Attic frost

nflliferJanuary 20, 2009

I have a 2 story built in 1977. One of the bedrooms upstairs has been closed and the heat turned way down. Entered and found a small amount of mold on one exterior wall. After investigating I believe the moisture is coming from the attic. I live in Minnesota. Opened attic and found frost almost everywhere!

Last summer roof was replaced, vents were plugged a new ridge vent was installed. Soffit vents were replaced last fall. I'm going to check to make sure no soffit vents are plugged with insulation, and check for a vapor barrier below the insulation.

Any other suggestions/ ideas? I've called 5 experts and they all said the same thing, but none of them work on this.

I'm guessing I will install a solar attic fan. Frustrated and dealing with a pregnant wife who is terrified of mold.

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Ventilation in the winter is unlikely to help. You are probably getting excess moisture in the attic from a bath exhaust fan or ice dams at the roof eaves. If the roofer didn't put Ice & Water Shield at least 3 ft horizontally back from the outside wall, he should come back and do it for no cost.

    Bookmark   January 20, 2009 at 10:48PM
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Your moisture is not coming from the attic, it is going into the attic from your living space. If you are in a cold climate, the outside air is BONE dry with a dew point below freezing.

The mold is from inside air having moisture in it and migrating to your unheated room, condensing on cold spots on your cold walls. Clean off the mold with bleach solution. Heat the room or open the door occasionally to reduce mold. Also, keeping the humidity down in your home with good cooking and bathroom vents will help.

The frost in your attic indicates that you may need better ventilation out your roof. This frost could lead to mold in your attic - another problem. A ridge vent could accomplish this. That still won't prevent the moisture/mold in your unheated bedroom. Only reducing your interior moisture levels will help.

    Bookmark   January 21, 2009 at 8:16AM
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Sorry I missed the part of your post about already having the ridge vent.

I would modify my previous post to say that you should focus on reducing interior moisture levels with bathroom and kitchen vents.

    Bookmark   January 21, 2009 at 8:19AM
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It sounds like a bath or kitchen fan is venting directly into the attic. If that is not the case go into the attic with a can of spray foam and foam around the vent stacks where they come into the attic,. Also look for any openings on top of interior and exterior partition walls for holes and seal, look to see if you have any return air ducts that run into the attic and seal around them. All it takes is a few small les to add of the a very large hole to let interior air into the attic.

    Bookmark   January 22, 2009 at 10:26AM
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Wanted to follow up. Was in the attic during the day and able to see all the soffit vents are free.

The moisture coming into my home and walls are from the frost (frost is in the attic, but it's from the attic being to warm)

I have mold on one interior wall, and about 6 brown tar streaks outside on my siding.

I went up on the roof and shoveled and swept the ridge vent. The vent was plugged completely. Some parts snow, others ice.

This morning I entered the attic again, with the roofing company. They were in shock, never had they seen anything like this. It has gotten better and the temp is much cooler. There are no bath fan leaks, I have the proper moisture barrier, it's all from warm air not being able to escape the attic.

Their solution is to shovel the vent when we get a snow/ cold patter like this. Really pretty rare for MN. They admitted they see similar problems with the vent, but typically it's in March when we get heavy snow.

They don't recommend installing any turtle vents. Reason is to mess up the ridge vent air vorex. They are going to get back to me, but thought a vent that closes, and operates on a light switch in the winter would be my best option.

Any other suggestions?

My advice is if you don't have a steep pitched roof and live in a snowy cold area, stay away from ridge vents.

    Bookmark   January 22, 2009 at 10:27AM
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Just trying to unnderstand.

1) as someone else said: Cold winter air is typically very dry so air infiltration from the the outside is not a likely source of moisture which would form frost. I believe that moisture in the air is from the heated interior spaces. What kind of heat do you have?

