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