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zeetwoeight 01-16-2011 05:35 PM

Condensation between foamboard and fiberglass insulation
Hi all,

Just stumbled upon this forum and figured I throw out a question about an issue I'm seeing in my basement.

I have put up polybead (foam board), adhesive to the concrete walls and then studded and fiberglass insulation. On one section of my basement, I'm getting condensation forming between the polybead and the fiberglass insulation. This is only happening where the basement wall has the exterior surface exposed to the air (it's in my basement stair well). The rest of the basement is completely underground and I don't see the issue anywhere else. I'm guessing that since this section is exposed to the air, the concrete is getting colder than all the concrete that's underground and I'm getting condensation. I know it's not a leak because I don't see any condensation in the summer and we've had quite a bit of rain as well. The polybead only get's wet when it's cold outside. The question is - how do I stop it? I was considering covering the exterior part of the wall with the blue foamboard (extruded???) to see if it will help but I need other suggestions as well. My concern is this condensation resulting in mold growth if I don't resolve it.
Thanks to anyone who can offer up help.


gregzoll 01-16-2011 05:58 PM

You need the wall to breathe. The reason that you are seeing the condensation, is because the warm air is meeting the cooler Foam board. leave a 1/2" to 1" gap between the foam & Roxwool. Better yet, you could skip the Fiberglass or Roxwool and just put up the Gypsum on the studs. has a lot of info on basement insulation. If you place the Blue insulation on the outside, you have to protect it both from UV and damage.

concretemasonry 01-16-2011 06:10 PM

What do you mean by "bead board" is it the cheap stuff they they use for packaging and meat trays at the superstore? The have now verififyable properties and has to be considered immaterial. Moisture will condense where ther is a dew point in the wall. In your case it shows up at the face of the fiberglass and the rest of the fiberglass will have some moisture is it that lowers the insulation value and hold moisture for mold to work with.

The simplest thing to do is to tear out the cheap beadboard and replace it with extruded (XPS and not cheap EPS). Lerave a gap between that and other insulation. It would probably be good to dump the old fiberglass because it is very difficult to dry out and reuse. - It does a great job of holding moisture due to its structure even if it does not absorb moisture.

Gary in WA 01-16-2011 08:34 PM

Fig.14, on page 14;

The insulation is not thick enough there to prevent condensation, as pointed out. There should be no air space to give convective loops; Nor should the foam board be open to the basement air, completely air sealed; ADA with drywall and a sill sealer under the p.t. bottom plate to stop air as well as a thermal/capillary break;
And, Welcome to the forum!

Where are you located to figure the required thickness?


zeetwoeight 01-17-2011 07:38 AM

Thanks folks. This is some very useful info for me to review.

With regards to my location - I'm in St. John's, Newfoundland, Canada.


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