2) For years, (cold or no cold) it was not common practice to install ridge vents. The intent of the ridge vent is cause air flow out the top (releasing the heated air) and subsequently draw in air from the soffit vents, thus cooling the attic space. Blocking the ridge vent would slow/stop this process but it would result in a situation similar to how attic spaces were "vented" for many years (usually just some louvered vents near the top of the end walls.) Not everyone had frost/mold in thier attic space.

As I see it.
Your attic is cold enough to support frost formation. There is too much moisture in the air which condenses and freezes.

I don't believe that you've identified the source of the moisture.
You might try putting a dehumidifier in the attic space to dry it out.

Anyway. good luck.

    Bookmark   January 22, 2009 at 10:52AM
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Attics are suppose to be a very similar temp to the outside temp during the winter. Heat loss from your home goes into the attic, and needs to go outside as quickly as possible. With the ridge vent blocked the heat would rise and condensate. The soffit vents are open, but the pressure from the warm air trapped in the attic appears to be preventing cold fresh air from entering.

A dehumidifier shouldn't work in an attic. The temp should be below freezing and cause the dehumidifier to freeze up. If I found one that would work below freezing, the moisture it collects should turn to ice.

On Tuesday night it was 48F in my attic, and around 10F outside. Heat loss appears to be normal for the interior of my home, it's just not escaping my attic quick enough.

They want to install an attic fan that I would turn on when my vent is covered.

    Bookmark   January 22, 2009 at 2:21PM
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You migt find this article interesting on the effectivness of ridge vents

Here is a link that might be useful: roof vents

    Bookmark   January 22, 2009 at 2:38PM
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what you say is true but you left out one really significant factor --> insulation.

Insulation on the attic floor should keep most heat inside the heated space. If your entire attic space is close to 40 degreees warmer than outside, I'd say you're doing a pretty good job of heating it (and also bringing in the moisture from below).

If you really want to just get the heat out then a power fan is a good idea, but your heating bills will likely go up too as your heat rises up there. Also if there's that much heat up there it should be melting any blocking snow and worming a hole to the exterior at the ridge. As we all know heat rises and if snow is in contact with something 32 degrees or higher it melts. Snow itself is a good insulator and usually the bottom layer is the warmest.

    Bookmark   January 22, 2009 at 3:49PM
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I've seen this problem when bath exhaust ducts terminate at an eave soffit and moisture gets blown back into the attic or when the wind blows snow into the eave vents. This is especially a problem in very cold weather like we have recently been having.

But I would still not rule out roof leaks in that area; sometimes it takes more than one source of moisture to create that much frost.

    Bookmark   January 22, 2009 at 5:04PM
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Thanks for the article rjoh... As for my insulation. When the roof was installed last March I had an insulation company present. I had planed to add insulation at the same time. The insulation company said I had plenty of insulation and only suggested replacing soffit vents. (which they did) When I was in the attic checking my soffit vents I did find a vapor barrier on the bottom of my attic, followed by 12 inches of blown in insulation, and then rolled insulation with a paper backing facing up. The attic is 40 degrees at the base, obviously the frost is freezing. I've literally spent 10 hours reading articles on line, and posting on other forums. Still a long way from any type of expert, but the understanding I now have is ridge vents are great for hot areas, terrible for snowy areas. Snow and cold plug them up.

As for warm air melting the snow... It probably would work for more mild winters, but Minnesota just had a 5 day stretch where we didn't crack zero... 32 degrees below the freezing point, and the wind chills hovered around 40 below.

    Bookmark   January 22, 2009 at 8:53PM
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Against my better judgement, I let the guys who redid my roof talk me out of any sort of ice dam barrier, "we don't need it in Kamloops BC" needless to say we've had a cold winter and I have some pretty impressive ice dams at the moment. We have also not seen the sun in a couple of weeks but I'm not sure if that's a good or bad thing from this perspective.

