The R-value of fiberglass insulation is a measure of the insulation's resistance to conductive heat flow. The higher the R-value, the more effective the insulation. R-values are affected by insulation's thickness and density.
Each type of material builder use in a building's walls, ceiling and floor has its own R-value. To calculate the total R-value for a wall of mixed materials, builders add the R-values for each layer of material. Compressing insulation to fit a space makes it less efficient. Joists and studs reduce insulation efficiency because they provide thermal bridges for heat to transfer between the interior and exterior of a building.
Fiberglass insulation comes in batts and as loose insulation. Installers roll out batts to fit the space to be insulated or use a blower to distribute the loose-fill type. The trademarked Blow-in-Blanket System does a better job of filling in space with insulation than other methods of installing fiberglass insulation, according to the Department of Energy.
Builders sometimes use rock wool and slag wool as alternatives to fiberglass. These materials contain an average of 75 percent of recycled content and do not need extra chemicals to make them fire-resistant. Manufacturers also use plastic fibers, cellulose, natural fibers, polystyrene and polyurethane to make insulation.