gets. Pressure is approximately halved for each additional increase of 56 km in altitude.

Air Handler

A steel cabinet containing a blower with cooling and/or heating coils connected to it.

Air Infiltration

Uncontrolled inward leakage of air (that may contain entrained water vapor) through cracks and insterstices in any building element and around windows and doors of a building, caused by the pressure effects of wind or the effect of differences in the indoor and outdoor air density. (from Gatley, "Understanding Psychrometrics")

Air Pressure Boundary

The air pressure boundary is the boundary (comprised of a series of planes to form a three dimensional boundary) that generates the largest pressure drop (usually much more than half the total) when the enclosure is subjected to a pressure difference.

Air Transported Moisture

Moisture movement associated with the movement of air from one place to another by means of a pressure differential.


(American Society of Heating, Refrigeration, and Air-Conditioning Engineers) Published standards for introducing Outside Air for occupant health and comfort; 62.1 for commercial, 62.2 for residential; now adopted into law as part of most building codes. The residential requirement, “Ventilation and Acceptable Air Quality in Low-Rise Residential Buildings” has been in existence since 2003. The standard describes three primary sets of requirements for acceptable indoor air quality in homes; they are whole house ventilation, local exhaust and pollutant control. The most common elements are the following: • Whole house mechanical ventilation using the formula ; Ventilation Rate= 7.5cfm ( number of bedrooms + 1) x (0.01 x Conditioned floor area or home) • Bathroom exhaust specified at low sone, 50 cfm intermittent or 20cfm continuous • Kithchens vented to the exterior at 100 cfm intermittent or equivalent to 5 air changes per hour if continuous. • Clothes dryers vented to the outdoors • Minimum filtration of ducted supply air systems of MERV 6


Reverse flow of combustion gases down the chimney of a vented combustion appliance, which is often caused by depressurization of the room where the appliance is located.


Process of adjusting the flow of air in duct systems, or water flow in hot-water heating systems.

Blower Door

A blower door is a calibrated fan (or blower) in a portable expanding frame, typically installed in an exterior door opening. Blower doors are used to determine the leakiness of a building envelope. A blower door measures the total air flow rate through all of the openings in a house by extracting air from (or blowing it into) the house and measuring the air pressure drop across the house. From this information, the area of al house cracks and openings can be calculated.


Amount of energy required to heat one pound of water one Fahrenheit degree.


British thermal units per hour. A BTU is the quantity of heat required to raise 1 pond of water by 1 degree Fahrenheit.

Building Enclosure

The system or assembly of components that provides environmental separation between the conditioned space and the exterior environment.

Building Science

The relationship of building materials and the building enclosure to heat, air, radiation, and moisture; controlling damage caused by these elements.

Cap Flashing

A flashing overlapping the vertical leg of base flashing to prevent water from migrating behind the base flashing.

Capillary action

(1) The ability of water to move through materials, even upward against gravity, through small tubes or spaces. (2) The movement of water within the confined spaces of a porous material or between two adjoining hydrophilic materials due to the attractive force of surface tension. Only significant in gaps of less than about 1/8” (3 mm).

Capillary Break

A hydrophobic material or non porous material (such as glass, plastic, or metal) or gap between parallel layers of material (often less than 1/16” or 1.5 mm) sufficient to stop capillary action.

CFM-cubic foot per minute

Cubic foot/feet per Minute; measure of Airflow. A unit of volumetric flow rate, often used as a metric of ventilation, airflow, or air leakage.


The number of cubic feet per minute of air flowing through the fan housing of a blower door when the house pressure is 50 pascals (0.2 inches of water). This figure is the most common and accurate way of comparing the air-tightness of buildings that are tested using a blower door.

Coefficient of Performance (COP)

A ratio calculated by dividing the total heating capacity provided by the heat pump, including circulating fan heat but excluding supplementary resistance heat (Btu's per hour), by the total electrical input (watts) x 3.412. (See Heating Seasonal Performance Factor, below.)


