Cellulose or fiberglass insulation?

Going for a Sunday scroll through Facebook, I came across an ad for an insulation company that installs loose-fill cellulose. I love cellulose, especially when it is properly, densely and deeply installed; but I know the cellulose-vs-fiberglass argument dates back to the Hatfield-McCoy feud (probably).

As one does, I went immediately to the comments about cellulose. Here are some:

“What an archiac insulation.”

“Try fiberglass blow insulation bro it would be much better for you and homeowners *you'll never go back to cellulose.”

“I wish this [naughty word] got banned and bring back asbestos or just spray foam seriously HATE CELLULOSE DIE DIE DIE.”

Well.

I never understood the hate some insulators have for cellulose. To be honest, I prefer it. Here’s why:

• It performs better at the truss heels than fiberglass. Most homes don't have a raised heel, so there's about 4 inches of space on a 2:12 pitch roof (common in the Phoenix Metro area, especially in 1950s-60s homes).

• Cellulose gives an extra R-4 at the heels, which is where the heat is most intense at the drywall.

• Less embodied energy.

• Far greater fire retardancy. (Fiberglass doesn't burn, but it does melt and exposes the framing to the fire.)

• Better sound absorption.

• Moisture absorption and release without losing R-value. This property leads to heat exchanges that can subtly improve a building's energy efficiency by buffering temperature and humidity swings. It's a well-documented phenomenon in building science, observed in cellulose and similar natural fiber insulations like hemp or wool.

• Better mold resistance (in most applications).

• Better air retardancy (aids in air sealing). I’ve blower-door tested thousands of homes that have been air sealed and insulated, and the ones insulated with cellulose always outperform those insulated with loose-fill or batt fiberglass.

• Zero itch, zero cough.

The bottom line is, whether you choose loose-fill fiberglass or cellulose to insulate your attic, ensure that it is done properly, in conjunction with proper air-sealing / air-barrier work. And the only way to ensure it is done properly is to have an experienced, qualified third party inspect and test it.

HEAT has performed thousands of home energy audits since 2009. We are independent, and we can verify whether you got what you paid for. Reach out to us!

This is insufficient yellow loose-fill fiberglass. I prefer cellulose, but no matter what insulation you choose, it needs to be installed properly, fully and deeply, and in conjunction with proper air-sealing. (Note the unsealed wire penetration and top plate. A home's insulation value is hampered by attic air and house air exchanging through holes like this.

Close-up of the penetrations. The insulation is discolored because air has been passing through it to and from the attic and house.

This is loose-fill pink fiberglass. Well, mostly pink. Note the discolored insulation around the exhaust duct and black pipe. That is a result of interior air moving into the attic, and vice versa. What good is a Yeti cooler if a dozen holes are drilled into the lid?

This brand-new home has cellulose -- my preferred choice for loose-fill insulation -- but it is comically insufficient. With most cellulose brands, R-38 would be 10.5 to 11 inches settled. This area near the truss has about 4 inches. Installation quality is key.

Large gap in the hallway wall's soffit cap, right next to the return duct.