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Energy audit found frost on rafters in the attic

We live in Minnesota. We recently had an energy audit. The inspector found frost on the rafters in one of the attics along with the plywood starting to separate. The house was reroofed in 2008 and I remember that they replaced quite a few sheets of plywood. There is some blown in insulation and batts. The bathroom vent is sealed and vented to the outside. We don't humidify the house other than a shower in the lowest level and cooking.
What would be the first thing to do to stop the frost and keep things warm in the house?
Thanks clj

Re: Energy audit found frost on rafters in the attic

That generally indicates a lack of ventilation. But look for your plumbing stacks where they come into the attic from below. There is probably a gap between the pipes and the hole they come through. If so, caulk those gaps.

Also look for open wet walls. A wet wall is a wall with two frames and plumbing running between them. Often the tops of there are left open. In this case, the plumbing vent stacks will be coming through those gaps. Cover the gap(s) with 4 mill vinyl and seal around the vent stacks. Insulate over the vinyl.

Re: Energy audit found frost on rafters in the attic

Keith, you may be right. One of the scans the audit company gave us was from the bathroom where all the pipes and stuff go up, it was a dark blue - cold. Once the weather is warmer and we can get into the attic we will try what you suggested.

Re: Energy audit found frost on rafters in the attic

The average house leaks like a sieve to the attic. Every place the envelope is broken to drop electrical lines into the wall, every cannister light mounted in the ceiling, around the metal bathfan boxes , attic hatches, pulldown staircases, etc. You just have to go up there and look for anything that breeches the ceiling and is leaking relatively humid air into that cold environment.

That being said, the attic is supposed to be cold and as dry as the outside air. The insulation is what is keeping the warmth in the house. The environment in the attic should be similar to the outside air. In super cold Minneasota winters, the attic will be very cold and have similar relative humidity as the outside air. If the attic is properly vented, there should not be ice forming even if moisture is leaking from the house envelope. However, that moist air that is leaking is also the warm air that you paid to heat up. Sealing the voids is killing two birds with one stone.

Were I building my dream house and were using normal attic construction, I would consider spraying a couple inches of closed cell foam on the back side of the ceiling before topping it off with conventional blown insulation. It would form a vapor barrier and also guarantee that there is no point where air could leak.

Re: Energy audit found frost on rafters in the attic

With today's codes losses to the attic are nothing compared to old houses. Fire-caulk at every penetration, all ventilation having to vent to directly to the exterior, and minimum insulation standards have made things a lot better above the ceiling than in the past. And roofing codes helping prevent ice-dam problems too.

These concepts can be easily applied to old houses- and they should be ;) No character of the house lost, but modern gains made that will pay off in savings and keep the old house intact for a lot longer.


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