Insulation and Ventilation Terms

A quick overview on the common terms you'll here in this industry

Insulation and Ventilation Terms

Humidity, relative

the amount of water vapor that is actually in the air compared to the amount of water vapor that could exist in the air at a given air temperature, expressed as a percentage. More moisture can exit in warm air than cold air, so raising air temperature while keeping the water vapor constant reduces relative humidity

Vapor Drive?

a condition that occurs when heat vapor pressure cause increase vapor diffusion. One example is when water vapor diffuses through permeable building material from heated interior ares in the winter. Another example is when solar radiation heat wet bricks forcing water vapor though permeable building materials in the summer. 

Dew Point (temperature)

the temperuatre at which water vaopr in the air may condense into liquid water. A higher dew point temperature indicates that the air contains more water vapor


the ability or proper of material to resist or allow the diffusion of water vapr through said material. Permeability is expresed as a number greater than zero.


the ability or property of a material to slow heat transfer. A high R-value number equals a higher ability to slow heat transfer. R-value is expressed as a number greater than zero and is used to compare the insulation value of materials

U-value (factor)

the ability or property of a material to allow heat transfer. it is the inverse of R-value and is expressed as a number greater than zero. The larger the number, the higher the rate of heat transfer and a lower R-value


openings in the wall and roof of a house such as windows, doors and sky-lights

Vapor Barrier

a technically questionable term often used when the term vapor retarder is intended

Vapor Retardere

 material that restricts the air flow of a water vapor. 

Vapor Diffusion

the process by which water vapor passes through a permeable material from an area of grater vapor presure to an area of lower vapor pressure

Ventalation (building)

the process of supplying outside air to a house or removing inside air from a house by natural or mechanical means.  It can be random and uncontrolled (air leaks) or designed and controlled (outside air ducts)