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Kramer
Member
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# Posted: 5 May 2010 20:58
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I'm finding it unclear if a vaulted ceiling actually has to be vented in all cases. Some say you can get by without venting it, providing the vapour barrier is of a high quality, others say as long as it is spray in foam (which acts as an impermeable vapour barrier) it will be okay, others say you must always vent it regardless of what you use.
What gives, and why?
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larry
Member
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# Posted: 5 May 2010 21:08
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it has always been told to me that the reason you vent is to allow air flow to the sheeting and also help asphalt type roofing...never did make sence to me.
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Kramer
Member
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# Posted: 5 May 2010 21:43
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Quoting: larry It has always been told to me that the reason you vent is to allow air flow to the sheeting and also help asphalt type roofing...never did make sence to me.
Doesn't really make sense to me either... You don't vent your walls. I can see with a regular trussed roof where you have a large air space that it makes sense.
Perhaps asphalt roofing is not as water tight as one would think? Do the recommendations differ with metal? (Might go the metal route)
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larry
Member
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# Posted: 6 May 2010 07:43
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i think your right about the roofing. it only seem that recently you here talk of venting. is this because building suppliers want to sell us more stuff? as far as the steel roof is concerned, it is attached to furring strips that are nailed to the roof sheeting so it will have an air gap.
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Kramer
Member
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# Posted: 6 May 2010 09:59
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Actually, not all metal roofing has to be attached like that. It can go right on top of sheathing, or an old asphalt roof (IIRC, but I'd be hard pressed to do that), depending on the manufacturer's recommendations. I don't know if the venting recommendations differ in that situation either.
I'm quite confused, and unfortunately all my official literature skips right over the topic of vaulted ceilings.
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SheldonDesigns
Member
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# Posted: 17 May 2010 19:33
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Venting the roof is primarily for colder climates where moist interior air (from human occupation, cooking, breathing, etc.) reaches the underside of the roof deck, condenses and eventually creates mold or rot.
A good vapor barrier on the warm side helps but unless it is perfect, some moist air will get through to the deck. The common industry standard opinion now is that a cathedral roof needs 50% of the vent area at the ridge and 50% at the eave... unless one uses spray foam. The spray foam will adequately seal all paths of vapor migration to the roof deck.
In warmer climates, venting the under side of the roofing or roof deck will tend to keep the roof cooler and add to it's life.
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nyantler
Member
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# Posted: 1 Aug 2010 20:02
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the reason you vent your roof is because heat rises.. and if not properly vented the roof will retain heat which conflicts with lower outdoor temps and causes condensation which cause mold and or rotting of lumber. This is especially true with metal roofing where condensation buildup will run down the underside of the metal like a faucet
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nyantler
Member
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# Posted: 1 Aug 2010 20:04
Reply
Also the best method for venting is a vented soffit with a ridge wentas the heat rises through the ridge vent it pulls in cooler outside air that helps equalize the temperature under the roof
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