A snakelike piece of copper tubing surrounded by rows of aluminum fin that clamp tightly to the tubing in order to aid in heat transfer.


The change of state from vapor to liquid. A common factor in moisture damage. Occurs on surfaces, which must be cooler than the air containing vapor next to it. Vapor supply to the condensation surface is usually by airflow but can be by diffusion.


The coil in an air conditioning system where the refrigerant condenses and releases heat


Transfer of heat, but not mass, from warmer to colder regions of a substance, or adjacent substances in contact.


Circulation caused by the buoyancy of heated or cooled fluids; more likely a reference to air than to water in HVAC.

Cooling Degree Day

The number of degrees that a day's average temperature is above 65 Fahrenheit, the temperature at which people will start to use air conditioning to cool their buildings.


Color Rendering Index. A metric of the ability of a lighting source to render colors correctly relative to ideal light. A CRI of 100 is the maximum value. For reference incandescent lights are in the 95-100 range and compact fluorescents are in the mid-80’s.

Data logger

An electronic device that records data over time or in relation to location either with a built in or external instrument or sensors. In home energy auditing, they may be used to monitor various data points, such as temperature, water, humidity, gas flow, power, solar radiation, and wind.


Removing water vapor from air.

Dew Point Temperature

Water vapor at saturation; 100% RH; colloquial: temperature at which water begins to condense out of air onto a surface.


Device installed over openings in room surfaces created for Air-flow that aspirates room air, causing it to mix with the air stream; normally used in ceilings; always a Supply; damper or shutter for regulating Airflow is described separately.


(1) The movement of individual molecules through a material. The movement occurs because of concentration gradients (and to a much lesser degree) thermal gradients, independent of airflow. A mode of water vapor transport in building enclosures that is much slower than airflow. (2) is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion.

Drainage Plane

Drainage planes are water repellent materials (building paper, housewrap, foam insulation, etc.) which are designed and constructed to drain water. They are interconnected with flashings, window and door openings, and other penetrations of the building enclosure to provide drainage of water to the exterior of the building. The materials that form the drainage plane overlap each other shingle fashion or are sealed so that water flow is downward and outward.

Dry bulb temperature

Ordinary air temperature; as opposed to wet bulb temperature.


Conduit through which air is transported; normally modified by state of air as in Supply Duct, Return Duct, etc.

Duct Blaster

a blower-door-like device used for testing duct leakiness and air flow in a forced air heating or cooling system.


The deposition of dissolved salts I the material (such as concrete or brick) being transported within water (usually by capillary action) on a visible surface after evaporation of water.


The ability of a material to emit radiant energy from its surface. Also called emittance.


Energy is power used over a specified time (energy = power x time) A 3 ton 13 SEER unit draws nearly 3,500 watts (power) while running on full capacity. If it runs for 3 hours a day at full capacity, how much energy does it consume in the 3 hours? (Answer: 10.5 kWh)


thermodynamic quantity equal to the sum of the internal energy of a system plus the product of the pressure-volume work done on the system; enthalpy cannot be directly measured; however, enthalpy differences between the initial and final statepoints of a process can be measured. (from Gatley, "Understanding Psychrometrics")


abbr. equivalent leakage area; typically expressed as the size of opening of a building enclosure at a 10Pa pressure diffference

Equivalent Leakage Area of a Building

Of a building. Quantitative expression of the airtightness of a building enclosure. EqLA is the method set by the Canadian General Standards Board in which a blower door depressurizes the building enclosure to 10 Pascals and the leakiness of the enclosure is expressed as a summary hole in square inches.


Or Energy/Enthalpy-Recovery Ventilator – a ventilator that recovers latent and sensible energy from the exhaust airstream and imparts it to the incoming airstream. The main difference between an ERV & HRV, is that in an ERV the heat exchanger transfers a certain amount of water vapor along with heat energy, while a HRV only transfers heat.


