Compressing Attic Insulation

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.

Compressing attic insulation.

To cut blanket style insulation place the product over a piece of plywood with its paper or foil side down. Work from the perimeter of the attic toward the door or hatch so that you don t trample all over the insulation you just put in. 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. When you compress fiber glass batt insulation the r value per inch goes up but the overall r value.

R value simply means resistance to heat flow. 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. Compressing fiberglass provides more r value per inch but reduces the overall r value. When compressed to less than its labeled thickness glass fi ber insulation will experience a reduction in r value.

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. 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.

Achieving greater r values in attics the higher the r value the better the thermal performance of the insulation. That means compressing fiberglass batts is ok if the cavity is completely filled. 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. 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.

For every x the insulation material is compressed the r value decreases by approximately one half of that percentage or x compressed. The north american insulation manufacturers association naima says.