Namesort descending Definition
Outdoor Air

Air outside the building.


An oxygen chemical compound of the form 03 instead of 02 (common atmosphereic oxygen). This 3-atom molecule is an even more active oxidizing agent than its more common 2-atom relative. At ground level, ozone is a pollutant and in the upper atmosphere it is a UV shield. Ozone's highly reactive nature tends to accelerate the breakdown of synthetic materials such as paints, plastics, and volatile organic compounds.


abbr. Pascal

A metric unit of measurement of pressure, defined as a Newton per square meter. One inch of water column is equal to about 250 Pa.

Parapet Wall

A low wall around the perimeter that projects above the level of the top of the adjoining roof level.


abbr. Partnership for Advancing Technology in Housing

Previously the Partnership for Advancing Technology in Housing, PATH is now an online resource for homeowners and homebuyers, the homebuilding industry and federal agencies.


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.


The physical property that defines the ease with which water molecules diffuse through a material. It is to vapor diffusion what conductance is to heat transfer. The unit of measurement is typically the "perm."

Related Resources: Info-312: Vapor Permeance of Some Building Materials; Info-500: Building Materials Property Table


abbr. cross-linked polyethylene (PE) tubing


abbr. polyisocyanurate

A closed-cell foam insulation material typically sold in rigid boards (but also available as a liquid, sprayed foam). R-value can range from R-5.6 to R-9, depending on the manufacturer, the age of the foam, and the presence or absence of foil facings. Foil-faced PIC has a greater R-value and is also vapor impermeable (less than 0.1 perm). The use of ozone-depleting substances as blowing agents is being phased out. Most U.S. PIC manufacturers now use pentane.


abbr. principal, interest, tax, and insurance


Sill plate or “mud sill”: a horizontal member anchored to a masonry wall.

Sole plate: bottom horizontal member of a frame wall.

Top plate: top horizontal member of a frame wall supporting ceiling joists, rafters, or other members.


An air compartment or chamber to which one or more ducts are connected, forming part of either the supply air, return air or exhaust air system. Plenums can include areas above ceilings, below floors, or in wall cavities. The practice of using unsealed building cavities to circulate air may result in indoor air quality issues.


A wood product made of three or more layers of veneer joined with glue, and usually laid with the grain of adjoining plies at right angles.


See PIC.


A condition where water stands on a roof, slab, or any other nominally horizontal surface for prolonged periods due to poor drainage and/or deflection of the surface.

Pot Life

The duration of time that the wet state material remains workable after it has been mixed.

Pressure Boundary

The primary air enclosure boundary separating conditioned air and unconditioned air. Typically defined by the air control layer system.

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.


A coating intended to prepare the surface of the substrate for the subsequent application of a paint, adhesive or adhered membrane.


abbr. permanent split-capacitor (motor)


The study of air and its energy and water vapor contents.

Puffer Stick

See Smoke Pencil.

Punched Window

A window installed as a “punched” opening surrounded by cladding, as opposed to being arranged in vertical or horizontal strips.


abbr. photovoltaic

Quick Set

See Flash Set.


Quantitative measure of an assembly or material resistance to heat flow for a unit temperature difference and a unit area. It is the reciprocal of the U-factor. The units for R-value are m2 K/W. As R-value increases, conduction through an assembly or material decreases for the same temperature difference. As an example of the context in which R-value should be placed, 25% to 40% of a typical building's energy use can often be attributed to air infiltration, and air conditioning loads are often dominated by solar heat gain.

The effect of thermal bridging and different framing details requires a metric more complex than just a single R-value to allow for meaningful comparisons. Five R-values have been and are used in the building industry. Oak Ridge National Labs (ORNL) proposed a number of definitions (Christian and Kosny 1995). We have found it useful to add some and extend their definitions.

  1. Installed Insulation R-value. This R-value is commonly referenced in building codes and used by industry. This is simply the R-value labeled on the product installed in the assembly.
  2. Center-of-Cavity R-value. The R-value at a line through an assembly that contains the most insulation, and the least framing, typically, the middle of a stud-bay in framed construction.
  3. Clear-wall R-value. R-value of an assembly containing only insulation and minimum necessary framing materials at a clear section with no windows, corners, columns, architectural details, or interfaces with roofs, foundations or other walls.
  4. Whole-wall R-value. R-value for the whole opaque assembly including all additional structural elements (such as double studs), and typical enclosure interface details, including wall/wall (corners), wall /roof, and wall/floor connections.
  5. True R-value. The R-value of an enclosure assembly that includes all thermal bridging, air leakage, wind washing, convective loops, radiation enhancements, thermal and hygric mass, and installation defects.

Each of these measures is progressively more realistic. The True R-value is very difficult to measure without field samples.


abbr. return-air pathway

A commercial product that serves the same function as a transfer grille or jump duct, but also adds some light and sound transmission resistance.


The movement of energy by electromagnetic waves; can occur through a gas or a vacuum.  One of the three modes of heat transfer (in addition to convection and conduction). At normal temperatures, radiation is in the infra-red region of the spectrum, whereas solar radiation is at visible wavelengths. Controlling radiation is important to both heat transfer through porous insulations and through windows, as well as playing a significant role in daylighting and solar heating.

Rain Control Layer

See Water Control Layer.

Rain Penetration Control Layer

See Water Control Layer.


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. It is another term for 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 (a Pressure-Equalized Rainscreen).

Rainwater Boundary

The rainwater boundary (comprised of one or several materials and formed into planes to create a three dimensional boundary) is the boundary beyond which rainwater is not intended to penetrate or beyond which damage can likely be expected to occur if penetration were to occur.

Raised Heel Truss

A truss that has been fitted with a vertical member between the top and bottom truss cords, raising the top chord. The primary advantage of this strategy is to allow full-depth, uncompressed wall-to-wall insulation of the attic floor, reducing heat gain/ loss and mitigating the potential for ice dams.


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. Residential Energy Service Network

In April 1995, the National Association of State Energy Officials (NASEO) 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.


abbr. relative humidity

See Relative Humidity.

Rigid Insulation

Rigid board material that provides thermal resistance. Foam plastics such as expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate are commonly used.

Rough Opening

The opening in a wall into which a door, window, or other enclosure component is to be installed.


abbr. supply air


A flexible, polymer-based elastomeric material that is used in the assembly of the building enclosure to seal gaps, seams, or joints, making them waterproof or airtight (as part of the air or water control layers) or to provide a clean finish. Sealants are typically applied wet and may be tooled before curing to form a durable but flexible seal.

Variant: Caulking