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Old 12-18-2011, 08:27 PM   #1
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Okay to blow cellulose into existing wall cavities that have fiberglass?


I live in a 1964 split level house in NJ. We're going to be having an insulation contractor do an overhaul in our two attic spaces. However, he also recommended they drill-n-fill blown in cellulose into the wall cavities along all the exterior sides of the houes. He said the cellulose would simply just compress the old/original existing fiberglass batts that are in there. This sounds kind of funny to me. Anyone have any comments about this? Is it okay to add cellulose into existing cavities that have fiberglass? Will it increase the R-value noticeably along the walls? I'm skeptical that they'll be able to find and fill every cavity along the outside of the house. What happens around doors and window and at the ends of walls where joist spacing breaks from 16" OC? I don't imagine they'd be able to find and get every cavity, so is it still worth it if they only get 90% of the cavities??? Thanks!

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Old 12-18-2011, 09:44 PM   #2
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Okay to blow cellulose into existing wall cavities that have fiberglass?


Just my opinion, don't bother.

You have insulation in the walls. That's good but your greatest heat loss is still the attic.

No matter what they do, with 2x4 walls, at best you can only get about R-13 in there. Then there is the problem of compressing the fiberglass. You don't want to compact the insulation, it is supposed to be "loose." It's the air pockets that actually do the insulating.

I doubt they'd actually manage to get the entire bay filled.

You probably already have 2" batts at a minimum, about an R-7. You may even have R-11 in there already. Without opening a wall, you won't know.

If there is NO insulation, then I'd go for it.

With the labor (and patching) costs, and some existing insulation in the walls I'd spend the money on the attic.

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Old 12-18-2011, 09:45 PM   #3
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Okay to blow cellulose into existing wall cavities that have fiberglass?


Dense packing a wall that is empty is hard enough to do properly without bridging and other issues.

I have personally never seen it done properly on a wall with existing batts but maybe your contractor has figured it out. I just have never seen it done.
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