Moisture and Thermal Resistance

U-value

A measure of thermal transmittance through a material. (latent heat). Gain or lose heat. You almost always want a low U value to keep heat on correct side of envelope.

Vapor Retarder (all climates) or Barrier (cold climates)

A membrane or layer of material intended to restrict the transmission of water vapor through a building assembly. It is placed on the warm side of the insulation (on the outside in hot-humid climates; on the inside in cold climates) behind the finished material. It inhibits the water vapor from condensing. can be made of aluminum foil or polyethylene film.

vapor retarder types

Class I, Class II, Class III

Class I Vapor Retarder

Vapor impermeable or vapor barrier. glass, sheet metal, polyethylene, rubber membranes

Class II vapor retarder

Vapor semi-impermeable. expanded polystyrene, heavy asphalt coated paper, plywood, bitumen kraft paper

Class III Vapor Retarder

Not classified as vapor retarder under IBC. Gyp, fiberglass insulation, board lumber, concrete, brick, house wrap.

U-value

The U-value is the amount of heat that can be lost from a building. The lower the U-value in this the better heat efficiency there is.

SHGC (Solar Heat Gain Coefficient)

The fraction of solar radiation (sun's rays) admitted through a particular glazing. Lower is better for hot climates, higher for cold.

R-value

The numerical value used to indicate the resistance to the flow of heat. Higher the better

K value

The thermal conductivity of a material.