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Dripping walls and wet sheathing in new house

A friend of mine has just built a log house with standard 2x4 (or 2x6) wall framing on the second floor in zone 6-7. The walls and ceiling have been insulated (roxul and foam in the walls and pink in the ceiling) but they have not yet put up a polyethylene vapor retarder. They turned on the heat this week and the walls started to drip and the sheathing behind the wall insulation became wet. Essentially, the moisture in the house turned to vapor and quickly condensed on the cold surfaces of the house.

I have told them that they need to either put up a sealed and caulked smart membrane, or an air barrier and ideally a layer of foam on the outer surface of the sheathing (R-10-15) (although they have already installed their rain screen and siding).

I would hate to see them close up their walls with poly (which they intend to do), and I was hoping this forum could provide some great advice that they could feel confident about. How should they proceed?

Asked by jessie pratt
Posted Dec 6, 2012 10:58 AM ET


4 Answers

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The problem is that the house has air-permeable insulation (Roxul and fiberglass batts), a high indoor humidity level, cold sheathing, and no interior air barrier. The result is predictable: the warm, moist interior air quickly finds the cold sheathing.

You need to lower the indoor humidity level by running the bathroom exhaust fans for 24 hours a day. If this doesn't work, you may need to rent a commerical-sized dehumidifier.

You don't want to close up the walls if the sheathing is dripping wet. In some situations, it may be necessary to remove some of the insulation to help the sheathing dry out. If you get a warm spell of weather, and the sheathing is somewhat dry, you can then re-insulate and quickly put up the drywall. The drywall is an air barrier; it will keep the interior air from having unfettered access to the cold sheathing.

Answered by Martin Holladay
Posted Dec 6, 2012 11:08 AM ET


Once the walls have dried out should they ensure air tight drywall installation?
And what about the poly? Should they put this up or go with a smart membrane (as well as the air tight drywall)?

Answered by jessie pratt
Posted Dec 6, 2012 11:21 AM ET


A house on the border of climate zones 6 and 7 can get away with interior poly, but only if the poly is installed when the wall cavities are dry. If the cavities are a little damp, MemBrain or a similar smart retarder makes a lot more sense.

It's also possible to go with airtight drywall and vapor-retarder paint, as long as your local building inspector agrees with that plan.

Answered by Martin Holladay
Posted Dec 6, 2012 11:39 AM ET


I almost always run a commercial dehumidifier--Drizair 110 or similar--in the later stages of a new build or large remodel, starting a few days or so before the insulators show up. It makes a huge difference in the interior humidity, and we don't get condensation on the windows when the drywall taping starts. Keep it going until the wall painting is done. I would go rent one and run it for a few weeks, and see what happens. You might be pleasantly surprised. The other option is to run the bath fans and turn up the heat a bit.

Answered by David Meiland
Posted Dec 6, 2012 9:42 PM ET

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