Attic frost

[BigJ273]

Have you sealed all the holes on the fan box in the attic? Including where the box meets the ceiling drywall? Likely it's mounted against a joist, and the slit between the two can let a lot of air out if not sealed properly. Silicone would do this (safer than foam that may expand to live wires inside the box).

I caulked all the holes (tabs pressed in to keep the thing in one box shape, as well as the grommet where the power comes in, AND the line where the box meets the drywall as well as where it meets the joist - as you can't reach where it meets the drywall there).

It’s a round box, fit into a round hole. Pretty snug all the way around, and insulation is stuffed all around it. I’m pretty confident it’s sealed up. That was my first thought. I added some insulation on top in the area where the frost was, but still left the soffit area open so air can pass through. Don’t know if that’s gonna help or not

 

[BigJ273]

Maybe bad/missing insulation behind the shower? Or maybe a leak in the window?

Could be missing or lacking behind the shower. We did replace the shower about 3 years ago, but didn’t change the insulation situation. And all the drywall is sealed up appropriately. But that’s not to say the insulation wasn’t always lacking. Like I said, this could have been going on for 40 years, I just haven’t ever been in the attic at 4 am on a cold morning to see it before.

 

[stoveliker]

It’s a round box, fit into a round hole. Pretty snug all the way around, and insulation is stuffed all around it. I’m pretty confident it’s sealed up. That was my first thought. I added some insulation on top in the area where the frost was, but still left the soffit area open so air can pass through. Don’t know if that’s gonna help or not

Insulation is a poor air sealer.
Soffit should be left open indeed.

 

[tlc1976]

Probably a long shot, but I notice a basement or crawl space opening below that general area too. Could there be a heat source under the house in that area? Or could the top side of that opening have damage on the sill plate where rodents have gotten in and chewed up the insulation?

 

[EbS-P]

Call your local library and ask if they have a thermal camera they check out. Worth a shot.

 

[BigJ273]

Call your local library and ask if they have a thermal camera they check out. Worth a shot.

Checked it out w a thermal camera. There’s a small heat source somewhere all the way back near the soffit. Again, this is the corner of the shower. Everything I can get to appears sealed up. I checked it again this morn, 27 degrees, the most frost I’ve seen yet. Still just that one joist area seems to be the most concentrated area of it. A few nails here and there, but nothing like this area. It’s on the wood in this area. I think for now I’m waving the white flag. I guess there’s bigger problems to have w a 40 year old house. My guess is it’s riding up the side wall in the bathroom area. Who knows how long it’s been going on. I’m certainly not tearing down drywall or anything crazy like that. I think I’ve done all I can do.

One more thing….if I can get in this area from the outside, I may pull the soffit and look. Maybe a family of mice?? Who knows…..

 

[BigJ273]

So I think I may have narrowed this down to a vent pipe coming through the ceiling in this joist area. I really don’t wanna spray foam it, in fear of causing another issue I can’t undo further down the road if need be. Is anyone aware of an airtight fabric or something I can stuff around the gap where it comes through the ceiling? I’ve stuffed insulation in there for the time being ti see if that helps, but I know it’s not airtight. Would much rather put something in there I could remove if need be instead of spray foaming.

 

[tlc1976]

So I think I may have narrowed this down to a vent pipe coming through the ceiling in this joist area. I really don’t wanna spray foam it, in fear of causing another issue I can’t undo further down the road if need be. Is anyone aware of an airtight fabric or something I can stuff around the gap where it comes through the ceiling? I’ve stuffed insulation in there for the time being ti see if that helps, but I know it’s not airtight. Would much rather put something in there I could remove if need be instead of spray foaming.

This is what I used to cover a duct in my attic. You can get various diameter sleeves. Or just the roll and wrap it and form it as you see fit. I used the foil tape on the seams to keep it in place.

 

[BigJ273]

I’m hoping this is the culprit. The toilet vent pipe coming up through the ceiling drywall. This is the same joist area where all the frost is forming. Just foamed it all up. Once the foam dries I’ll lay all the insulation back on top. Fingers crossed

 

[peakbagger]

Good detective work.

BTW I ran into someone that claimed that Home Depot rents thermal scanners https://www.homedepot.com/p/rental/FLIR-COMMERCIAL-SYSTEMS-INC-Thermal-Camera-FLIR-i7/316822080

My guess is that its $80 well spent.

 

[Sawset]

likely an air leak from along the walls of the home to the attic, very common. humidity in the
house rise up with warm air leaks through and then you have frost in attic.

Had this happen in our farmhouse. The construction allowed an in wall open space from main floor to attic. It's a two story, and the path from lower level to attic was a chinese puzzle, up one wall, across a ceiling, up more walls. I saw the same localized frost melt on the roof and frost in the attic. But the source was several rooms and floors away on the opposite side of the house. It did take some detective work to figure out.

 

[BigJ273]

I FINALLY got this figured out at the end of winter last year. When we replaced the shower 3 years ago, they had to move the drain over. When they did that, they cut a new square hole into the subfloor obviously to accommodate the drain pipe. They never replaced the insulation around the pipe. The air from the crawl space was rising up into the hole, up the back wall of the shower, and into the attic space, creating the moisture when the warm air mixed w the cold attic air. I filled the hole w insulation, and no more issues throughout the rest of the winter.