Blown in insulation: cellulose v fiberglass

booksatJune 16, 2006

I'm going to have insulation blown into the walls and attic of our 100+ year old house. I've had a few estimates, some using blown in cellulose and some using blown in fiberglass.

What is the difference(s) in using cellulose and fiberglass? Has anyone had either blown into their "old" house?

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I've used both and didn't see a big difference in terms of the installation. You need two people, one to run the hopper and one to spray the insulation. You need some physical dexterity to walk around in an attic but spraying the actual insulation isn't much more complicated than using a garden hose. Summer is a lousy time to work in an attic. It gets hot, very hot, like 120°+ hot. Do it early in the morning or wait for cooler weather.

Cellulose is just ground up newspaper with chemicals added to make it vermin and fire proof. The fiberglass industry will tell you the chemicals are water soluble so if the insulation gets wet, the protection may be lost. I'm inclined to think that's a bit of hype.

Fiberglass is, well, fiberglass. It's a bit more expensive than cellulose.

Cellulose also settles but you just add more to take that into account. The instructions will tell you how much to use.

Walls involves drilling a couple of holes into each stud bay. "Ask This Old House" did a segment on blown in cellulose that gave a good overview of walls. You might want to watch for it.

Respiratory protection is a must.

The bales of insulation aren't so much heavy as bulky. You'll probably want to have them delivered. The blowing machine is bulky and heavy too. You'll need a truck if you want to pick-up the machine.

Good luck!


    Bookmark   June 17, 2006 at 7:47AM
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The biggest difference between cellulose and fiberglass is how they react to water vapor.
Fiberglass does not absorb water vapor. If the freeze point is within the insulation, any water vapor will form frost crystals on the fiberglass strands. When the temperature then moves above 32F the frost normally melts wetting the insulation. This tends to mat it down and can cause damage if enough free water is produced.
Cellulose is hydrophilic. It can absorb water vapor, hold it, and release it again as vapor. If it gets enough water vapor it may saturate and form frost crystals, just like fiberglass. The ability to moderate water vapor by absorbing and releasing reduces the chance of free water forming at the freeze point in the insulation layer. It does not eliminate it, but fiberglass cannot absorb any water vapor without forming frost.
The most tightly insulation houses I have seen use wet applied cellulose. Not an option in an existing structure without major destruction (all the walls out).
Next is blown cellulose. When correctly 'overfilled' the settling is nearly non-existent (I have opened walls filled even 15 years ago that remained full).

    Bookmark   June 17, 2006 at 12:41PM
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Make sure there is no operating knobe & tube wiring in the walls. It cannot be buried in insulation.

    Bookmark   June 17, 2006 at 12:42PM
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The most important thing, no matter what type of insulation you choose for your attic, is to spend time sealing air between the main body of the house and the attic. Every leak you seal means that much greater efficiency.

    Bookmark   June 18, 2006 at 1:53AM
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