Attics and ceilings
The most common blanket products for attics and flat ceilings are R-30 and R-38 batts. As mentioned earlier, high-performance batts
Figure 8.7 Installing batt insulation. (CertainTeed)
are also available as R-30C or R-38C. Because the high-performance batts achieve the recommended R-values in less space, they allow room for ventilation between the insulation and the roof deck without the need for roof baffles or the installation of larger roof joists (Fig. 8.8).
In order to achieve R-values of 38 and higher, two layers can be used and their R-values combined. For example, an R-19 batt added to an R-30 will yield an R-49. When installing a second layer, always use unfaced insulation, because using a second vapor retarder will trap moisture between the two layers. It is also recommended that the second layer be applied across the joists in open attic spaces (Fig. 8.9).
A combination of blankets and blown-in insulation also can be used. A batt or roll can be installed during the initial construction process, and a layer of blown insulation can be added at a later time.