This summer we put an addition onto our farmhouse. In the construction, we put 1" rigid foam insulation first, then the OSB siding. So the foam is against the wall studs. It is now winter and in starting to hang drywall, I noticed some wetness in spots along the bottom wall plate. (it is a bit warmer today-above freezing) In pulling away the batt insulation, I noticed that there is condensation against the pink foam in many areas. I am trying to figure out what is causing this and what to do about it. Is the outer OSB which is not permeable, not allowing moisture to pass through and keeping it at the inner surface of the foam? Or is it moisture from the warm air meeting the cold air on the exterior creating the condensation? Any help/ideas would be greatly appreciated!
After pulling the insulation away from a cavity, the area is dry with no more condensation. Would I want to put a 4mil poly barrier over the insuation (between the insulation and the eventual drywall?
You don't say what region you live in, but I gather it's a cold one. Yes, the foam board, not the OSB, is perhaps creating at least a partial vapor retarder in the wrong place in the wall for a northern climate, and the addition of a second vapor retarder will compound the damage, trapping moisture between the two, and leading to possible issues of mold and structural decay.
Does not the insulation have an integral vapor retarder?
If you were to use those materials, the foam board should have been placed, in my opinion, on the inside face of the studs, right behind the wallboard, not outside behind the siding.
Architect (NY) and Home Designer (PA)
Posts: 2483 | Location: Tobyhanna, PA | Registered: 24 October 2005
I do live in Wisconsin. It is common to put rigid foam on the outside prior to putting on the siding- (I've never heard of anyone putting rigid foam on the interior though). However it's generally done with the OSB against the wall and then the foam and then siding. We put the foam first as we thought it would provide insulation against the wall studs so that you would not have cold penetrating through the studs to the interior wall.
Also, we only have tyvek on the exterior as of now- no siding yet. I believe rigid foam is permeable and should allow water vapor to pass through. I am thinking that the condensation is forming due to this being where the warm air of the interior is meeting the cold air of the exterior. (When it was colder, I suppose it was frost and now that it's warmer the frost is melting and hence the condensation) But would putting the 4 mil poly block the moisture of the warm air passing to the exterior area?
Do I understand correctly that at the moment you have only insulation on the inside of the wall, and no wallboard yet? If that is true, then I agree, a vapor retarder just behind the wallboard should stop the migration of moisture through the wall.
However, given the construction as you describe, the foam board would act as at least a partial vapor retarder, and that would account for the moisture you found. The manufacturer may state that it is permeable, but my experience is that it isn't VERY permeable.
Hopefully, your interior vapor retarder will stop enough migration of moisture so that the problem doesn't occur again.
Architect (NY) and Home Designer (PA)
Posts: 2483 | Location: Tobyhanna, PA | Registered: 24 October 2005
Yes, currently I don't have the drywall on yet- I had just finished putting the ceiling drywall on and was starting the walls when I noticed what was going on.
Just curious, would this same problem still occur if I had put the OSB on first and then the foam? I was doing research on this problem and I swore I came across something that said OSB had very little permeability (is that a word?)
Or is it moisture from the warm air meeting the cold air on the exterior creating the condensation?
The problem is caused because you do not have the interior walls covered with drywall and warmer, moister interior air is condensing against colder stud cavity surfaces with temperatures below the dew point of the interior air.
Once you seal the wall and reduce the air infiltration to the stud cavities, your problem will cease.
I would be very concerned about the way you installed the structural sheathing over the foam insulation, however.
Stuctural sheathing should never be installed that way, It is designed and intended to be installed directly and tightly to the studs with the foam insulation placed on the exterior of the sheathing.
By framing the way you have, you have weakened the wall's ability to resist lateral load and it could cause the structure to fail in high wind or snow load conditions.
Its likely to late to address this problem from the outside, but you may wish to install internal let-in bracing in the open studs to correct the problem before you install your drywall.
Also be sure to install the vapor barrier on the inside of the house, before the insulation goes up. Without it condensation will form in the wall cavity where it gets cold enough to condense. What is happening now is the visual surface area of the foam boards has cooled down to a point in which the humidity within the home has condensed on its surface.
Posts: 993 | Location: New Jersey | Registered: 31 January 2006
Now I'm a bit confused. Do I install the vapor barrier just behind the drywall or pull all the batt insulation out, and push the barrier all the way into the wall cavity against the rigid foam? Also, can I just use 4-6 mil poly for this and how do I secure it in place- staples or some sort of special tape seal product?
Place the vapor retarder as close as possible to the surface of the warm in winter side of the wall, directly behind the drywall. Fibreglas insulation with a paper facer rated as a vapor retarder should be adequate. The impregnated facer has a rating of 0.40 perms while 7/16" OSB is rated at 2.0, meaning that the paper will be the more effective vapor retarder and should prevent moisture laden air from reaching the surface of the OSB.
Posts: 216 | Location: Annville, PA | Registered: 03 July 2006
Actually, I already had the insulation with the paper facer installed. I was just starting the drywalling when I noticed the moisture problem. I thought the paper facing was supposed to vapor as well and that's why I was so alarmed at the amount of condensation I was finding. Just to clarify though, the rigid foam is first on the outside then the OSB, so the OSB is furthest from the interior.
Kraft paper facing on the insulation is a vapor retarder. It is not an air barrier.
Without drywall being placed over the insulation, you are allowing air infiltration into the stud cavities at a high rate.
It is this infiltration of warmer, moister air that is temporarily causing the condensation problem.
Once you install the drywall and in essence cut off the air infiltration to the stud cavities, the vapor retarder can do its thing and your problems will cease.
I also want to clarify that one should NOT install OSB over rigid foam insulation as you have done.
Rigid foam should be furthest from the interior.
OSB should always be placed directly and tightly fastened to the studs.
Your application of the two materials is backwards.
This message has been edited. Last edited by: homebild,
Hey thanks for all your help/input on this! I just wanted to get the best approach before I sealed off the walls with drywall as from that point on, I couldn't do anything about it.
Originally posted by Dpots: Just to be on the safe side, could I add a 6mil poly over the insulation- even with the batting with the paper vapor barrier?
NO!.
You will create a double-vapor retarder condition which can actually trap moisture in the wall once condensed.
1 vapor retarder in the form of either 6 mil polyethylene OR kraft faced paper should be used.
One must not restrict the flow of water vapor back into the room from the stud cavities. This will prevent the cavities from drying out in the event some condensation does occur in the future.