EXTERNAL STATIC PRESSURE - The sum of the static and velocity pressures of a moving air system at the point of measurement.


Water changing from liquid into vapor.


The heat transfer coil of an air conditioner or heat pump that cools the surrounding air as the refrigerant.


Room air lost from a space through cracks due to positive pressure.

Exhaust Air

The air flow leaving the conditioned space and discharged to the outdoors.

Expanded Polystyrene Insulation

A rigid cellular foamed plastic insulation material manufactured by expansion of polystyrene beads within a mold. This mold creates an open cell structure filled with air. EPS Type I is the most widely used insulation. Type I has a density of 1 lb/ft3 (16 kg/m3) ), Type II is a denser, more durable insulation of 1.5 lb/ft3 (24 kg/m3) density.

Extruded Polystyrene Insulation

A rigid cellular foamed-plastic insulation material manufactured by extrusion of polystyrene in the presence of a blowing agent. The blowing agent dissipates out of the closed cell structure over time, creating a structure that resists liquid water penetration and vapor diffusion. The manufacturing process for XPS insulation results in a smooth surface skin. Typical density of 2 lb/ft3 and R-value of 5 per inch (0.029 W/mK).

Face Seal

A building enclosure rain control strategy that relies on the exterior face of the enclosure to act as a perfect barrier to rain penetration. This method typically relies on exposed sealants to provide rain tight joints, and hence is highly reliant on workmanship and maintenance to achieve performance. Failure is defined as water penetration of the face. It is a sub-set of perfect barrier rain control strategies.

Face velocity

Is the average velocity of air passing through the face of an outlet or return.


A deficiency in the installation of membranes (roofing, self-adhering membranes, etc.) that results in a fold in the edge of the membrane, through which air and/or water can penetrate.

Flow hood

A diagnostic tool used to measure air flow through ducts, supply registers, and return grilles.

Flow Plate

or TrueFlow Air Handler Meter is a device designed to provide a simple and accurate measurement of air flow through residential air handlers. Flows can be measured between 400 cfm and 2,200 cfm

Freeze Thaw

Damage that occurs to stone, clay, or cementitious products due to high moisture content (near saturation) combined with cycling above and below the freezing temperature. Typically manifested as spalling of the surface.

Geothermal/Geo-Exchange Equipment

Heat pumps that uses the ground to transfer heat to and from the refrigerant in the unit. The unit circulates water through a heat exchanger in to a closed loop buried in the ground or by pumping water from a well through the unit.

Gum Lip

A method of sealing a flashing to a wall surface whereby the top edge of the flashing is bent outwards to form a sealant-filled reveal (typically at the vertical termination of a waterproof membrane).

Habitable Space

Building space intended for continual human occupancy. Such space generally includes areas used for living, sleeping, dining and cooking, but does not generally include bathrooms, toilets, hallways, storage areas, closets, or utility rooms.

Heat Transfer

The science of estimating the amount, rate, and direction of heat flow; an inexact science bounded by the Laws of Thermodynamics.

Heating Degree Day (HDD)

A form of degree-day used as an indication of fuel consumption; in United States usage, one heating degree-day is given for each degree that the daily mean temperature departs below the base of 65°F. The number of degrees that a day's average temperature is below 65Fahrenheit (18Celsius), the temperature below which buildings need to be heated.


Abbr. Home Energy Rating System. Developed by RESNET, DOE and others for rating buildings for energy efficiency.


Abbr. Heat Recovery Ventilator- This device bring fresh, outside air into a home while simultaneously exhausting stale indoor air outside. In the process of doing this, an HRV removes heat from the exhaust air and transfers it to the incoming air, pre-heating it.


the water vapor component of moist air.


Abbr. Heating Ventilation and Air Conditioning


A term used to describe a surface or material that attracts and holds water, transiently bonding with it via hydrogen bonds. Liquid water on a hydrophilic surface will tend to absorb into the material—i.e., "wettable."


