The R value of spray foam depends on the type. Some closed-cell foams ("2-lb foam") may start at 7 per inch but after some offgassing over time ends up at around 6.5/inch. Water-blown closed cell foams typically start and end at 6/inch. There are a number of open-cell ("half-lb") foam products around (eg. Icynene), for which the R value typically is 3.6-3.8/inch. Blown-in cellulose has a nominal R value of about 3.6 or so. A shortcoming of all insulation materials blown or injected into an existing wall is that it's easy to miss parts of the cavities that are blocked off by framing members.
For all cavity insulations, the center-of-cavity total R value is only part of the story. The R value of framing lumber is only R 1 to 1.2 per inch, and the exterior shell of a framed house typically is 20-25% solid wood! The "thermal bridging" of all that wood reduces the "whole wall" R value considerably from what the center-of-cavity R value is. One technique for mitigating the thermal bridging of the framing lumber is to apply a couple of inches of rigid foam to the exterior, but that involves removal of siding, removal and reinstallation of windows to bring them out to the new outer plane, and reinstallation of siding.
|