Cellulose or fiberglass blown in insulation?

Short answer: Both work. Settled cellulose gives about R-3.2 to R-3.8 per inch and settles 10 to 20 percent; loose fill fiberglass gives about R-2.2 to R-2.7 per inch and settles 2 to 4 percent [1]. Cellulose slows air better and needs fewer inches; fiberglass weighs less and settles less. We install both.

U.S. Department of Energy, Building America Solution Center [1].

Cellulose

Recycled newsprint, finely shredded and chemically treated to resist fire, corrosion, fungal growth, and vermin [2]. NIST lists ammonium sulfate, borates and boric acid as the common treatments [3]. The DOE says it does a good job of slowing airflow, even at loose-fill densities and is the favored loose fill in DOE-funded weatherization [1]. The catch: it has to stay dry. The DOE warns that wetting can cause cellulose to absorb large quantities of water [2], and it must be kept away from chimneys and metal flues.

Fiberglass

Lighter, settles less, and does not soak up water the way cellulose can. It takes more inches to reach the same R-value. CertainTeed blown in insulation is what went into Dan Inman's home in Athens.

Which we pick

For walls, dense packed cellulose is our usual call, since it fills around wires and slows air. For attics it depends on the attic: what is there now, how much room there is, the ceiling, and the roof. We tell you which one we would use and why. Attic insulation.

More questions

Is cellulose a fire hazard?

Cellulose is chemically treated to resist fire, per the DOE, and NIST lists borates, boric acid and ammonium sulfate as the usual treatments. It still has to be kept away from chimneys and metal flues.