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I'm in the process of renovation of a 1945 house. Originally built with a flat roof, tar and gravel. At some point a gable roof was constructed over the original flat roof. Space between the ceilings and flat roof is 10 inches, there is insulation in that space ,about 2 inches of paper backed fiberglass. I would like to add more insulation with out removing the existing ceilings. My plan was to blow in cellulose on top of the tar/gravel roof. Does this make sense.

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Brr! That would have our local fire department freaked as they had to deal with this, one serious attic fire if it ever gets lit. I realize it's not an answer to the question, but it wouldn't be allowed under current building codes because of the above concern. –  Fiasco Labs Jan 5 at 18:53
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@FiascoLabs, I don't understand how this would be against code. Can you please cite a code that prohibits this? Is it because it would be between the "old roof" and not really inside the house? –  auujay Jan 6 at 15:56
    
Does the original 10 inch "attic" space have vents to the outside? If it does, insulation above that space won't do much good. –  littleturtle Jan 7 at 21:21
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2 Answers 2

I'd blow cellulose into the 10' space, from above. I'd worry about moisture and rodent problems with cellulose between the old tar & gravel and the rafters. The tar & gravel makes for a moisture barrier you don't really want.

Check your venting in this attic space, and consider a humidity controlled fan. You probably have K&T wiring in the 10' space, if so see http://diy.stackexchange.com/a/20279/5960

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This does make complete sense, you'd have a nice layer of insulation there if you fill that void. As it stands, that pocket might be in danger of forming condensation when your inside temp differs greatly from your attic temp.

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