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Above grade wood frame walls..

2025 Views 9 Replies 4 Participants Last post by  Gary in WA
Just want to make sure I got this right. I was going to install unfaced rock wool bats with no vapor barrier to these wood frame walls. Even the section above the concrete wall. Is that all I need, would it be OK if I first cut and installed xps to the stud bays and then the roxul bats? I am trying to insulate thermally and for sound as well. I can here a mouse fart outside this wall in the corner if I am standing in the room. I can also see daylight at the end of the concrete wall where it turns into a full framed wall! I figured I would just use spray foam for that.

I know what to do with the concrete wall just as soon as I figure out how to move the baseboard heater that is attached to it. (another thread).

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Forgot to add the pic..


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Maine1 said:
Forgot to add the pic..
Also, the siding is cedar with plywood sheathing. I dont knowwhat type of wrap is under the siding.

And I live in Maine... Brrrr
Foamboard on the inside of the sheathing could be risky. Find out the wall make-up exactly- before adding a low perm f.b. to dry only to the exterior.

Foamboard on the inside of the sheathing could be risky. Find out the wall make-up exactly- before adding a low perm f.b. to dry only to the exterior.


Best to know what the housewrap or other WRB layer is like on the outside.

The plywood will have about the same vapor permeance rating of the 1" XPS in this case (Class II).

In the colder climates, most of your drying happens to outside in the winter months but if you are very detailed on your air barrier on the inside, very little moisture should be getting into that wall. That is not really the issue in this case.

If the exterior wall has some funky vapor control layers (not likely), it is possible that you could create a vapor trap of sorts where the wall previously dried to the interior in the summer/wet months. That is not a good idea regardless, however, if it currently is the status quo, putting that foam board on is going to prevent that and you are going to start having rot issues to the exterior.

The exterior cladding should be pretty open and should more readily dry to outside than through what is likely some sort of asphalt paper and plywood combination.
Thanks for the advice, I really appreciate it. I was planning on adding the XPS mainly just for sound deadning. I think I will be better off using just the Roxul R- 39 by itself. Hopefully that will add enough of an acoustical barrier to deaden some of the outside noise. Spray foaming all the gaps should help too. It just seems like the outside walls of this house are paper thin as far as noise goes.
Seal all the gaps and go all Roxul. The delta in noise will be non-existent.

The noise you will get after that will be via the framing of the wall.
Depends on how far you want to go-

#6 or #8; for Residential Sound Control.pdf

Or add some XPS strips on the studs for a thermal/sound break and:

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