Builders often talk about the R-value of their walls. But if a builder claims to have an R-20 wall, what does that mean?
Building codes commonly include a table listing the minimum prescriptive R-values for walls and ceilings in different climate zones. For example, Table R402.1.1 in the 2012 International Energy Conservation Code (IECC) informs builders that the minimum prescriptive R-value for walls in Climate Zones 3, 4, and 5 is “20 or 13+5.”
This type of table raises many questions. For example, if a builder chooses to comply with the R-20 option, how is R-20 calculated? The code provides some guidance on the issue, but not much. According to a footnote at the bottom of the table, the “first value is cavity insulation, second is continuous insulation or insulated siding, so ‘13+5’ means R-13 cavity insulation plus R-5 continuous insulation or insulated siding.”
Complying with the code usually means installing insulation between the studs
The code language governing the prescriptive R-value requirements has changed in recent years. If you want to know the specific language that is enforced in your jurisdiction, you’ll have to consult your local code book. For example, the prescriptive requirements in the 2009 International Residential Code (IRC) note (in section N1102.1.1), “Computed R-values shall not include an R-value for other building materials or air films.”
A footnote to Table N1102.1 in the 2009 IRC — equivalent in most respects to the prescriptive table (Table R402.1.1) in the 2012 IECC — notes, “R-19 batts compressed in to nominal 2Ã—6 framing cavity such that the R-value is reduced by R-1 or more shall be marked with the compressed batt R-value in addition to the full thickness R-value.” This footnote is confusing. Who should do the marking? Does this instruction mean that the builder has to mark the insulation in every stud bay? How do…