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alberta boy 03-23-2010 08:16 PM

cathedral ceiling vented or not in canada
Hi I am building an A frame cottage with vaulted cathedral ceiling in Manitoba Canada, the question I have is , I am insulating it with roxul insulation (green Stuff) Does my roof need to be vented or can it be just sealed and insulated ? We are going with shingles on the roof also . I heard that the venting can cause condensation and water problems but non vented roofs cause heat damage to the shingles . Is any of this true ?

jlhaslip 03-23-2010 09:15 PM

from their site

maintain ventilation space. Can't have too much.
I would strap across the top of the rafters to get cross-bay ventilation in addition to the 1 1/2 inch above the batts because of the cathedral ceiling.

Lack of ventilation will cause condensation. You want the air space above the insulation to be the same temp as outdoors. Soffit and ridge venting is required.

Bob Mariani 03-24-2010 06:32 AM

you will find arguments both ways. In areas with a large change in outdoor and indoor temperatures with very cold temps and a lot of snow it is best to have adequate ventilation. Normally with open cell insulation you can eliminate this venting, but under some conditions like keeping the room to hot and excessive snow loads on the roof (6' +) you will get ice damns. So on the safe side I always recommend balanced venting. What is even more important is to get enough insulation over the exterior wall top plates and still not block the air flow from the soffit vents to the ridge vents. Avoid uncovered fiberglass in this area since wind will simply flow through it freely. Also be sure every penetration to the attic floor (if you have one) is sealed. Avoid recessed lights in a cathedral ceiling. If you cannot, use low profile IC cans with LED or CFL lamps.

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