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Calculating "mixed media" R value?

523 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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Heh and here I was trying to be to the point :vs_smirk:

Code for Alaska is R30, but R35-40 is recommended [from online sources anyway, I haven't called anyone about this yet as I prefer to have a good enough idea what I'm gonna do before I yap at the super busy folks up here. Earthquake repairs have them over booked and frazzled heh]

I'm not set on anything in particular. I was looking at "slow rise" closed cell for the exteriors because that's supposedly the best compromise for insulation + sound on exterior walls. Foam seals air holes better I'm told, so I'm good with that too. That said, I just want the Rvalue and the sound coverage - ya know, all the things...

I have projects that are slightly different; improving my existing house and building a new detached shop.

The existing home has 2x4 16OC walls centered on 8" wide CMU perimeter - excepting the 21' x 20' attached garage, we have a 6' unfinished basement/crawl space that's quasi-heated via the living spaces baseboards.

I mostly want better sound absorption as we lost a couple huge spruces in the front yard last summer and now we can hear traffic a lot more. Unfortunately it's a 1970s home so the rooms are all on the smaller side, two of the upstairs bedrooms are barely functional as more than closets, and two are a tight fit for King beds with more than a dresser, so I can't really add a second wall inside as is oft recommended. (That would actually take the two bedrooms down to 7' wide...)


As for the shop, the current one's slab got wrecked by the quake and split in half. We figure since we gotta go through the nightmare of emptying the thing out, we might as well demo it and make it bigger/more suited to our specific wants/needs. AKA I want an actual woodshop and he wants the ceiling height for a car lift. We have also been considering replacing our septic system as it's 17-18 years old, and I'd really like to bury our power lines. So basically we're looking at digging up the entire back yard anyway.

The shop will be all new construction and I'm trying to figure how thick to plan the walls. Typically we do 8" CMU perimeter up here, and I want a 12" thick reinforced concrete slab.

Sound absorption is important here as the neighbors don't need to listen to my power tools. I also wasn't planning to keep the Nat Gas connection to out there. Black pipe had rusted out and we had a leak with more than a few people smoking over by the leak for who knows how long o_O - I'm keen to get rid of that particular black pipe entirely, it goes currently goes through the basement of the house. So the shop'll be 100% radiant heat, but it's going to have large & high spaces so I want to go big on the insulation.

Thing is I'm not sure I can go over 8" cavities because of the CMU's. I've been told I can't double them up (something about high water table and frost heave? I don't recall exactly as it was like 15 years ago.) Fully poured foundations aren't really a thing up here, concrete is typically kept to a bare minimum because (I'm told) it doesn't shift well with the ground. They're probably already going to be grumpy that I've got such a big slab, I was thinking I might break it into two "pours" since I want a wall between my husbands area and mine anyway (we do not share shop space well at all) and I might as well have another support point for the roof/snow load.

... And yep, wall of text. Sorry >.<
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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.

I'll have to read through that after my nap, thanks.

We can't ditch studs for thermal bridging and sound bridging improvements because of the earthquakes and snow loads so rigid foam is our best option.

The vast majority of houses up here are 2x4 16OC construction so you're probs right on the R30 code thing.

The newer ones are being built with 2x6 and 2x8 and rigid foam on the outside is all the rage for everyone now - all the neighbors are doing it so I figure I best join the party :p
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