Why is frost forming on roof sheathing in my attic & how do I fix it?
November 22, 2007
Q: Last winter I noticed signs of white frost forming on the underside of the roof sheathing in the attic. Is this normal or is there something I can do to prevent it? Is this a job that the average homeowner can tackle?
A: Warm air from your home carrying high humidity during the winter months will be pushed up in and around light fixtures and the attic access allowing it to condense and freeze on the underside of your roof sheathing and turning in frost. The reason this is simply that your home has not been draft proofed to stop this air leakage and is possibly lacking sufficient roof ventilation. For more information on draft proofing your home contact Natural Resources Canada @ 1-800-387-2000 and ask them to send you the pamphlet on “Air Leakage”. Follow the outlined instructions on how to draft proof properly. To correct the roof ventilation, calculate the square footage of the insulated area and divide this by 200. This will give the amount of square feet of sufficient roof ventilation. 50% percent of this calculation is required in the soffit venting (overhang) and the other 50% on the roof, preferable along the ridge if you have a gable roof. You may be required to use mushroom or gable vents depending on the roof design. Contact the HouseSmart Referral office if you would prefer to have a company perform the draftproofing for you at 604-542-2236.