Cellulose vs. Fiberglass attic insulation: Which is better?

Cellulose wins

 Last Update: April 2010

Both cellulose and fiberglass insulation have their pros and cons. But on the balance, cellulose seems to be a better bet. It's not better in every single aspect, but it's better on more points than fiberglass. Here's why:

• Cellulose insulates better than fiberglass. (Boston Globe)
• It's cheaper than fiberglass.
• Cellulose insulation is typically made from mostly recycled materials.
• Houses that catch fire burn more slowly with cellulose installed than with fiberglass.
• Cellulose provides better sound insulation than fiberglass (i.e., homes are less noisy with it).
  
• Fiberglass is a suspected carcinogen and carries a hazard warning label. Cellulose is much more benign.
• 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)
• Fiberglass settles much more than cellulose, like a box of cereal. So over time you actually get fewer inches of fiberglass than you paid for.

Let's look at a couple of these of these in more detail.

Cellulose insulates better than fiberglass

A home insulated with cellulose used 21.3% energy for heating than one insulated with fiberglass, according to research conducted by the School of Architecture and Planning at the University of Colorado, Denver. (PDF)

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 mass 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 I haven't seen anyone challenge the results of the test, I think the test results are fairly trustworthy.

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)

 

References

• Wikipedia
• Boston Globe
• University of Colorado, Denver (PDF)
• University of Maryland
• National Research Council of Canada
• Community Environmental Center
• Better Insulation.com