Basement insulation
kickstarter
| Posted in General Questions on
Looking for recommendations on insulating a basement in climate zone 5a. It is currently unfinished, but will be finished at some point.
Area is roughly 2000 sqft. The basement is full walkout. The wall that is walkout is wood framed with R-11 fiberglass batt insulation with brick on the outside. The rest of the basement is concrete walls. The floor is not insulated.
How would you insulate the basement? I was thinking some type of foam board insulation on the walls, then framing with 2×4, then fiberglass batts. I’m not sure if the one wall being R-11 is going to make this approach less effective though.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
First thing, I very much doubt that R-11 is adequate for an exterior wall under your local codes. If the basement is conditioned space, the IRC requires R-30 if standard wood framed construction. In your case, it might be easier to use R13 plus R10 continuous insulation. You can use high density FG batts to bump up 2x4 wall insulation to the R13 requirement, then use R10 rigid foam insulation on the inside of the framed wall and the basement wall. Then a 2x4 basement wall with R11 or R13 batt insulation would be fine. I would use foil or plastic faced insulation, though 2" XPS insulation would also be an adequate vapor retarder. Taped seams are a must.
Note that walls with brick veneer do not dry quickly and can cause moisture issues in some cases. Detailing is very important, with water barrier detailing behind the veneer and proper flashings and weep holes in the veneer system for drainage and (limited) ventilation. The weep holes must be above the finished grade. Adding weep holes at the top of the veneer is not required, but it does improve ventilation and drying.
Many builders in our area use a "simulated performance model" to get away with using cheap, outdated practices and still meet code. Multimillion dollar homes with 2x4 framing and R13 batts. Their computer software lets them get away with it.
It's better to put all your R value in as rigid foam against the walls and skip the batts entirely. Using batts in a basement is asking for trouble. If you don't have any issues with water getting in, I'd use polyiso against the walls, then 2x3s "on the flat" against that to provide 1.5" for electricals (you can use 1.5" deep 4" square boxes that way, and mud rings to bring the devices up flush with the surface of the drywall). This gives you a minimum depth wall assembly, so it saves space, and it's very robust in terms of moisture tolerance too. Code in your area is probably R15 if your area is on the newer revisions, and that's going to be 2.5" of polyiso.
The existing wall won't change the effectiveness of the insulation you install in other areas. If you can upgrade that wall at all (R15 is easy by replacing the existing batts), that's great, but if you can't, that's OK too.
Bill
Thanks Bill. I was thinking about removing the unfinished drywall on the wall that is already framed and adding rigid foam in front of the framing. Problem is there are windows, so I'm not sure how the added thickness of the rigid foam would affect things. The batts are also faced, so I'm guessing if I added foam in front of the framing, the batts would need to be unfaced.
Also, would you do any type of insulation on the floor before finishing it (probably some type of LVP/vinyl)? It's just concrete slab. If so, anything that wouldn't sacrifice too much ceiling height?
You want the rigid foam DIRECTLY AGAINST the concrete wall. You do NOT NOT NOT want to put the rigid foam on the opposite side of the studs from the concrete, since that would create a moisture trap between the foam and the concrete and you risk rotting out your framing that way.
You can lay down rigid foam over the floor and put a plywood or OSB subfloor over the top of that. You can use EPS or XPS for this, high density EPS is a bit cheaper and more green. Just be sure you get something rated for compressive strength for this application, and the usual 10PSI stuff would be fine here.
Bill
Good point. Thanks.
Any recommended thickness for the EPS/XPS and plywood/OSB? I want to minimize loss of ceiling height.
It's nice to have R10 underfoot, but even R5 would be a big improvement.
Standard subfloor is 3/4" tongue and groove plywood or OSB. You really do want the T+G version here since there is no room for blocking under the seams between panels when you're going right over the foam with the subfloor.
Bill