Expert: Daniel Humphrey - 12/15/2008

Question
I am working on finishing my basement.  My block walls were dry loc'd a few years ago to help keep moisture from coming in the basement. I have now built stud walls 1 to 2 inch off the block wall (to leave an air space) and then installed fiberglass insulation with a vapor barrier, no drywall up yet. Today I noticed water puddling coming from underneath one of my new insulated stud walls. I tore off some of the insulation where the water was coming out and found it to be from condensation. The insulation near the block and the block is quite wet, lots of condensing, some of the moisture on the insulation was ice crystal yet. What occurred in the last day was our temps went from 20 deg F to 38 deg F. I checked the rest of my basement for this problem but it is only happening on the exposed walkout wall where this no ground cover.  All the other walls that are ground covered are fine.  Is this a one time thing, or is there a way to stop this from occuring?  I don't want to put up the drywall right now because I am worried about mold if I get condensation like this.  Right now I left the insulation I took down to check it off and put a dehumidifier running full blast down there hopefully to get rid of moisture and dry out behind the rest of the insulation on that wall. How do I fix this issue, or should it be only a one time thing. - I'm in need of help, I don't know what to do here. Thanks for your time.

Answer
Dear Trav,

You've correctly identified the cause of the problem being warmer, moist air hitting the colder block and condensing. It will do this to some extent whenever the dew point of the air touching the wall is above the temperature of the wall.

In this respect, the insulation is working against you because it keeps the block wall from warming up. The ground-covered walls are fine, of course, because they happened to be above the dew point, or at least warm enough not to create frost. It could be that only in these extreme temperature changes will the block have enough condensation or frost to matter.

So to keep this from happening you need to prevent 100% of the interior air from getting to the cold part of the block, and that would be very difficult with your present system, requiring a perfect air/moisture barrier behind the painted sheetrock. Even so, whatever air is in that air gap will condense its moisture under those conditions. The air gap is not really functional unless air can come and go, anyway, taking room moisture out of the wall system. It would be better to have exterior air washing that wall, just as exterior air washes through an attic, or no air gap at all. Research shows that if the moisture can penetrate more than 1/3 into the insulation system. it will condense in there.

The other approach to solving this problem would be to thicken the block wall with an insulative material that provides an interior wall surface warmer than the dew point. You were working toward that with your stud wall, but the air gap foiled you. If you can, coat the block with urethane foam to fill that gap. Rigid foam glued to the block, in place of the air gap, might be as effective if you can't get the urethane.

Good luck,

Daniel