Insulating basement stud wall
user-3338613
| Posted in Green Building Techniques on
There’s a ton of information out there regarding how to insulate concrete walls. My basement (actually I think it’s technically a really tall crawlspace with vents) has wooden stud walls sitting on concrete walls where the wall goes above grade (house is built into a hill). I don’t have traditional rim joists on 3 sides of the house, it’s just a half wooden wall. How could I insulate and seal the stud part of the wall without resorting to spray foam? Could the rigid foam boards be attached in whole sheets directly to the studs?
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Where is this house located? (Climate zone, or postal code.) The climate makes a difference on what's relatively moisture safe, and what isn't, so the answers will vary.
Cary, North Carolina. Looks like the southern edge of climate zone 4
David,
It sounds like your walls are balloon-framed (that is, the stud spaces extend continuously from the crawl space up to the floor above, without any plates). Do you know whether your house has any wall insulation on any of the floors above the crawl space? If your house has balloon framing, you should be able to look up the stud spaces -- if necessary, with a flashlight and a mirror -- to see what you've got.
If your house has balloon framing, spray foam is an easy way to limit air currents in your stud bays. If you are dead-set against spray foam, you'll have to painstakingly seal each stud bay (between floors) with rectangles of rigid foam. Each rectangle needs to be sealed at the perimeter with caulk, canned spray foam, or high-quality tape.
Once you have created this air seal in each stud bay -- in other words, once you have solved the balloon framing problem -- you can insulate these framed walls with the same techniques used to insulate any framed wall.
Thanks for your detailed response Martin. I think I threw you off target accidentally when I said I don't have traditional rim joists. What I should have said is that I have rim joists, but on three sides of the house they are sitting on wood framing instead of concrete blocks.
You say that I can insulate these framed walls with the same techniques as any framed wall. Does that include attaching whole foam boards to the studs, or do I need to put the insulation inside each stud bay?
I'm including a picture of the rim joists, as well as a picture of the overall crawlspace for reference.
Thanks,
Dave
Rim Joist
Overall
David,
One of your photos seems to have been garbled as you uploaded it to the photo sharing site -- in any case, my computer only displays half the photo. The part I can see (shown below) is a pretty sloppy example of fiberglass batt installation -- so, to prepare you for the work you're planning to do, I'll have to ask two questions:
1. Are you the one who installed the fiberglass batts in the photo?
2. Do you understand the basic principles behind a quality installation of fiberglass batts?
.
David,
Q. "You say that I can insulate these framed walls with the same techniques as any framed wall. Does that include attaching whole foam boards to the studs, or do I need to put the insulation inside each stud bay?"
A. You need to establish an air barrier -- and that air barrier should be continuous with the air barrier for the wall on the floor above. This can be tricky. You don't want any hidden air leakage paths.
If you leave your stud bays empty, you can easily be encouraging leaks through these hidden pathways.
It's best to fill the stud ways with insulation -- for example, dense-packed cellulose, mineral wool, or high-density fiberglass batts (carefully installed, without any voids or compression).
If you also want to install a continuous layer of rigid foam on the interior side of your studs, you can. This article explains more about that approach: Walls With Interior Rigid Foam.
1. Are you the one who installed the fiberglass batts in the photo?
No, it was the builder. This is what almost every crawlspace and basement I've seen looks like, though. Those wire hangers compress the fiberglass. The idea is to remove all of the floor insulation once the basement/crawl is conditioned though.
2. Do you understand the basic principles behind a quality installation of fiberglass batts?
The very basic principles? Yes. Avoiding voids and avoid compressing the fiberglass because the air inside is what is doing the insulating.
Everything I've read discourages the use of fiberglass in the basement/crawlspace. Does that not apply when it's only used in above-grade sections like this?
Thank you for your advise
David,
This is a framed above-grade wall -- so yes, it's perfectly OK to install fiberglass in the stud bays.
I'm glad to hear that you weren't the person who installed the fiberglass insulation in the photo. I hope my question didn't offend you -- it may have come off as rude. (That wasn't my intention.)
The IRC calls for R10 continuous insulation on the foundation too, which could be done with 2.5" of EPS or 1.5" of foil faced polyiso strapped to the CMU with 1x furring through-screwed to the CMU. (The 3/4" air gap between the foil facer & wallboard provided by the furring brings the R9 nominal for 1.5" polyiso to the R11-12-ish range at center cavity, and clears the R10 hurdle whole-wall even with the thermal briding of the furring factored in.) If the interior edges of the stud wall are co-planar with the CMU (as it appears in the picture) you might as well run all the way to the ceiling with it rather than detail-in a ledge.
You definitely want to neatly install some fluff for a full cavity fill. Can-foam the electric & plumbing penetrations in the framing (studs included), and caulk the sheathing to the framing in every stud bay, and between any seams in the top/bottom plates before insulating. Be sure to tuck the corners & edges in to eliminate gaps at the sheathing/framing boundary, and tug it gently back so that the fiber is slightly proud of the interior edges of the studs.