Basement walls
I am building a ranch home and most of the basement (2800sqft) will be finished as living space. The builder has the waterproofing membrane spray on the outside of the basement, the plan calls for 2×6 with r21 insulation batts, and a certainteed membrain. There are drain tiles on the interior of the basement walls and the exterior. There will also be 10 mill poly underneath the basement slab when it will be poured.
Unfortunately I didn’t run across Martin’s articles on basement walls until I started looking at insulation. Since we are in the middle of building it is now too late to consider placing foam boards on the basement walls with framing behind it.
My question is will there be any mold issue with the above setup? I am being told that with the membrain and a whole house dehumidifier/humidifier the chances of that is unlikely unless there is a crack in the future. I am also being told that if I am still concerned I can go with spray foam which will be as good as the foam board and the spray will also get behind the studs,but it is very cost prohibitive $6-7k just for the basement! All of this stuff is new to me and I wish I had read Martin’s articles beforehand but can anyone give me recommendations on if I need to dig deep and go for the spray or the original setup good enough. Just trying to make sure I am doing this right since this is my family’s dream home.
Thanks!
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Replies
D.L.,
If you are just talking about a few 2x4 stud walls, those are pretty easy to remove and re-install. Just disassemble the 2x4s, install some rigid foam against the concrete, and then re-install the stud wall.
Martin's suggestion should be cheaper than the spray foam.
The membrain + dehumidifier might work, but depending on your climate and some other details it could be an ongoing expense to aggressively dehumidify, and it's not very robust if water gets in there, which can happen in the long term despite best efforts.
Might as well do it right while you still have the chance.
Hi Martin,
None of the 2x6 studs have been put up yet. I was told the plumber ran his pipes and it will be costly to have him stop his work and dissemble everything. Will have to find out more tomorrow what stage he is in.
I am mainly concerned about mold issues since kids will be living in the basement. Am I being too paranoid or is the original setup good enough that I shouldn't worry about it? I'd hate to stop the building progress we have made.
D.L.,
You didn't tell us your climate. When builders try to insulate basement walls with fiberglass batts, the worst mold problems tend to be in cold climates -- for example, Minnesota. In milder climates or dry climates, builders have fewer problems.
It's hard for me to visualize why a few plumbing pipes -- presumably pipes that were installed before the stud walls have been installed -- preclude the installation of rigid foam against your concrete walls.
Hi all,
I am in Missouri which is zone 4, so we do have cold and hot weather which is pretty humid at times. This year we've had alot of rainfall as well. I spoke to my builder and the issue is that the basement architectural plans are 2x4 framing in the basement. It is a completely finished basement so adding 2" foam behind the framing will push everything off by 2". The cost to redo all of the plumbing and architectural plans is going to be higher than using the spray foam (not to mention halting the build process).
From what I am being told the current build with the outer membrane and interior/exterior drain tiles should handle all most of the water issues, anything else like water vapor would be dehumidified through the inside vapor retarder certainteed membrain. So at this point I guess my only option is to decide if I am comfortable with the current build and if not then I have to find a way to fit spray foam into the budget. I don't know how easily mold issues can crop up so any thoughts on if I should go spray foam or keep my current build?
2" foam + 2x4s installed flat is the same total thicknesses as 2x 4s the normal way. It sounds like your builder is being a little stubborn and making excuses. I am not sure how bad the mold risk is, but I am not convinced that the membrain eliminates the risk. And I am not a fan of spray foam in general, especially in case like this where there seem to be other perfectly good options.
In zone 4A it doesn't take anything like 2" of foam for dew point control on R21 2x6 construction. As little as 3/4" polyiso or 1" EPS would be enough.
To meet IRC code min without studs you could put up 2.5" of EPS held in place with 1 x 4 furring through-screwed to the foundation with TapCons, and for less money than a 1" shot of closed cell polyurethane.
Or, 1" of polyiso and a 2x4/R13 studwall beats the thermal performance the 2x6/R21 wall sans-foam, and it does it in 4.5" instead of 5.5" from foundation to gypsum. This stackup would have HUGE dew-point margin at the foam/fiber boundary in your climate.
With any of the rigid-foam + stud options you could skip the MemBrain. That is not that it's a huge cost adder, but with sufficient foam for dew point control on the above grade section there's not much benefit to be had there.
There are SO many ways to deal with this that it's a bit ridiculous for the contractor to be whining about moving plumbing that doesn't really have to be moved.
Not only does the MemBrain add cost, but it also would slow drying if you had moisture coming in from the outside due to a crack some time in the future.
MemBrain would be far more vapor-open than the interior latex paint in a leak scenario, and would not appreciably slow drying.
