Hey Everyone – I hope you can help.
I’m insulating a finished basement and I have some concerns about trapping moisture behind the drywall. Here’s the deal:
The walls are poured concrete and it’s a walk-out basement, so they get pretty cold. Especially right now we’re having a cold snap in Wisconsin, down to zero overnight. The exposed concrete walls have a lot of condensation on the inside right now.
I put 2x4s flat against the concrete and filled in the spaces with 1-1/2 inch foam board. I intend to cover the whole assembly with 4 mil plastic and then apply drywall. There’s a treated 2×4 sitting on the floor, running under where baseboard will go, and the vertical 2×4 studs rest on top of it. As I said, the exposed concrete has condensation on it. The concrete underneath the areas that have foam board up already actually have frost beginning to form.
I’m afraid if I cover the foam board right now, the moisture will be unable to escape and start developing that musty basement smell. Do you think it will dry out behind there once the warm air can’t get to the concrete? Do you think this insulation system is a good idea in general?
Replies
I would put the polyethelyne directly on the block wall with construction adhesive. I would make sure to seal all the seams in the poly. Then I would build a free standing wall away from the basement wall a couple of inches. Then insulate the free standing wall with fiberglass........then drywall.
Well that's the way my basement is done and there's no musty odor. It smelled awful before the basement was finished......lots of prior water problems.
I would think that poly between the drywall and foam would trap the moisture and the foam would be constantly wet........
Hope this helps!
jocobe
Thanks for the responses. I went back through the archives and I see that this is one of those questions that comes up over and over. Several posts suggested looking at http://www.buildingscience.com, and I found some very good reference material there. They apparently have studied the problem rigorously, and reject the use of a vapor barrier.
In my case, I have a dry basement, and the only moisture problem is condensation on the cold concrete surfaces. It seems logical that if you can prevent any air from reaching the surface of the concrete, there can't be any condensation. The method they recommend seems realistic to accomplish this:
Glue EPS foamboard directly to the concrete wall, taping all seams and sealing all edges with adhesive or caulk. Fasten 1x3 furring on top of the foamboard by nailing through it into the concrete. Fasten 1/2" drywall to the furring. No additional vapor barrier. The reasoning for no vapor barrier is that any stray moisture must be able to escape. On an exterior stud wall, it could escape to the outdoors, but on a concrete basement wall it can only escape to the interior.
Here's my example of how NOT to do it: I started by fastening 2x4s directly to the concrete and then filling in the bays with EPS foamboard. I used dots of adhesive and no perimeter line of adhesive or caulk. This actually held the foamboard and the studs ever so slightly off the concrete, creating a tiny airspace behind and entry points at all the edges for inside air. Immediately upon starting the project, a cold snap hit and the basement walls got VERY cold. In 24 hours I saw water puddling at the bottom of the wall, and upon pulling off a section of foamboard, saw quite a lot of frost buildup under the foamboard. Also saw frost underneath after pulling off a couple of studs.
Although the uncovered concrete gets wet from condensation, at least it can also dry to the warm inside air. By putting up the foamboard all I did was to cut off the condensation from the warm interior air, giving it no way to dry out - thus frost. I shudder to think what would have happened if I had gone ahead with a plastic vapor barrier and drywall over this wet assembly.
I'm with you. No vapor barrier. If moister condenses inside the wall, it must me allowed to dry to the inside. Be sure your foam board is no also a vapor barrier (plastic or aluminum foil skin).You get out of life what you put into it......minus taxes.
Marv
I just did a basement with a new ... to me .. method that's got good reviews ....
sheets of expanded or extruded insulation ... dircetly against the block ... friction fit ... with just a few clamps ...
2x PT plates ... tight against the bottom of the boards ....shot into the floor
steel stud walls setting on the plates ....
attached to the joists at the top and the base plate at the bottom ...
holds the insulation board tight against the wall ...
tyvek taped the insulation sheet seams ....
and now with all seams and such taped shut ...
the insulation is your vapor barior.
No poly needed .... nothing to trap moisture.
I liked it as far as constuction .. and will use it each and every time from now on after reading the research that went behind the system ...
It's been discussed here before ..... if the search function worked ... you'd be in business.
Jeff
Buck Construction Pittsburgh,PA
Artistry in Carpentry
when we do finished basements we use a 3" EPS PerformGuard bonded to OSB..
notched around the joists...
Mike Smith Rhode Island : Design / Build / Repair / Restore
Mike, in the first pic, do you attach those so that they can be easily removed for termite inspection. If not, how do they a thorough termite inspection. I assume the borate in the PerformGuard would keep them from tunneling up through that, but the little buggers have other ways of getting up to the tasty stuff above...
no... the base of the osb was treated with BoraCare... and the slab was poured against it..
if you finish a basement there is no way you can provide for termite inspection.. so all of our contact points are either PT, on-site treated with BoraCare, or PerformGuard
this basement is like a walk-in cooler except it keeps the cold out instead of in...the panels go to the bottom of the subfloor, so they also form a box that we blew with dens-pak cells by blowing thru the sub-floor from above...Mike Smith Rhode Island : Design / Build / Repair / Restore
Nice Job,I'm feeling toasty just looking at it
Clay
Mike, are those panels site built, or do you buy them that way? Kind of like 2/3 of a SIP?Andy Engel, The Former Accidental Moderator
i can order EPS panels almost any way i want from Branch River...
any density, with/ or /without PerformGuard
with blueboard bonded to one side or both sides, with gypsum... with firecode.. with plywood or osb..
one side, two side...they don't care.... these were 3" PerformGuard with 1/2 osb (4x8)... i could also get 9' if i needed them
the only thing i didn't check on is if i could get them with PT ply... but i intended to treat the base with BoraCare so....Mike Smith Rhode Island : Design / Build / Repair / Restore