Frost in the attic after air sealing
I own a 1960s ranch in Minneapolis, and last year I spent a considerable amount of time in the attic performing air sealing. I pulled up almost every square inch of insulation and got all of the interior stud wall top plates, plumbing penetrations, electrical penetrations, and drywall joints. I also sealed at the exterior wall top plate. also sealed the attic hatch with weatherstrip. Now, I didn’t have a “before” blower door test done, but the “after” blower door test didn’t show any leakage through the dropped soffit, kitchen vent, or attic hatch. (ACH50 of 5.1).
Today is a cold day here, about -3 F. I went up to the attic to see how it looked and I saw frost on the roofing nails and in some spots of the sheathing. Needless to say, I was disappointed. So, I got out my foam gun, and crawled over to the spots with frost, expecting to find a nice size air leak that I missed. But when I moved the insulation….nothing. Just drywall, solid. No obvious air leak. So, I’m stuck.
At this air temperature, is frost normal? If it’s a sign of continued air leak, how should I track it down?
Thanks.
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Replies
If the attic is ventilated and at sub-zero temps the frost is only on the nails I wouldn't sweat it too much. If that nail-point frost grows to some huge rime-icing sculpture by March there may be problem potential, but I'm thinking probably not.
You have soffit vents, but is there corresponding ridge vent to depressurize the attic a bit, pulling outdoor air into the soffits?
Yes, I do have roof vents as well. When I had the new roof installed, I made sure to have the roofer include enough of them to meet the guidelines from Building Science Corp. The only shortcoming is that they're not continuous soffit or ridge venting, but I do have enough NFVA with it being split 60/40 as suggested.
Thanks.
Jay,
I agree with Dana that your attic is probably OK. Keep an eye on it, though.
Air sealing an attic can be tricky. Even a conscientious homeowner (or weatherization contractor) can miss something, so it never hurts to keep visiting your attic, looking for clues.
Your blower-door results of 5.1 ach50 for an old house may be as good as you will ever get -- but those results aren't particularly stellar.
Ok. Thanks for the info. I realize that 5.1 ach50 isn't that great....but for now, I was happy with it, especially considering the age of the house. I will have an opportunity to improve it when I re-side the house within the next few years.