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Calculating "mixed media" R value?

518 Views 6 Replies 4 Participants Last post by  Mystriss
I'm trying to get to R38-40, but I want better sound absorption.

I have 4" thick walls and my thought is to do 2" rigid foam under the siding, 1" of high quality closed cell spray foam, and 3" of rockwool comfortboard 80.

I calculate the rigid foam is R14
I calculate the spray foam is R7
I calculated the rockwool at R12

Do I just add them all up?

And if so, that'd only be R33 so how can I make up the missing R5 :( I've already added 2" to the wall thickness in this plan; if I go much thicker it'll start to look stupid at the doors/windows. I don't really want to give up the rockwool and fill the cavity with foam because I'll lose the sound insulation (though R42 is pretty spiffy lol)
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I think you are safe just adding them, I am sure there is some scientific calculation but.
I saw something the other day, where they suggested 2 layers of 1 1/2 foam on the outside in different orientations. It allows you to move the buck for the windows and doors out 1 1/2" giving you better insulation around the doors and windows.
Found it.

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