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codyg 01-06-2011 12:30 AM

Attic Frost
I am getting about 1 inch of frost on my rafters and board. The attic is about a 4/12 pitch 56 x 28 feet. I have two roof vents one whirlybird and gable end vents, also ran 4' sheets of the venting styrofoam to my soffit for extra air flow, spray insulation about 16-20 inches thick yet i still get the frost build up. I live in northern canada the air is very dry and cold in the winter but when we get a chinook the frost starts to melt and leaks water into my house looking for ideas? i can provide pictures if needed


Slyfox 01-06-2011 07:21 AM

There are several issues that will result in frost forming in your attic/crawl space.
Roof Leak.
Non-sufficient ventilation.
Warm air escaping the living space.
Bathroom/Kitchen/Laundry room vents exhausting into the attic instead of out of the home.

Pictures would surely help.

codyg 01-08-2011 12:01 AM

I have tonnes of air flow in attic everything seems to be sealed off well not sure what else i should look at there is quite a bit of moisture in my house but the ceiling was just recently re-vapour barriered so i do not think it is coming from there.

mem 01-08-2011 08:46 AM

Slyfox is right on the money. Your frost is nothing more than moisture from the air condensing and freezing on the cold roof structure, but you knew that. The moisture has got to be coming from somewhere. What exactly did they do to re-vapor barrier your ceilings?

Barring a roof leak, I would hazard a guess that the frost is heavier towards the eaves. With the venting styrofoam installed I would presume you have soffit vents. They may be clogged. Also, gable end vents are not the best option for use with soffit vents as they tend to 'short circuit' the air flow from the soffits. Air exhausting from one gable end will more easily be replaced with incoming air from the other gable end rather than from the soffit vents. This leaves the area near the eaves with a higher moisture content, condensing and freezing. If this is the case, and photos would be helpful, the solution would be to block off the gable vents and add sufficient venting to the peak with, ideally, a ridge vent or enough roof vents to get a minimum of 9 square inches of free vent area per foot.

codyg 01-08-2011 11:07 AM

I have two roof vents as well as a whirly bird so there is a lot of air flow in the attic i just redid all my soffits this past summer and re-drywalled my ceiling complete reno of the house u could say there is a 6mm vapour barrier between drywall and straping and tuck taped galor did it myself. the frost builds up more towards the peak as i can look down the styofoam draft vents and see very little frost its been snowing lots here so hopefully gets some better light so i can get decent pictures to show


Gary in WA 01-08-2011 12:08 PM

Wire the movement stopped of the whirlybird. Use the bathroom and kitchen fans more often and longer because you are not exhausting the moisture in the attic coming from below fast enough.

Air seal the attic, then use the fans below less often.
The w.bird is pulling air from the easiest/closest inlet, the gables as they are a larger volume (surface area) compared to the soffits. Without the action of the w.bird, the gable vents act as exhaust because they are above the inlet and have air moving through them from ‘natural” and “stack effect” air. With a wind directly towards a gable vent, inlet wind will go through the attic to the other gable acting as an exhaust as it’s on the leeward side, reducing the air from the soffits;

Page 603-618;

Check your attic NFVA for a balanced system, you sound way undersized (especially the exhaust where most the frost is);

List the sizes and location of all vents and we can help you more…..


SPS-1 01-08-2011 12:54 PM

I also have to figure you are getting moisture from the living space entering your attic. Is the frost evenly dispersed throught your attic, or is it more concentrated in some areas, such as over a bathroom maybe, or close to the eaves ?

mem 01-08-2011 04:48 PM

It is very difficult to completely air seal off the ceiling from the attic space. Eliminating every single crack or gap around electrical boxes, can lights, wiring, plumbing and whatever else is next to impossible. All you can really shoot for is to minimize the migration as much as you can. By the way, what do you mean by "tucked taped galor"?

You say the worst accumulation of frost is near the peak. I'm guessing just above the level of the gable vents, eh? This would indicate little air movement in that area and a continuous ridge vent with all other vents blocked off (except the soffit vents, of course) would go a long way to help alleviate the problem.

GBR posts some very good links here. Your best advice will probably come from the last paragraph of his 57_e.pdf link tho.

codyg 01-17-2011 12:14 AM

The whirly bird i installed after the first year of frost so it has not made a difference as far as sealing of all excess air flow too the attic is done to the best i can by tuck taping around outlets and so on. there is one bathroom fan that is run through the attic out to the gable end has hard pipe and insulation around it nothing else vents in the attic.

The draft comes up well through the soffits as it is -35C right now and i was up there and you could say it was a bit drafty.

The only other thing i could come up with a was looking for some thought is that i have an external chimney on the outside of the house and a little bit of heat comes off of it in the attic maybe 3-5C if that would this cause my frost problems the air up here is quite dry?

Extra notes i run two wood stoves in my house one main and the other just for comfort in rec room.

mem 01-17-2011 11:20 AM

I imagine the air up there is very dry at -35C. So where do you suppose the moisture is coming from? It's got to be from your living spaces. Your wood stoves are not helping either, as the two main by-products of combustion are carbon dioxide and water vapor. Do you have pots of water on the stoves for comfort? But you have to be comfortable in your home and you seem to have done what you could to seal things off. The solution is better ventilation.

As mentioned earlier, the gable end vents are probably short circuiting the air flow from the soffits. The incoming air never reaches the peak and moisture collects there and freezes on the roof underside. The roof vents you have seem to be woefully inadequate as well, and not the best option anyways.

I still think your best solution would be to install a continuous, baffled ridge vent. This would insure the incoming air follows along the bottom of the roof decking and carry the moisture laden air with it out the peak. You would also need to block off the roof vents and gable vents after the ridge vent is installed. Be sure to do your homework to size the ridge and soffit vents appropriately for your region, and pay attention to suggested ratios of ridge to eave vents. The soffit vents should be somewhat larger capacity the the ridge to prevent air being sucked up through the ceiling.

This site looks to provide some good information:

And where are those pictures you promised? They may provide more detail of your situation and help people here help you better.

handy man88 01-17-2011 06:29 PM

It could be caused by your bathroom exhaust fans vented to the soffits or worse, into your attic.

codyg 01-20-2011 02:39 PM

for some reason it will not let me upload pictures not sure why ?

mem 01-20-2011 03:39 PM

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