But with R4+ foam on the exterior side of either a 2x6 / R21 or a 2x4 / R13 to R15 wall there's just no point to it in zone 4A. The vapor retardency of the foam will limit the rate of moisture coming into the house from ground moisture wicking up the wall from the footing, and there is sufficient R on the exterior of the fiber layer to prevent dangerous levels of wintertime condensation at the top of the wall.
The 1" polyiso (or EPS) and 2x4 / R13 to R15 solution isn't going to be a lot more expensive than the original plan, and a heluva lot cheaper than a closed cell spray foam solution, and it's an inch less overall depth. The R6 polyiso should run no more than 75 cents per square foot, installed, R4 EPS would be more like 50 cents. Assuming a ~40 x 70' foundation that's 9' tall that's ~$1000-1500 all-in for the rigid foam, installed, the same number of board cuts (but fewer board-feet of lumber) and cheaper cavity fill. If they nick you $2K for the change-order their making pretty good money on that.
The spray foam up-charge may seem nuts, but that's what it costs for ~2000 square feet of R20-ish closed cell foam. If the studs aren't already installed, there's no reason to go there. If the studs WERE already installed and full of pre-installed wiring & plumbing threaded into the framing, 1" flash of closed cell foam and compressed unfaced R19 or R20s would be fine, but that would still cost $500-1000 more than 1" polyiso and 2x4 / R15 (that would outperform the flash & batt.)
Dana, I stand by my statement that MemBrain inhibits drying of a below-grade wall. I agree that a 1" layer of polyiso or EPS against the wall solves the problem, so this is a tangent to answering the question.
Drying a below-grade wall is harder than a normal wall, and I don't think MembBrain is particularly useful for basements even though I like it for other applications. Here's why:
Consider the original design--fiberglass stuffed stud wall directly against the concrete with MemBrain on the interior. I think the groundwater in that area is about 52 F, and the footing could be at a similar temperature. So if water leaks or wicks in at the footing, we might have a wet surface at 55 F. The liquid water only evaporates if the air in the wall is dry enough to have a dew point below 55 F. For good drying we'd want the dew point substantially lower. If the room is at 72 F, 55 F dewpoint corresponds to 55% humidity at the inside of the MemBrain. We now have a dilemma: to rapidly evaporate the water from the cold wet corner of the wall, we need the humidity at the MemBrain well below 55%, but to have the MemBrain open up, we'd like the humidity above 55%. It won't completely prevent drying, because, for example, at 50% humidity, we'd get a little evaporation from the cold wet puddle, and the Membrain permeance is still substantial even though it isn't as wide open as it is above 60% humidity. But we can't go down to 30-40% humidity to get good fast drying, and at 50% humidity, it's not "far more vapor open" than paint--it's about the same. So it slows drying for two reasons. First, it only lets vapor out when the wall air is too humid for fast drying, and second, it retards the vapor transmission even when the humidity is higher than ideal.
The scenario where MemBrain works well is an above grade wall, where the outside is hot in the summer. Then if you have 70% humidity at the MemBrain, you have 70% or lower humidity throughout the wall, such that you get decent drying throughout (and even faster drying at the hot side).
any builder can move plumbing and walls 2" without redoing plans. that argument is false.
Hi all, I appreciate all of the input and after a lot of reading I found that some people turn their 2x4 studs sideways to get more space in their basement. After re-reading Charlie's comment #6 I realize he meant the same thing. This gave me the 2" I needed to fit the foam and after verifying with my architect it should work since none of the walls are load bearing. I spoke to my builder and there are only a couple of pipes that can easily be moved off of the wall to make it fit.
Since I now have the 2" to fit foam boards should I go Polyiso over XPS? From what I've been reading it seems that Polysio foil is preferred over XPS because it has higher r value which should be around r13. Is there any reason to consider XPS besides cost? I noticed that the foil is on both sides, would the polysio somehow absorb water if there was a leak that touched the foam at this open area or through a rip on the foil?
There was another Q&A thread where someone had condensation build up on the foil but I think it was from not sealing properly, is XPS easier to seal vs Polyiso or should be the same? Just trying to get all of the Pros and Cons to make a decision, want to do it right the first time so i don't have to revisit this again in the future.
Thanks!
D.L.,
Either polyiso or XPS will work in this application. Polyiso has a higher R-value per inch, and is much more environmentally friendly, than XPS. If you choose foil-faced polyiso, it is much easier to tape than XPS. Ordinary housewrap tape should work fine, although you may prefer to use a foil-faced tape.
Make sure that you install the rigid foam insulation in a continuous layer -- don't cut it into strips and install it between the 2x4s.