Attic frost

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BigJ273

Minister of Fire
Feb 15, 2015
711
Maryland
So, while letting the dog out this morning, I noticed a spot on the roof that was not as frosted over as the rest of the roof on the shingles. Temps got down into the 20’s last night. My curiosity took me into the attic, where I located frost on the underside of my roof, near the soffit. I attached a photo

Clearly warm air was making its way up into the attic (no rain recently and a new roof-3years ago). I suspect it traveled down to the lowest, coldest point, st the soffit intake, where it froze. Several nails were frozen over also.

I’ve never seen this before. Not saying it’s never happened, I’ve just never noticed it. I suspect the attic door is the culprit. It’s in the area of the frost, a few feet away, and doesn’t have any insulation. We’ve been getting the house up to about 77 before going to bed. I’m wondering if that warm air is seeping in, finding the coldest point near the soffit, and freezing. I’m gonna get an attic door insulator today to see if that helps.

The only other thing that has changed is we recently switched our heat from oil to propane, oddly the tanks sit under the soffit in this area. But I doubt they produce anything that could cause this. They have all the required installation clearances.

The bathroom fan is vented through the roof, and insulated. It appears to not have any leaks. It’s about five feet away but no moisture in its immediate area. I did replace the soffit vent in the area of the frost, cause it was looking a little rough.

Any thoughts??

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If that’s the only spot with the ice (and no frost on the roof) then the heat (warm air) must be localized to that location. I’d be pulling the baffle and the insulation to see what hiding under/behind it. I want to start air sealing my attic.
 
If that’s the only spot with the ice (and no frost on the roof) then the heat (warm air) must be localized to that location. I’d be pulling the baffle and the insulation to see what hiding under/behind it. I want to start air sealing my attic.
I pulled it. Nothing shocking, thank god. Insulation was not even wet.
 
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I'm assuming the attic door is in the same joist space as that frost? I wonder if the air is going straight up, then going down along that section between the joists, and freezing like you mentioned?

I've got to reseal mine as well... they leak pretty bad.
 
It is in the same joist space area. I just assumed the leaking warm air would rise up through the ridge vent, and not pool down near the soffit. That’s the confusing part for me
 
Does seem strange. You say out noticed the outside was 'not as frosted' but when you went in the attic this area WAS frosted, but not others? So sort of a reverse of the outside?

It would seem like the warm air should rise and go out the vent. But it would really be humidity in the air creating the frost - so if you have moist air in the attic, that moisture can condense or frost anywhere it is cold enough. That could even come from typical day/night cycles, not necessarily humidity from the house.

Curious if there is any tree or shade over that part of the roof which might lead to it getting a bit colder than average? Or maybe it has more northerly exposure? Anything which would make frost preferentially form there?

Either way, two things to do with moisture in the attic... stop it from getting in...as you mention a better seal on the attic door. Other thing is to get it out. You mention ridge vents, so hopefully they are clear. Are the soffit vents clear so air can easily pass in? Looks like you have those air duct diverters - so good deal there. But sometimes the soffit vents can get plugged, painted over or have new soffit installed with no venting. So something to consider there, too.
 
No trees on that side of the house. It is on the north side. And yes, on the outside it looked like the roof was warmer in that spot. So possible warm air (from the door area) hitting the underside warming it up, then mixing with the freezing air coming on through the soffit.??? Idk. The soffit vents are clear, all looks good there. The only way moisture could form on the underside of the wood is if warm air hit the cold surface, at least I would think. Then it’s freezing. And the only place warm air could be coming from in that area is the door. There was no insulation on the door, just wood. And I’m wondering if the fact that we had it up to 77 with the wood stove made a difference with REALY warm air seeping up there. I checked the entire attic, this was the only spot. Oddly, it is directly over our shower, but no one had showered yet, not since yesterday morning. I’m really not thinking it is the cause, because I’ve never seen it before

I bought an attic door insulator today. And it’s supposed to get into 20’s again tonight. So I guess there’s only one way to find out!
 
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No trees on that side of the house. It is on the north side. And yes, on the outside it looked like the roof was warmer in that spot. So possible warm air (from the door area) hitting the underside warming it up, then mixing with the freezing air coming on through the soffit.??? Idk. The soffit vents are clear, all looks good there. The only way moisture could form on the underside of the wood is if warm air hit the cold surface, at least I would think. Then it’s freezing. And the only place warm air could be coming from in that area is the door. There was no insulation on the door, just wood. And I’m wondering if the fact that we had it up to 77 with the wood stove made a difference with REALY warm air seeping up there. I checked the entire attic, this was the only spot. Oddly, it is directly over our shower, but no one had showered yet, not since yesterday morning. I’m really not thinking it is the cause, because I’ve never seen it before

I bought an attic door insulator today. And it’s supposed to get into 20’s again tonight. So I guess there’s only one way to find out!
Sweet, I look forward to hearing how it works out!
 
