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Hi, redoing roof on a low sloped simple gable roof(3/12)with on-the-ridge chimney. Considering using metal roof with insulation on the outside directly over tongue and groove deck. My dilema is the type on venting I require. I want to vent the air space between the rigid board insulation and the metal which will be spaced by strapping. The roof on one side of the house has open rafters/tongue and groove catheral ceiling. The other half has an attic space.(not vented - this is how it was built 35 yrs ago.) Since it snows alot in this area and the snow accumilation on the roof is at times 2-3 feet high, I suppose a ridge type vent like Cor-A-Vent, for example, would not work under these conditions? What would you recommend?
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The building code calls for venting both vaulted ceilings and attics. Vaulted ceilings require a minimum of 1" vented air space between the roof covering and insulation. I would install an air barrier over the existing deck, then ridgid insulation (butted ticht and taped, then an underlayment, then strap the roof vertically over the rafters and then horizontally to accept the new metal roof.
For the attic side it requires 1 sf of free air for every 150sf of ceiling space with 50% provided at the ridge. A vented ridge detail will provide venting for both the vaulted and attic bu you will need to cut the ridge open on the attic side and provide eave or soffit intake air. Ask the manufacturer to recommend ridge venting accessories. Hope this helps.
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You mentionned that I should install an air barrier first over the
existing roof. Question: should I be using a vapour barrier instead?Like a plastic sheet since it ends up on the warm side?
But rigid insulation is already considered as a vapour barrier.(unless I use Cladmate which is not a vapour barrier created for cold climates. The underlayment , should I use roofing paper OR sheathing paper since sheathing paper has a better permeability rate than roofing paper. Its all too confusing, the details , at least...thxPat
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Just wanted to add one idea as a cost saving measure. Since one half of roof has an attic space and already has insulation, would it be feasible to add rigid insulation to the other side of the roof only ?( the cathedral part of the roof) Then strap the whole roof to receive the metal. What do you think? thx Pat
2/5/2007
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2/6/2007
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