Will Packing Down Fiberglass Insulation Impact Its Effectiveness?

Home Construction

• Packing R-19 insulation designed for a cavity depth of 6 1/4 inches into a 3 1/2-inch space created by typical 2-by-4-inch construction decreases the insulation’s effectiveness. While a higher R-value provides a higher insulating effectiveness, ensure the chosen insulation is specifically designed for the cavity in which you desire to put it. For homes built with 2-by-4 framing, the insulation needed for the cavity is R-13 or R-15 in a width between studs, such as 15 1/4 inches for studs framed 16 inches on center or 23 1/4 inches for homes framed 24 inches on center.

Choosing Insulation

• Glass provides little to no R-value and, because fiberglass insulation is made from glass, its effectiveness comes from the trapped air during the manufacturing process. Contrary to belief, more insulation isn’t necessarily better, especially when dense insulation packs over light insulation, such as in an attic, and reduces its ability to provide thermal resistance. Determining what insulation is best requires taking several things into consideration: insulation location accessibility, how much is needed, the space available and how much it will cost.

Compressed Insulation

• Calculating the R-value of insulation depends on its thickness and thread density. For instance, in an attic requiring an insulation value of R-60, two R-30 batts are placed one atop the other without packing to create the R-60 value. Compressed insulation doesn’t provide the rated R-value because the air gets pushed out of the insulation. When you squeeze insulation into a space, you reduce its thermal resistance, sometimes removing it altogether, if you squeeze it to the point where there is little trapped air.

Thermal Bridging

• Insulation placed between studs does not provide a thermal resistance for the studs themselves. The same is true for insulating between ceiling rafters and floor joists. The actual R-value for the wall or ceiling incorporates the R-value of the wood and the R-value of the insulation together. The heat flow through these structures is called thermal bridging, resulting in the overall R-value of the wall being different from that of the insulation. Adding a sheathing on the exterior wall that also has insulative properties increases the overall R-value of the wall.