Now that it's mentioned, I've seen what looks like tar staining on my siding on the north side of the house, where we tend to get ice damming the worst (I purchased a roof rake to pull the snow off there because it was tending to drip on the driveway and form ice, I think I'll have to use the rake more) and right now we're getting icicles coming from BEHIND the gutters. Gutters need to be replaced but I've been putting it off because I'd like to get the fascia done, and don't want to do it myself because a) front of house is scarily high and b) I want the continuous stuff. When roof was redone they put on bigger, more generous drip edges than were there, so they overhang gutters in places.

As a stopgap I've put some aluminum sheeting under one small valley that did not have enough drip edge, poor design and execution. It was a good move because it's kept the icicles out and away from the fascia, neighbour with similar roof has quite a lot of damage and weathering to the woodwork there, and a huge sheet of ice.

I have the new vented soffits everywhere (not well attached, they have drooped in a couple of spots but I guess that's another story) and two whirlybird ventilators (I don't think a ridge vent here is an option) that seem to work, I also have gable vents that may or may not be cosmetic, they have some sort of plastic covering inside over what appears to be flyscreen, I wondered if the plastic covering can be removed, then thought perhaps it's like tyvek paper? Any idea? If the covering can be removed, that should add some ventilation. There is a reasonable overhang above them, but I want it to be bug proof, I've also been concerned about wasps entering roof space via the ventilators on still days, and have noticed a little detrius underneath them which made me wonder if they might leak or allow snow in under some circumstances? They appear to be in good order and the rotating collars (where you adjust their angle etc) have all been sealed. I think I may have seen some sort of weather/bug covers for them which can be fitted inside, I had also considered just stapling bugscreen from underneath....

I am also wondering about the bathroom vent, it does vent outside and I'm pretty sure tube through attic is insulated, however, I've noticed that there's always a clear spot on roof around it in terms of snow, and wonder if that's a bad thing as it infers melting...and the spot where the damming seems worse is downhill from it.

The vent on the roof itself for the bathroom is one of those brown low-profile vents where it turns over on itself and points downhill, needless to say that's where the bulk of the snow-free area is.

Should I change the vent for something that sits more above the roof? Would insulating the underside of the roof in that spot helps?

Apart from the possible staining on the cladding where the damming appears worst, I've also noticed regular dark marks on the ceiling inside where this spot is. Doesn't appear mouldy or anything but they do look like they could coincide where the rafters are. I have a laser non-contact thermometer, I think I'll look and see if they are cold spots.

Assuming the roof is in order (and is warrantied at the moment, was only redone one or two years ago) I know insulating the roof space better (only one layer of fibreglass batts in there, I plan to get blown-in once I add the recessed lighting I want to put in the ceiling) is probably one of the more important things.

Just frustrated. I am the sort of people who plans and is prepared to go to further lengths for the unexpected, it's frustrating when I'm dealing with people like the roofers who insist x or y isn't necessary because our weather/conditions don't do x or y, then it's a big surprise when we get freak weather.

I don't like those sorts of surprises but I had enough problems convincing people I need an expansion tank on my hot water system (ended up doing it myself) - I would have been better off doing my roof myself - would have taken a few days but I know I'd have a roof I could rely on...

    Bookmark   January 23, 2009 at 12:18AM
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"rolled insulation with a paper backing facing up."

You got a problem there. You have 2 vapor barriers in the attic. That maybe part of your problem. I would remove or slash the paper vapor barrier on the fiberglass insulation.

The second vapor barrier may be trapping moisture that gets past the first vapor barrier. I would check the blown in insulation and see if you got any wet spots.

Everything i have ever read about insulation is you do not
put kraft faced insulation as the second layer in the attic. If it is the first layer the kraft face, the vapor barrier goes to the warm side. That is one of the biggest mistakes people make in floor insulation. they put the kraft paper down so the can staple it in place. It creates a dead area where moisture gets trapped and causes problems.

    Bookmark   January 23, 2009 at 4:01PM
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