Materials that do not attract liquid water, they will force liquid water to form beads on their surface, act as capillary breaks and are non-Materials that do not attract liquid water, they will force liquid water form beads on their surface, act as capillary breaks and are non-hygroscopic.


A term used to describe pressures developed by a non-moving fluid (typically water, in our cases), such as the buildup of subsurface water against a foundation wall.


Materials that interact with water vapor by adsorbing water vapor as a function of the relative humidity of the surrounding air.


Abbr. indoor environmental quality

Indoor Air

Air within a conditioned space


Outside air introduced naturally through cracks due to wind and/or negative pressure.

Infrared Camera

Also called a thermographic camera, an “IR” camera is a device that forms an image using infrared radiation, similar to a common camera which forms an image using visible light.

Insulated Sheathing

Non-structural insulating board products with varying R-values and a wide variation in vapor permeability and drainage characteristics. Materials include expanded polystyrene (EPS), extruded polystyrene (XPS), polyisocyanurate (most often foil-faced), rigid fiberglass, and mineral wool.


Thermal: Any material which significantly slows down or retards the flow or transfer of heat. Building insulation types are classified according to form (e.g loose-fill, batt, flexible, rigid, reflective, and foamed-in-place) or material (mineral fiber, organic fiber, foam plastic). All types are rated according to their ability to resist heat flow (R-Value or RSI).


An interface between elements. Joints may be required to allow movement of different parts of a building assembly, or may be required to make construction sequences practical. In all cases, the functional requirements of the enclosure must be maintained the same as for the body of an enclosure element, although aesthetic requirements may be relaxed. A joint may pass through the entire enclosure assembly, in which case it is a building movement or assembly joint, often commonly (and imprecisely) referred to as an expansion joint. Control joints are surface cuts or intentional geometric features which control the location of shrinkage cracks. Construction joints are formed between successive building elements parts during construction work.

Jump duct

(1) Colloquial; transfer air duct. (2) A flexible, short, U-shaped duct (typically 10-inch diameter) that connects a room to a common space as a pressure balancing mechanism. Jump ducts serve the same function as transfer grilles. Used when return ducts are not located in every room.

Kiln-dried Lumber

Any lumber placed in a heated chamber to reduce its moisture content to a specified range or average under controlled conditions. For softwood framing lumber, the moisture content of KD lumber is somewhat based on regional conventions but is most often an average of 12% by weight. In comparison, the moisture content of thoroughly air-dried softwood framing lumber is 15% to 20%.


one kWh is equivalent to 1,000 watt power draw over a 60 minute interval. One kWh is equal to 3600 kilojoules or 3412 BTUs.

Latent heat of air

(1) Energy removed from air due to the condensation of moisture. (2) Change in enthalpy during a change of phase.

Liquid Flow

Typically the flow of liquid moisture via gravity or capillarity


Short for low emissivity, which means the characteristic of a metallic glass coating to resist the flow of radiant heat. A thin metallic oxide coating increases the U-value and/or decreases the SHGC of the window by reducing heat flow from a warm(er) surface to a cold(er) glazing surface. The best location for the coating should be calculated but is often based on whether the primary heat flow control direction is from the inside out (heating climates) or the outside in (cooling climates).


Measuring device for fluid pressures. A digital manometer measures the air pressure relationship between two spaces, providing the magnitude of the pressure difference (if one exists) rather than just the direction.


Heavy-consistency compound that may remain adhesive and pliable with age. Is typically an airtight waterproof compound applied to exterior walls and roof surfaces, or to provide a robust and durable air seal to air duct distribution systems.

Mechanical Ventilation

Controlled, purposeful introduction of outdoor air to the conditioned space.


Full-scale, but limited extent demonstration of a construction assembly, such as a window in a wall, a roof parapet, cladding interface or other construction assembly; prior to mass construction.

Moisture meter

A moisture meter is a device which can check for moisture content in drywall, insulation, wood, concrete and plaster. It can also assist in tracing leaks and finding exterior moisture sources.


