Insulation is often chosen mostly on the basis of how well it stops the flow of heat—its R-value. Insulation with a high R-value—such as spray polyurethane foam or rigid polyisocyanurate—is at least twice as effective as, say, loose-fill fiberglass. Wall or roof assemblies with higher R-values typically cost more but translate into lower heating and cooling bills.
In reality, choosing insulation is rarely as simple as comparing R-values, and relying on R-values alone can be misleading and short-sighted. To be sure, the thermal properties of insulation are important, but builders and homeowners have many variables to consider:
- How much does it cost?
- How difficult is it to install?
- How available is it?
- How safe is it?
- Is it an air or moisture barrier?
- What are the environmental consequences of choosing a particular kind of insulation?
A good place to start is the International Residential Code (IRC), which spells out the minimum amount of insulation for basements, walls, and roofs. This information is summarized in a table (R401.1.2) that breaks down minimum R-values by application (where it will be used) and location (the climate zone where the house will be built). Take, for example, wood-framed walls. In climate zone 1, which encompasses the warmest areas of the continental U.S., R-13 insulation meets code minimums for exterior walls. If you live in climate zone 6 or 7, you will need insulation between the wall studs plus a layer of continuous insulation to control thermal bridging. The code offers two possible ways of doing that: R-20 insulation in the wall cavities and a continuous layer of R-5, or R-13 in the wall cavities plus R-10 of continuous insulation.
These values are code minimums for new construction, but many builders are aiming for better performance than that. Insulation for a “Pretty Good House,”…