IRC Insulation Thickness & R-Value Requirements
- Rigid foam insulation.Jupiterimages/Photos.com/Getty Images
The International Residential Code sets insulation requirements for new homes and additions to existing homes. These requirements are in the form of R-values; R-value is a measure of thermal resistance. Different insulating materials have different R-values. The higher the R value, the greater the insulation. The IRC does not specify insulation thickness or type of insulation. R-values specified in the code are minimums; builders are free to exceed these values. - The IRC specifies insulation requirements according to climate zone. There are 17 climate zones within the United States. Climate zones are numbered 1 through 17, with zone 1 being the warmest, and zone 17 the coldest. Higher R-values are required in colder zones. The IRC lists climates zones for all counties within the United States.
- The IRC specifies different R-values for different areas of the building envelope. Ceilings require the greatest amount of insulation, while basement walls need the least. For example, in zone 13, ceilings require R-38, walls R-18, floors R-21, and basement walls R-10.
- When a crawlspace is vented, and the floor above is insulated, the crawlspace walls need not be insulated. Conditioned crawlspaces are heated or cooled, and must have their walls insulated according to code. Additionally, exposed earth at crawlspace floors must be covered with a continuous vapor retarder having a maximum permeance rating of 1.0 perm.
