Is Compressed Fiberglass Insulation Really a Problem?

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Is Compressed Fiberglass Insulation Really a Problem?

Or is this just another myth in the world of building science?

Posted on Jun 7 2017 by Allison A. Bailes III, PhD

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, 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 insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. , the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. 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 2x6 wall, it will be compressed 0.75 inch because a 2x6 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 sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. . 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.


Image Credits:

  1. Energy Vanguard
  2. Owens Corning

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