Compressing Attic Insulation

Find your zone on the map and then use the chart to determine the level of insulation you need to properly insulate your attic walls floors and crawlspaces.

Compressing attic insulation.

Example let s look at an r 19 batt in a 2x6 cavity. To cut blanket style insulation place the product over a piece of plywood with its paper or foil side down. R value simply means resistance to heat flow. 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.

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. For every x the insulation material is compressed the r value decreases by approximately one half of that percentage or x compressed. That means compressing fiberglass batts is ok if the cavity is completely filled. When compressed to less than its labeled thickness glass fi ber insulation will experience a reduction in r value.

Achieving greater r values in attics the higher the r value the better the thermal performance of the insulation. The north american insulation manufacturers association naima says. Subtract the cavity depth from the thickness of the batt in inches. 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.

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. 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. These thermal defects not only undermine the r value of your insulation but they also make rooms uncomfortable and increase your utility bill. Lay a two by four on top temporarily compressing the insulation to a manageable.

Compressing fiberglass provides more r value per inch but reduces the overall r value. 6 25 batt 5 5 cavity depth 0 75.