We’ve all seen tables that list the R-value per inch for a variety of insulation types. These tables are handy, of course, but because old information is endlessly republished, in some cases long after the old information becomes obsolete, the tables are often flawed. Most authors make little effort to update these tables in light of the latest information from researchers.
Many authors, including me, have reported that polyisocyanurate has an R-value of R-7 per inch. That information is both obsolete and misleading — a lower value is more accurate — yet it still shows up in many online tables and articles.
In some cases, older R-value tables have values that are misleadingly low. For example, in the old days, most fiberglass batts had relatively low R-values — in the range of R-3.2 to R-3.4 per inch. But insulation manufacturers now offer high-density fiberglass batts with R-values of R-3.7 per inch or more.
The information in the table below reflects recent manufacturing changes and researchers’ findings, making it more up-to-date than information in some online tables. Moreover, the table includes information that is often lacking from R-value tables, including information on cold-weather performance and thermal drift.
A few general principles and hints
Where can I find brand-specific R-value information? In the U.S., insulation manufacturers are required by law to report the R-values of their products to builders and homeowners. Acceptable laboratory test methods for determining R-value, and requirements for R-value labeling, are specified by a federal law (16 CFR 460) called the R-value Rule. Because of this law, most major insulation manufacturers publish trustworthy R-value information.
If you want to know the R-value of a specific product, the information is usually available with a simple web search. (For example, this useful document reports the R-values of all of…