Compressing Attic Insulation

Subtract the cavity depth from the thickness of the batt in inches.

Compressing attic insulation.

The north american insulation manufacturers association naima says. To cut blanket style insulation place the product over a piece of plywood with its paper or foil side down. We can compress insulation and increase the performance of it right up to the point where the main mode of energy transfer is through the material of the insulation itself. The chart below indicates the compressed r value of standard building insulation products when installed in framing cavities with a depth that is less than the labeled thickness of the insulation.

Example let s look at an r 19 batt in a 2x6 cavity. When compressed to less than its labeled thickness glass fi ber insulation will experience a reduction in r value. Always cover the tops of the ceiling joists to make sure the insulation is deep enough to reach your target r value and to prevent thermal bridging the heat loss that occurs through the wood framing. Simple thermal analysis reveals that if you have 5 missing insulation in an attic bare sheetrock you ll have a slightly more than a 54 drop in r value.

Lay a two by four on top temporarily compressing the insulation to a manageable. When you compress fiber glass batt insulation the r value per inch goes up but the overall r value. If you know you aren t going to convert the attic in the future that leads you one direction compressing the insulation is fine maybe you don t care so much about head height and can add some more thickness to improve the insulation etc and if you know that an attic conversion is viable and that you are likely to want to pursue that in the future you can plan accordingly. R value simply means resistance to heat flow.

Compressing fiberglass provides more r value per inch but reduces the overall r value. For wool types of insulation fiberglass mineral wool steel real wool etc while it varies based on material this is typically much denser than we are able to get it via compression on the job site. For every x the insulation material is compressed the r value decreases by approximately one half of that percentage or x compressed. Achieving greater r values in attics the higher the r value the better the thermal performance of the insulation.

These thermal defects not only undermine the r value of your insulation but they also make rooms uncomfortable and increase your utility bill. That means compressing fiberglass batts is ok if the cavity is completely filled.