A type of fungus that is different from plants, animals and bacteria. Molds are decomposers of dead organic material such as leaves, wood and plants. Molds sometimes can infect living plants and animals. The spores and hair-like bodies of individual mold colonies are too small for us to see without a microscope. When a lot of mold is growing on a surface, it often appears black or green. The color of mold is influenced by the nutrient source and the age of the colony. Mold growing on fabric is called mildew.


The natural air changes per hour in a building, as calculated by dividing by the LBL ACH50 Factor. The number of cubic feet of air flowing through a house from indoors to outdoors during typical, natural conditions. This figure can be roughly estimated using a blower door, and can also be denoted as (natural air changes/hour or NACH or ACHnat)


abbr. net zero energy house or building. A building that has the capability of producing as much energy as it consumes on an annual basis.

Occupiable Space

Any enclosed space inside the pressure boundary and intended for human activities, including but not limited to, all habitable spaces, toilets, closets, halls, storage and utility areas, and laundry areas.


or Pa; A unit of measurement of air pressure.


A hole passing through the building envelope in which ducts, pipes, wires, structural elements, and windows are run between inside and outside. Windows are also a penetration.


Unit of measurement typically Perm; A measurement of how much water vapor a material will let pass through it per unit of time OR Semipermeable: speed at which water molecules pass through a material by diffusion (see diffusion).


Relatively large chamber in an air duct or pathway experiencing low air velocity.


(units of energy): Electrical power is usually measured in watt (W), kilowatt (kW), megawatt (MW), etc. Power is energy transfer per unit of time.


A force encouraging flow by virtue of a difference in some condition between two areas.

Pressure drop

The decrease in pressure due to friction of a fluid or vapor as it passes through a tube or duct or/and lift.

Pressure-equalized Rainscreen

A specific type of drained wall system that uses spatial compartmentalization, careful venting to air leakage ratios, and stiff components to encourage short-term equalization of drained cavity (also the air chamber) pressures with exterior wind pressures to reduce the pressure across the screen (or cladding). Such systems rely on effective drained to control rain, and if functional, merely reduce the rainwater that the drained system must accommodate.


The science of the relationship between air and its energy and, water vapor contents.

PV or PhotoVoltaic

Capable of producing a voltage when exposed to radiant energy, especially light.


A measurement of thermal resistance to conductive heat flow through a material. R-value is primarily for labeling insulation products. It is the inverse of the U-Value; R-value = 1/U-value. While many in the building community consider R-value to be the primary or paramount indicator of energy efficiency, it only deals conduction, one of three modes of heat flow, (the other two being convection and radiation). As an example of the context in to which R-value should be placed, 25% to 40% of a typical home's energy use can be attributed to air infiltration.


Transfer of heat through space without the benefit of an intervening medium. Can occur through a gas or a vacuum.


A building enclosure rain control strategy that accepts that some water will penetrate the outer surface (the cladding, which “screens” rain) and removes this water back to the exterior by gravity drainage over a drainage plane, through a drainage gap, and exiting via flashing and weep holes. In essence it is a drained system, however, some use the term only for systems that have larger drainage gaps (e.g., 1/2") or just for systems that are also ventilated (a ventilated drained approach) or just for systems that attempt to pressure-equalize.

Relative Humidity

The ratio (expressed as a percentage) of the amount of moisture within the air to the maximum amount of moisture that the air could possibly contain at a specific temperature.


abbr. the Residential Energy Services Network. n April 1995, the National Association of State Energy Officials and Energy Rated Homes of America founded the Residential Energy Services Network (RESNET) to develop a national market for home energy rating systems and energy efficient mortgages.


Building enclosure element intended to slow penetration. Example: thermal insulation.

Return Air

Air drawn into a heating unit after having been circulated from the heater's output supply to a room.

