Cellulose vs. Fiberglass attic insulation: Which is better?

Cellulose wins

Last Update: February, 2015

Both cellulose and fiberglass insulation have their pros and cons, but I feel that cellulose is the better bet overall. Here's why:

Cellulose insulates better than fiberglass, especially in very cold weather. (Green Building Advisor, Boston Globe, BuildItSolar)
It slows air flow better than fiberglass. (Green Building Advisor)
It's cheaper than fiberglass.
It's made from mostly recycled materials.
A test house burned more slowly with cellulose installed than with fiberglass. (more below)
Cellulose provides better sound insulation than fiberglass (i.e., homes are less noisy with it).
Fiberglass is a suspected carcinogen. (sources)
Many fiberglass installers "fluff" the fiber with extra air, so you don't get as much insulation as you're supposed to. This kind of trickery can't easily be done with cellulose, and certainly not to the extent that it can with fiberglass. (BetterInsulation.com)

Let's look at #5 in more detail.

Houses with cellulose installed burn slower than those with fiberglass

The Maryland Fire and Rescue Institute (of the University of Maryland) built three small houses and then burned them to the ground to see how insulation choice affected burn time. One house had no insulation, another had fiberglass, and the third had cellulose. The results? The house with no insulation and fiberglass insulation collapsed about the same time, after 40 and 42.5 minutes respectively. The house with cellulose insulation lasted another 24.5 minutes longer than the building with fiberglass.

How could this be, when cellulose is made from old newspaper and fiberglass is made from, well, glass? The first thing is that cellulose insulation is treated with fire retardants like borate which make it harder to burn. But still...how does it outperform fiberglass? The answer is probably that when you try to burn cellulose, it chars black but the material is still there. When you burn fiberglass, it melts away, allowing the fire to more easily spread. There's a bit in the video below where someone takes a blowtorch to some cellulose insulation he's holding in his hand, and the cellulose won't ignite, it just turns black.

Similar research by the National Research Council of Canada showed that cellulose insulation increases a building's fire resistance by 22 to 55% compared to traditonal fiberglass. (source)

The video below which shows the house-burning test results was apparently produced by a cellulose company, so we could assume some bias, but since the test itself was conducted by the University of Maryland, and since the fiberglass industry hasn't responded with their own test showing different results, I think the results of this test are fairly trustworthy.

However, cellulose doesn't have superior fire performance in every case. Federal code does say that cellulose is a fire hazard if it's installed too close to the sides or on top of recessed light fixtures, or too close to exhaust flues for things like furnaces and gas water heaters. (CFR) And electrical fires are probably more likely in older homes with substandard wiring when cellulose is installed. (Firehouse magazine)

Other

Since cellulose is so common and so important, its production is regulated by the federal government. That is, there are minimum standards for quality. It's a crime to sell cellulose that doesn't meet this standard. (source)

The fiberglass industry association (NAIMA) snail-mailed me a massive letter taking issue with this page. I found most of it to be B.S., but I made a few changes to the article, mostly minor. Green Building Advisor got a similar letter.

Sources re: carcinogenity and health effects of fiberglass

"Studies have shown inhaling these fibers can reduce lung function and cause inflammation in animals and humans....Fiberglass emits a synthetic material called styrene, which is a possible carcinogenic according to the IACR and U.S. Environmental Protection Agency." (Am. Lung Assoc.)
"Certain glass wool fibers (inhalable) are reasonably anticipated to be human carcinogens...." (Natl. Institutes of Health, 2014)