I’ve been guilty of perpetuating a myth. Not long ago I wrote an article in which I said installing insulation, “cavities [should be] filled completely with as little compression as possible.” But is compression really such a bad thing? Here on GBA, commenter Dana Dorsett wrote, “Compression of batts is fine (resulting in a higher R/inch due to the higher density) as long as the cavity is completely filled.”

He’s right. Compression isn’t the problem. Incompletely filled cavities are a problem. Gaps are a problem. But you can compress fiberglass insulation quite a bit and it still works just fine. The North American Insulation Manufacturers Association (NAIMA) has a little two-page document about compressing fiberglass insulation. Here’s what NAIMA says: “When you compress fiber glass batt insulation, the R-value per inch goes up, but the overall R-value goes down because you have less inches or thickness of insulation.”

The document includes a general chart for how to tell what your R-value is with different levels of compression. Owens Corning also has a compression chart for R-value (see Image #2, below).

So, you don’t get the full R-value on the label, but the insulation still works perfectly well if all you’ve done is compress it. Of course there are limits. If you use a hydraulic press to compress it so much that it approaches the density of solid glass, things change. We’re talking about reasonable amounts of compression.

Here’s something you may not know. The standard R-19 fiberglass batt is 6.25 inches thick. If you put that batt in a closed 2×6 wall, it will be compressed 0.75 inch because a 2×6 is 5.5 inches deep. That means the batt labeled R-19 really gives you R-18 in a closed cavity.

One place where you’re pretty much always going to end up with compression is around windows. If you use backer rod in the gap around a window and then fill the remaining space with chinked fiberglass, “it’s damned near impossible to compress the fiberglass ‘too much,’ without using a hammer!” That’s what Dana Dorsett wrote in his GBA comment to me.

Another is behind electrical junction boxes. If you install fiberglass correctly, you need to cut notches in the insulation where it goes around junction boxes. You can then take that little rectangular piece of insulation and put it in the space between the junction box and the exterior sheathing. You don’t need to worry about removing some of the insulation so you can do it without compression. Just put the whole piece back there and let it be compressed.

So, compress if you need to. Just make sure the space is completely filled. That’s the real measure of a good installation.

Allison Bailes of Decatur, Georgia, is a speaker, writer, building science consultant, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.