Rigid Insulation

Rigid board material that provides thermal resistance. Foam plastic such as EPS, XPS, and polyisocyanurate are commonly used


A flexible, polymer based elastomeric material installed wet and used in the assembly of the building enclosure to seal gaps, seams, or joints and to provide a clean finish, or waterproof, or airtighten the joint.


Cooling Efficiency. "SEER" is a measure of cooling efficiency for air conditioning products. SEER stands for Seasonal Energy Efficiency Ratio. The higher the SEER rating number, the more energy efficient the unit. The government's established minimum rating typical residential split air conditioning system is 13. SEER is calculated by the amount of heat removed, divided by the electrical input power.

Sensible heat

Energy removed from air due to cooling (as opposed to Latent Heat). OR- Heat that can be measured or felt. Sensible heat always causes a temperature rise.


abbr. Specific Leakage Area.

Smoke pencil

A smoke pencil is typically a hand-held device that emits a chemical smoke when squeezed. Smoke pencils can be used to quickly and accurately determine the air pressure relationship between two spaces--for example, between the indoors and outdoors or between a bedroom and the space inside the wall separating the bedroom from the hallway. If smoke gets sucked into a crack or opening, the smoke pencil is located in a region of positive air pressure with respect to the crack.

Solar Heat Gain Coefficient (SHGC)

The ratio of solar heat gain through a window to incident solar heat. Includes both transmitted heat and absorbed and reradiated heat.

Solar reflectance or Albedo

of an object is the extent to which if diffusely reflect light from the sun. It is therefore a more specific form of the term reflectivity. The percent of total solar energy reflected by a material. This is associated with many products toady as an SRI on a scale of 0 to 1. (zero absorbing all visible light and 1 reflecting all)


Measures the loudness of appliances like range hoods. One sone is about equal to a refrigerator running, while four is equivalent to normal conversation. This is a subjective unit of loudness, as perceived by a person with normal hearing, equal to the loudness of a pure tone having a frequency of 1,000 hertz at 40 decibels.

Specific Heat Capacity

The amount of heat necessary to change the temperature of one pound of a substance 1 F.

Stack Effect

air movement caused by warmer air rising and cold air falling. This generates small but steady pressures in direct relation to the size of the temperature difference and the height of the column of air. The resulting pressure differences can lead to air leakage and generate unplanned air flows within buildings that result in indoor air quality problems.

State Change

Matter exists in 3 states: solid, liquid, and gas; a state change is a transition from one state to another. Examples include boiling/evaporation and condensation (liquid/gas); freezing and melting (soild/liquid); and sublimation/frost (solid/gas). Energy must be absorbed and released into the material for a state change to occur.

Static pressure

The normal force per unit area at a small hole in the wall of a duct (in respect to HVAC).


A potentially harmful accumulation of water-soluble salts that re-crystalize just beneath the masonry, stucco, or concrete surface as moisture in the wall evaporates leaving the salts behind.


The ductwork that carries conditioned air from the air handler to the rooms in the house.


An agent (e.g. detergent) that when mixed with water, breaks the surface of water drops, thus enabling easier absorption of water through a material. Without surfactants, water would have a great tendency to remain as drops on the surface of a given material.


A measurement of heat intensity

Terminal Velocity

(fluid mechanics) The velocity with which a body moves relative to a fluid when the resultant force acting on it (due to friction, gravity, and so forth) is zero. In context of this class: Air begins to fall vertically when it slows to a certain speed. (usually at a velocity 50-75 feet per minute)

Thermal Boundary

The layer in a building enclosure that controls the transfer of energy (heat) between the interior and the exterior. It is a component of the building enclosure and it may, but does not have to align with the pressure boundary.

Thermal Bridge

A material with higher thermal conductivity transferring heat through an assembly faster than one with substantially lower thermal conductivity. For example, a steel stud in a wall will transfer more heat than the surrounding insulation, reducing the overall thermal performance of the system.


Is the distance (measured in feet) that the air stream travels from the outlet to the point of terminal velocity. Throw is measured vertically from perimeter diffusers and horizontally from registers and ceiling diffusers.


