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Blown in cellulose....replace existing fiberglass or blow right over?

Blown in cellulose....replace existing fiberglass or blow right over?


Old 02-06-12, 07:33 AM
Thread Starter
Join Date: Dec 2004
Location: USA
Posts: 267
Blown in cellulose....replace existing fiberglass or blow right over?

Ive been thinking about adding some R-value to my attic, in the form of blown in. After years of running ceiling boxes, network cables, etc, some of the existing fiberglass has been pretty torn up, moved, etc. Most of it is fine, but some Im sure is causing air infiltration both through the disturbed insulation and new electrical boxes.

I was also able to find a few spots that were missed with fiberglass (the 2nd floor of my house was added on in the early 90s, but the way they built it was a little odd shaped so there are some hard to get to spots in the attic, which leads to those weird spots missing insulation)

Since Ive always thought one of the biggest advantages of blown in insulation was to aim it right where you need it, and fill small penetrations, I was figuring it would work best if I removed the fiberglass completely then blew cellulose in its place filling gaps, etc. Larger gaps I would fill myself with expanding foam, ridgid foamboard, etc. before adding blown in on top.

Ive gotten one quote so far and they said removing the fiberglass is a waste and the weight of the blown in will take care of filling anything. Does that sound right? It didnt to me, I figured you wanted the blown in as the first layer and *maybe* fiberglass over it (if I DIY-ed this I would opt for 18" of blown in if I could get it, located in southern CT)

Thanks in advance.

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Old 02-06-12, 04:57 PM
Join Date: Sep 2011
Location: canada
Posts: 56
Hi Chris, you are right to remove the existing fiberglass but not because you can't blow in over top. You should take the time and air seal all penetrations before you insulate. This will make more difference than just adding insulation and will keep your attic dry and therefore mold free. You can re install the glass if you want before the blown in. The cellulose should remain fluffy and will be installed to a depth that will settle normally to the R value you are shooting for. If you put the glass over top, you will compress it and lose some insulation value.

Fiberglass and cellulose are not air / vapour barriers but closed cell foam can be if it is around 2" or more. That may be something for you to consider - 2+" of closed cell and blow as much as you want on top. It's not going to get too much better than that.

Good Luck with it!

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