A Glossary Of Building Science Terms

Author: Muntin Professional Services Inc. | | Categories: Architectural Professionals , Furniture Design , Home Inspections , Mold Inspector


Every industry has its own language and terms. These words and phrases can be confusing to anyone who is not part of the daily operations of that specific industry, and the home inspection business is no exception.

To help you understand the terms, acronyms, and phrases regularly used when working with a home inspector, the experts at Muntin Professional Services Inc. have created this handy reference guide. Here you’ll find valuable information allowing you to comprehend and communicate your needs and concerns effectively.

ACH or Air Changes Per Hour is a common unit of measuring the ventilation rate for a space, or air leakage rate for a building. It is defined as the volumetric flow rate divided by the volume of the area considered.

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

 It is the process in which fluid molecules are concentrated on a surface by chemical or physical forces or both.

b. It is surface adherence of a material in extracting one or more of the substances present in an atmosphere or mixture of gases and liquids, unaccompanied by physical or chemical change. (Taken from Gatley, “Understanding Psychrometrics.”)

AFUE or Annual Fuel Utilization Efficiency is 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
This term is 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
This is a material with sufficiently low air permeance and adequate strength that it can be part of an air barrier system. The 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 acting to control air-flow across a building enclosure, or between a conditioned space and an unconditioned space. In multi-unit or townhouses and apartment constructions, 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 a 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
It is replacing the volume of a space in one hour. That is space, volume in cubic feet divided by sixty minutes yields 1.0 air change in CFM.

Air densities
The density of dry air at sea level is 0.0745 pounds per two feet at seventy Fahrenheit or about one eight-hundredth of the density of water. But as altitude and temperature (or both) increase, the density drops dramatically. This is because the density of air is proportional to the pressure and inversely proportional to temperature. So, the higher you go into the atmosphere, the lower the pressure gets. The pressure is approximately halved for each additional increase of fifty-six kilometers in altitude.

Air handler
It is a steel cabinet containing a blower with cooling and heating coils connected to it.

Air infiltration
It is the uncontrolled inward leakage of air (that may contain entrained water vapor) through cracks and interstices in any building element and around windows and doors of a building, caused by the pressure effects of wind or the impact 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 utilizing a pressure differential is called air transported moisture.

(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:

a. Whole house mechanical ventilation using the formula ; Ventilation Rate= 7.5cfm ( number of bedrooms + 1) x (0.01 x Conditioned floor area or home)

b. Bathroom exhaust specified at low sone, 50cfm intermittent or 20cfm continuous

c. Kitchens vented to the exterior at 100cfm intermittent or equivalent to five air changes per hour if continuous.

d. Clothes dryers vented to the outdoors

e. Minimum filtration of ducted supply air systems of MERV 6

It is the 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.

It is the 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. They measure 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 all house cracks and openings can be calculated.

BTU or British thermal units refers to the amount of energy required to heat one pound of water at one degree Fahrenheit.

British thermal units per hour or BTU/hr is the quantity of heat required to raise one pound of water by one degree Fahrenheit.

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

Building science
This is the relationship of building materials and the building enclosure to heat, air, radiation, and moisture, controlling damage caused by these elements.

Cap flashing
It is a flashing overlapping the vertical leg of base flashing to prevent water from migrating behind the base flashing.

Capillary action
 The ability of water to move through materials, even upward against gravity, through small tubes or spaces.

b. 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 one-eighth of an inch or three millimeters.

Capillary break
It is a hydrophobic material or non-porous material (such as glass, plastic, or metal) or gap between parallel layers of material (often less than one-sixteenth of an inch or one and a half millimeter) sufficient to stop capillary action.

CFM-cubic foot per minute
CFM cubic foot or feet per minute is a measure of air-flow. A unit of volumetric flow rate often used as a metric of ventilation, air-flow, or air leakage.

This is the number of cubic feet per minute of air flowing through the fan housing of a blower door when the house pressure is fifty 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)
It is a ratio calculated by dividing the total heating capacity provided by the heat pump, including circulating fan heat but excluding supplementary resistance heat (BTUs per hour), by the total electrical input (watts) x 3.412. (See Heating Seasonal Performance Factor, below.)

It is a snake-like piece of copper tubing surrounded by rows of aluminum fin that clamp tightly to the tubing to aid in heat transfer.

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

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

It is the transfer of heat, but not mass, from warmer to colder regions of a substance, or adjacent substances in contact.

It is the circulation caused by the buoyancy of heated or cooled fluids. More likely, a reference to air than to water in HVAC.

Cooling Degree Day
It is the number of degrees that a day’s average temperature is above sixty-five Fahrenheit, the temperature at which people will start to use air conditioning to cool their buildings.

Color Rendering Index or CRI is a metric of the ability of a lighting source to render colors correctly relative to ideal light. A CRI of one hundred is the maximum value. For reference, incandescent lights are in the ninety-five to one hundred range, and compact fluorescents are in the mid-eighties.

Data logger
It is 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.

It refers to the removal of water vapor from the air.

Dew point temperature
It is water vapor at saturation that is 100% RH. In colloquial terms, it’s the temperature at which water begins to condense out of the air onto a surface.

