A second aspect of spray foam is the enhanced thermal insulation characteristics. The stated R-value, or thermal resistance value, of insulation is measured under laboratory conditions. Real-life in-use R-values are quite different. An R-13 rated insulation batt installed improperly may only provide R-9. Whole wall R-values may be even less because of voids, wood, headers, etc. in the wall. Spray foam can provide a higher whole-wall R-value because of its ability to better fill wall cavities around electrical, plumbing, and other obstructions within the wall. The Oak Ridge National Lab has tested several whole-wall R-values for various wall/insulation combinations. Some of their results have been published in publications such as Energy Design Update, and should be available on their web site soon.
The R-value of an insulation system also depends upon the lack of air movement through the insulation. Most insulation products use entrapped air as a barrier to heat transfer. Therefore, to get a high R-value, air cannot move within or through the insulation.
In a whole-house situation, part of the energy use is in infiltration air. Air flow retarder products such as house wraps were developed to reduce the amount of infiltration air. These air barriers help reduce infiltration as well as air movement through the insulation.
Typical loose fill or batt insulation works well if installed correctly, and if installed in conjunction with an air barrier. Good installation is difficult to do, however. The insulation is often packed too tight or too loose, cut too short or too long, gapped around plumbing and wiring, or left out because of access problems.
Spray foams claim a couple benefits. First, they fill gaps and voids better. Second, they perform well as air flow retarders. The result is a higher in-the-wall R-value. Infiltration is also reduced, so that component of a building’s energy use is reduced. Both of these benefits result in raising the "effective" R-value of spray foam when compared to typically installed loose fill or batt insulation.
Spray foam products must still be sprayed correctly, and dense-pack blown cellulose can make some of the same claims. Spray foam is also self-supporting, which enables its use on the underside of roofs and floors.