No trees on that side of the house. It is on the north side. And yes, on the outside it looked like the roof was warmer in that spot. So possible warm air (from the door area) hitting the underside warming it up, then mixing with the freezing air coming on through the soffit.??? Idk. The soffit vents are clear, all looks good there. The only way moisture could form on the underside of the wood is if warm air hit the cold surface, at least I would think. Then it’s freezing. And the only place warm air could be coming from in that area is the door. There was no insulation on the door, just wood. And I’m wondering if the fact that we had it up to 77 with the wood stove made a difference with REALY warm air seeping up there. I checked the entire attic, this was the only spot. Oddly, it is directly over our shower, but no one had showered yet, not since yesterday morning. I’m really not thinking it is the cause, because I’ve never seen it before

I bought an attic door insulator today. And it’s supposed to get into 20’s again tonight. So I guess there’s only one way to find out!
could be moisture built up over time absorbed into the wood/insulation from the shower area. Do you have a moisture meter? Take a reading just below the roof decking on those joists, then take a reading 15' away.
 
could be moisture built up over time absorbed into the wood/insulation from the shower area. Do you have a moisture meter? Take a reading just below the roof decking on those joists, then take a reading 15' away.
I do. The insulation and drywall are completely dry. No indication of moisture buildup or staining
 
29 degrees this morn when I got up. Removed the insulation cover and went into the attic. No signs of moisture or frost at all. Hopefully that resolved it. Still amazes me that a little bit of heat like that going through the door area like that can produce that much moisture, and settle in the soffit area.
 
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Nice fix!
Soooo….gonna have to keep an eye on it. I’m fairly certain when I initially went up this morn I checked everything real good, including nails, and everything was dry. But after being up there for a while, the nails began dripping, just one or two of them towards the very back near the soffit air entrance. If my theory is correct, that is hopefully because the door was wide open while I was up there for about ten minutes, allowing the warm air up

This would confirm my initial theory, just at a much faster rate due to the door being wide open, and not just in adequately sealed. That is of course, only true if everything was in fact completely dry when I initially went up. Pretty sure it was 😉, but when I saw the drops I started second guessing myself.

Unfortunately it doesn’t get below 30 here for another week or so. So I’ll keep an eye on it!!
 
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Soooo….gonna have to keep an eye on it. I’m fairly certain when I initially went up this morn I checked everything real good, including nails, and everything was dry. But after being up there for a while, the nails began dripping, just one or two of them towards the very back near the soffit air entrance. If my theory is correct, that is hopefully because the door was wide open while I was up there for about ten minutes, allowing the warm air up

This would confirm my initial theory, just at a much faster rate due to the door being wide open, and not just in adequately sealed. That is of course, only true if everything was in fact completely dry when I initially went up. Pretty sure it was 😉, but when I saw the drops I started second guessing myself.

Unfortunately it doesn’t get below 30 here for another week or so. So I’ll keep an eye on it!!
Got a spare GoPro? Just leave it up there for a few hours ;) LOL
 
I would throw a remote thermostat with humidity sensor up there and see if you can pin down anything.
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I would throw a remote thermostat with humidity sensor up there and see if you can pin down anything.
That’s my plan. I think Santa is bringing me some. I’ve sealed up everything I can see or find. I’m thinking it has to do w it being right in the center of the north facing side of the house. It has decreased since I added the door insulation, but I’ve still had a few dripping nails. It’s probably always done it, I’ve just never noticed. The house is 40 years old. The roof is only about 4 years old, has ridge vents and proper soffit.
 
25 degrees this morn, 92 percent humidity. Frost covering everything outside. I decided to check the attic. It was again frosted over in the same spot, the sheathing underside, and the nail heads. I even noticed a few more nail heads that weren’t frosted over previously, were today. I’m perplexed. All on the north side. Temp inside the house was 68, wood stove ran all night, so it was dry heat, no humidifier on.
 
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. and yep those attic access pull downs leak like sieves. you could maybe rent a thermal scope to pin point hot spots (leaks) up there or any where else on your place..
 
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See if the top board of the wall under the ice has a hole in it where romex goes down to e.g. an outlet. (And then seal the hole.
 
What's outside of that spot on the house? Window, door, patio door, etc? I had an issue where i was seeing mold forming on the underside of the sheathing on my roof in one particular spot on the north side also, and noticed a lack of frost on the roof on cold mornings but couldnt figure out why. Then i had the roof replaced but the top side of the plywood sheathing was perfect but the underside had evidence of moisture. I came to the conclusion that me having the sliding door open a bit in my bedroom was allowing warm, moist air to flow out of the house, up into the soffit where it's condensing on the cold sheathing.
 
What's outside of that spot on the house? Window, door, patio door, etc? I had an issue where i was seeing mold forming on the underside of the sheathing on my roof in one particular spot on the north side also, and noticed a lack of frost on the roof on cold mornings but couldnt figure out why. Then i had the roof replaced but the top side of the plywood sheathing was perfect but the underside had evidence of moisture. I came to the conclusion that me having the sliding door open a bit in my bedroom was allowing warm, moist air to flow out of the house, up into the soffit where it's condensing on the cold sheathing.
The bathroom window is on the outside in this area. Never open in the winter tho of course. The shower is directly under this spot, so my assumption is that has something to do w it. Although, it has occurred on mornings when no one has even showered yet. The bathroom vent is pumped directly upwards through the roof, and is insulated. The lack of frost on the roof is not near the vent, but more towards the shower area. I’ve attached a pic from the outside. The circled area is the warm spot that doesn’t have frost in a cold morning when the rest of the roof does. My guess is it’s something to do with warm air coming up from the bathroom area somehow, even when the shower has not been used recently. I’ve checked all the areas around the fan, and it’s all sealed up. But really, without something Infrared, it could be coming from anywhere. Probably the wall area on the outside portion of the house, rising up and hitting the cold air being pulled into the soffit. But again, it’s just this one little spot. Perplexing

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