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Calculating "mixed media" R value?

517 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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Are you trying to meet a code level requirement?

Codes usually specify cavity insulation, but when you add a continuous layer on the outside you gain a lot more than cavity improvement. Something like 20% of the wall is framing, not looking at windows now. But that framing is reducing the whole wall R-value.

Then the 2" on the outside of the framing may also covering any rim areas like in a basement ceiling.

So yes you just add up the layers in the cavity, but adding in the framing get a bit more complicated, it's an area weighted process.

More description on how your walls are built and I can go further.

Note, extra effort on air sealing helps both R-value and sound suppression.


And good morning
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Sometimes codes are based upon what can practically done as opposed to best practices. R-30 would be a 2x6 (5.5") wall with 2" rigid over the exterior and filled with something in the range of R-3.5 per inch batt or blown insulation. Getting a wall up to the 35-40 range would need a thicker wall or insulation with a greater r-value or redefining the desired assembly r-value.
This link:
Page 11 goes through an example of calculating the "whole wall r-value" for a 2x6 wall. It shows how a cavity rating of 23.81 drops to 16.1 when the thermal bridging is included.

Now, add 2" (of polyiso) over all framing (+R-13) and the whole wall improves by 80%.

See if that helps your planning.

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