Heat pumps and air conditioners are generally sized in tons. Typical sizes for single family residences are between two and five tons. Each ton equals 12,000 Btuh. It is important to note that actual capacity is not constant and will change based on outdoor or indoor temperatures. The published capacity rating of air conditioners and heat pumps is based on performance at the ARI standard temperature levels of 95 F outside, 80 F inside.

Transfer Grille

An intentional opening used to relieve HVAC supply pressure, preventing unintentional pressurization or depressurization of spaces.


Quantitative measure of heat flow or conductivity, the reciprocal of R-value. While building scientists will use R-values for measures of the resistance to heat flow for individual building materials, U-factor is usually used as a summary metric for the ease of heat transfer through building assemblies


(1)The amount of heat that will flow through a square foot of building cross-section experiencing a temperature difference of 1° F. (2) Quantitative measure of heat flow or conductivity, the reciprocal of R-value. While building scientists will use R-values for measures of the resistance to heat flow for individual building materials, U-factor is usually used as a summary metric for the ease of heat transfer through building assemblies

US Army Corp of Engineers Air Leakage Test Protocol

abbr. USACE standard. Airlfow testing procedures developed for testing air tightness of the building enclosure under ASTM test procedure E-779. This test allows for a maximum leakage ratio of 0.25 cfm per ft2 of enclosure area a t pressure differential of 75 Pa. See website : http://airbarriernews.com/industry-news/


Any gas at a combination of temperature and pressure placing it very close to its dew point (saturation).

Vapor Barrier

A material that has a permeance of 0.1 perm or less. A vapor barrier is a material that is essentially vapor impermeable. A vapor barrier is a Class I vapor control layer. The test procedure for classifying vapor barriers is ASTM E-96 Test Method A—the desiccant or dry cup method.

Vapor Permeance

layer property that describes the ease with which vapor molecules diffuse through it. It is defined as the quantity of vapor flow across a unit area that will flow through a unit thickness under a unit vapor pressure difference. It is to vapor diffusion what conductance is to heat transfer. The unit of measurement is typically the "perm" (gr/h•ft2•in. Hg); in metric/SI units, it is stated in terms of ng/(s•m2•Pa).

Vapor Permeance Classes

Test procedure for determining vapor permeance class is ASTM E-96 Test Method A (the desiccant or dry cup method). ▪ Class I: Materials that have a permeance of 0.1 perm or less (Note: This is the definition of a "vapor barrier.") ▪ Class II: Materials that have a permeance of 1.0 perm or less and greater than 0.1 perm (Note: This is the definition of a "vapor retarder.") ▪ Class III: Materials that have a permeance of 10 perms or less and greater than 1.0 perm ▪ Class IV: Materials that have a permeance of greater than 10 perms


The intentional flow of air into occupied spaces or behind cladding/roofing to move heat and moisture in a desirable manner.


abbr. Volatile Organic Compound. Often considered a pollutant detrimental to indoor air quality.

Water Resistant Barrier

A sheet, spray- or trowel-applied membrane or material layer that prevents the passage of liquid water even after long or continuous exposure to moisture.

Weep Holes

An opening placed in a wall or window assembly to permit the escape of liquid water from within the assembly or component. Designed correctly weep holes may also act as vents.

Wet bulb temperature

Temperature of wet wick slung through the air so that it cools by evaporation; surrogate for enthalpy; fortuitous phenomenon, has no scientific basis.


The phenomenon of air movement driven by wind pressures wind passing through the thermal insulation within enclosures, causing significant loss of heat flow control and potentially causing condensation. Typically occurs are exposed building edges, such as at the outside corners and roof eaves because of the large pressure gradients at these locations. This can be thought of as the “wind blowing through the insulating sweater” effect.


abbr. Wärme Und Feuchte Instationär. A modeling program for simulating one-dimensional dynamic heat and moisture transfer through building enclosure.