Fiber Glass

Fiber glass R-value changes with density.

Light, fluffy blown-in fiber glass may have an Rfactor of 2.2 per inch, or less; high density glass batts may have an R-factor of 4.0 per inch. The dishonest practice of "fluffing" blown-in fiber glass both reduces the R-factor of the material and increases the amount

of settlement that will occur. Depending on the density of the material, 12 inches of blown fiber glass may have an R-value anywhere

between R-26 and R-38. Under winter conditions the R-value of fiber glass is further reduced. The actual R-value of blown-in fiber glass in an extremely cold attic may be 30, 40, or even 50 percent lower than the purchaser thought he was buying. The more you need it the less insulating performance it delivers. The stated R-value of fiber glass batts is based on full thickness. Batts stuffed into wall cavities are often compressed to less than full thickness and lose R-value.