Attic insulation question - fiberglass or cellulose?

[Louis Brandt]

Hello,
I’m sure that a lot of you woodworkers are experts in construction and construction materials, so I’ll ask you this.

My wife and I have a 35 year old, 2200 square foot, ranch style, single level house, with a hip roof, with a 6-12 pitch. We live in the south. We currently have about 4 to 5 inches of blown-in rock wool (I know, they no longer use that type) insulation in our attic. We’d like to add another 12 inches of blown-in insulation. We don’t want to use batts, because no one in our area installs batts, and we don’t want to do it ourselves.

Our question concerns the comparison of blown-in fiberglass versus blown-in cellulose. We have read that cellulose has a lot of merit, and according to some things that we’ve read, is superior to fiberglass, but since we don’t know anyone who’s used cellulose, we have no way to compare it. The insulation contractors that we’ve spoken to in our area indicate that they can blow either cellulose or fiberglass, depending upon our choice.

Can any of you help us with this decision? We’d appreciate any input from those of you who have experience with either cellulose or fiberglass or both.
Thanks,
Louis

[David G Baker]

I have never used blown in fiberglass but I did put 14 inches of blown in cellulose in the attic of my pole barn and have been very satisfied with it. I keep the building at around 48 degrees in the cold months and when I work in the building I turn the thermostat up to 65 degrees. It takes about 20 minutes to come up to temperature and the building stays at temp for quite a while. The heater only comes on a couple of times an hour. The building is 30'x40'x10', has 1 inch double foil covered rigid foam board on the walls.
I had a house in California that had rock wool in the attic and the only problem I had with it was when I re did the ceiling of my kitchen. I had to rake the wool out of the joists or it would have filled the kitchen.
I personally like fiberglass batts, I would roll them across the joists above the rock wool. I am surprised that there isn't anyone in your area that uses fiberglass batts.

[Lee Schierer]

I've never cared for the blown cellulose insulation. There were a lot of fire retardent problems early on and it is a pain to work around when remodeling. Plus it settles over time reducing the insulation value. I installed batts and unfaced rolls of fiberglass 90 degrees to the existing insulation in my two attics. It made a major difference on heat retention and coolness of the house in the summer. It goes pretty fast.

If you go the fiberglass route make sure the added insulation is unfaced as you don't want a second vapor barrier. Also make sure the installer leaves air gaps in the rafters to let air flow between the underside of the roof and the top of th insulation. This is critacl to shinlge and roofing life and will significantly reduce the chances of ice dams forming.

[Jim Becker]

I'm more inclined with fiberglass for blown-in. In fact, that's what I recently had put in the attic of the original portion of our home; both the 1950s structure and the 1750s (!) structure. (The addition got all closed cell spray foam) When I worked in the business for a couple years, the owner wasn't very glowing on cellulose, either. But that was a long time ago and, perhaps, the product has improved, etc.

[Von Bickley]

In our area, the only blown-in insulation that I have seen has been fiberglass...

[Larry Edgerton]

I use celulose in all of the homes I build. It is a recycled material which is a plus for the planet, and is less of a health hazzard for any of my guys that have to work around it. It has a slightly higher R value than glass.

You may not have that option being as you are in the South, but find a contractor if you can that has a machine that has some manner of mixing binder in the insulation. There are some celulose brands that have binder premixed, and just require injection of water, and some that mix the water and the glue, but either way do not accept some guy with a hardware store machine in the back of his truck. A real machine costs many times what the hardware machine costs, and always include a grinder that grinds the chunks up before it hits the blower. This lofts the materal so you use less, and the insulating value is more. The binder [glue] makes sure it stays tha same R value, be it glass or celulose. Batts are in no way comparable to blown in done right.

Find someone that does the wet celulose in the walls, they will have the expensive machine with the grinder and binder mixer, but they can still blow an attic, they just cut down on the moisture.

[Kev Godwin]

I have done many thermal analysis tests on several buiildings (residential and commercial) with different insulation materials used in traditional attics. Both fiberglass and re-cycled cellulose can perform well depending on the application. I have had a few cases where enough air movement thru the attic has mixed extensively with fiberglass insulation thus reducing the thermal performance. Cellulose tends to perform better in this particular situation since it does tend to settle into a matted layer with retained air content. (Much less mixing with the air moving thru the attic.)

Attics that have lesser air flow can have good success with either system. Exterior walls with sheathing on the interior and exterior are another topic entirely.

[Brian Elfert]

The builder used blown-in fiberglass in the attic of my hosue in 2001. He said fiberglass settles less than cellulose.

[Ben Franz]

IMHO fiberglass holds up better over the long haul - there are "itch-free" materials available that make it less unpleasant if you ever end up in the attic. I did one remodel of a house that was 35-40 years old and we discovered that the cellulose fire retardant had lost it's effectiveness. I was cutting a nail with a recip saw at an end of the roof that was open to the exterior because of the addition. There was a fairly strong wind and a spark from the nail started a smoldering fire in the cellulose material. I had to wet it down with a garden hose to get it to stop - scary. The homeowner was a retired engineer and decided to have all the existing cellulose removed and replaced with fiberglass. Maybe it's kind of "belt and suspenders" but I won't ever use that stuff.