It is a device installed over openings in room surfaces created for air-flow that aspirates room the air, causing it to mix with the air stream. It is normally used in ceilings. It is always a supply, damper, or shutter for regulating air-flow as described separately.

 It is the movement of individual molecules through a material. The change occurs because of concentration gradients (and to a much lesser degree) thermal gradients, independent of air-flow. A mode of water vapor transport in building enclosures that is much slower than air-flow.

b. It 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, house wrap, 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 in a shingle fashion or are sealed so that water flow is downward and outward.

Dry bulb temperature
It is ordinary air temperature, as opposed to wet bulb temperature.

It is a conduit through which air is transported. It is normally modified by the state of air as in the supply duct, return duct, etc.

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

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

It is the ability of a material to emit radiant energy from its surface. It is also called emittance.

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

It is the 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. The enthalpy cannot be directly measured. But, enthalpy differences between the initial and final standpoints of a process can be measured. (From Gatley, “Understanding Psychrometrics”)

EqLA is an abbreviation for equivalent leakage area. It is typically expressed as the size of the opening of a building enclosure at a ten Pascals pressure difference.

Equivalent leakage area of a building
The EqLA of a building is a quantitative expression of the air-tightness 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 ten Pascals, and the leakiness of the enclosure is expressed as a summary hole in square inches.

ERV or an Energy or Enthalpy-Recovery Ventilator is 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 and HRV is that in an ERV, the heat exchanger transfers a certain amount of water vapor along with heat energy, while an HRV only transfers heat.

ESP or external static pressure is the sum of the static and velocity pressures of a moving air system at the point of measurement.

It refers to when water changes from a liquid into vapor.

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

It refers to room air lost from a space through cracks due to positive pressure.

Exhaust air
It is the air-flow leaving the conditioned space and discharged to the outdoors.

Expanded polystyrene insulation
It is a rigid cellular foamed plastic insulation material manufactured by the 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
It is 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
It is 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
It is the average velocity of air passing through the face of an outlet or return.

It is 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 or water can penetrate.

Flow hood
It is a diagnostic tool used to measure air-flow through ducts, supply registers, and return grilles.

Flow plate
A flow plate or true flow 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 400cfm and 2,200cfm.

It is a 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 or geo-exchange equipment

These are heat pumps that use the ground to transfer heat to and from the refrigerant in the unit. The unit circulates water through a heat exchanger into a closed-loop buried in the ground or by pumping water from a well through the unit.

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

Heat transfer
It is the science of estimating the amount, rate, and direction of heat flow. It is an inexact science bounded by the Laws of Thermodynamics.

Heating degree day (HDD)
It is a form of degree-day used as an indication of fuel consumption. It is often used in the United States. 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 65 Fahrenheit (18 Celsius), the temperature below which buildings need to be heated.

HERS or Home Energy Rating System was developed by RESNET, DOE, and others for rating buildings for energy efficiency.

HRV or Heat Recovery Ventilator is a device that brings 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.

Humidity is the water vapor component of moist air.

HVAC is the abbreviation for heating ventilation and air conditioning

It is 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 be absorbed into the material, i.e., “wettable.”

It refers to materials that do not attract liquid water will force liquid water to form beads on their surface. They act as capillary breaks and are “non-wettable” materials that do not attract liquid water, as they are non-hygroscopic.

It is 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.

It refers to materials that interact with water vapor by adsorbing water vapor as a function of the relative humidity of the surrounding air.

IEQ is the abbreviation for indoor environmental quality.

Indoor air
It refers to the air within a conditioned space.

It occurs when the outside air is naturally introduced through cracks due to wind or negative pressure.

Infrared camera
An infrared camera is also called a thermographic camera, or an “IR” camera. It is a device that forms an image using infrared radiation, similar to a common camera that creates an image using visible light.

Insulated sheathing
These are non-structural insulating board products with varying R-values and a wide variation in vapor permeability and drainage characteristics. The materials used to make it include expanded polystyrene (EPS), extruded polystyrene (XPS), polyisocyanurate (most often foil-faced), rigid fiberglass, and mineral wool.

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

These are interfaces 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 needs 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
 Colloquially, it’s a transfer air duct.

b. It is 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. They are 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 1,000Watt power drawn over a sixty-minute interval. One kWh is equal to 3600 kilojoules or 3412 BTUs.

Latent heat of the air
 It is energy removed from the air due to the condensation of moisture.

b. It is a change in enthalpy during a change of phase.

Liquid flow
It is typically the flow of liquid moisture via gravity or capillarity.

Low-e is 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 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).

It is a 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.

It is a heavy-consistency compound that may remain adhesive and pliable with age. It 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
It is a controlled, purposeful introduction of outdoor air to the conditioned space.

It is a full-scale but limited extent demonstration of a construction assembly, such as a window in a wall, a roof parapet, cladding interface, or another construction assembly; before mass construction.

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

It is 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 can sometimes 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 grows 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.

NACH is 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 per hour or NACH or ACHnat.

NZEH is a net-zero energy house or building. It is a building capable of producing as much energy as it consumes on an annual basis.

Occupiable space
It is an 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.

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