Fiberglass Insulation Myths

I love a good myth as much as anyone, and there are some doozies out there in the world of fiberglass. Here's my number one all-time favorite:

Myth: Fiberglass-insulated homes are too leaky, resulting in high energy bills and comfort problems.

Whoa, there! Yes, fiberglass is an air-permeable insulation and many houses where it's been installed are definitely too leaky. But this is like blaming your stale, moldy candy bar on the ingredients rather than the fact that the one you bought came wrapped in tissue paper. A properly designed and installed building enclosure needs to have well thought-out control layers for heat, air, and moisture. Fiberglass is a perfectly fine control layer for heat, but it's not going to stop air movement. That's the job of the air barrier. You need both.

Myth: Fiberglass insulation causes cancer.

In the late 1980s, the National Toxicology Program and the state of California declared fiberglass insulation a possible carcinogen. That was based on early studies that later were shown not to be accurate. In 2011, both removed fiberglass insulation from their lists. Read more about it in this National Insulation Association article. Oh, by the way, fiberglass is second only to cork as a healthy insulation material. See the table in Lloyd Alter's article on the HIRL survey to see how it ranks.

Myth: Blown fiberglass attic insulation loses half its R-value because of convective loops.

I heard this one for many years before I finally wrote about it. Yes, there was a study from the early 1990s that showed blown fiberglass in attics could lose 50% of its R-value when the attic temperature dropped. The problem was the way the insulation was made, and the fiberglass industry fixed the problem. See this article for more details.

Myth: Compression in fiberglass insulation is a bad thing.

This is another persistent one, and I have to admit that I fell for it for a while, too. The truth is that compressed insulation does have a lower R-value than that same insulation expanded to its full thickness. Here's what I wrote in my article on this topic in 2017:

Compression isn't the problem. Incompletely filled cavities are a problem. Gaps are a problem. But you can compress fiberglass insulation as much as you want. The North American Insulation Manufacturers Association (NAIMA) has a little two-page document about compressing fiberglass insulation (pdf). Here's what they say:

"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."

As long as you're filling the cavity and getting the R-value you want, compression doesn't matter.