GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Which is the best blown insulation, Fiberglass or cellulose?

GBA Editor | Posted in Energy Efficiency and Durability on

I live in Phoenix Arizona and am going to add several inches of insulation to my attic and would like to know which insulation is best to use.

I had been told fiberglass was best by on installer, but after reading some of the posts here it appears the cellulose is the better.

Some suggested would be appreciated.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Riversong | | #1

    Loose-fill fiberglass has very low R-value (as little as R-2.4/inch), is highly permeable to air movement which can all but eliminate it's minimal thermal value as well as allowing air-borne moisture to exfiltrate and condense. Fiberglass is a known carcinogen and some of it contains formaldehyde, a primary sensitizer to other chemicals. And mice and other rodents love to nest in it.

    Cellulose is mostly recycled newsprint, with non-toxic-to-humans boric acid as a fire retardant, making it one of the most fire-resistant of insulations. It has very low ecological footprint, embodied energy and global warming contribution - in fact, less than any insulation except straw bales. The boric acid is a fungicide, so it inhibits mold growth, and is an irritant to rodents, discouraging them from nesting. It also kills most common household insects, including roaches and termites.

    Additionally, cellulose has good R-value (3.6 or better per inch), inhibits air movement, absorbs excess moisture for safe storage and release, actually protecting wood framing from moisture. Objectively, it's the best commonly-available insulation on the market.

  2. Christopher Briley | | #2

    I concur with Robert.

    I’ve never understood why blown fiberglass exists. It’s always seemed like the fiberglass industry felt threatened by cellulose so they invented this product to say “we can do that too.”

    Also, breathing cellulose fiber, is not good for you. Breathing fiberglass fiber, is extremely bad for you. I don’t understand why they take a blanket product and turn it into a blown product that performs worse and is more hazardous to install.

    I vote cellulose.

  3. Armando Cobo | | #3

    Here you can see the R-values for insulation: I imagine your house has a low pitch roof and vented roof, so finding out what type of roof vents you have is crucial. If you have soffit vents, you need to make sure you do not cover those vents with insulation and create attic ventilation problems. Many homes in AZ have no soffits and ventilation is higher up on the roof line, but still, make sure any kind of vents you have they remain clear.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |