venting a roof in cold wet climate
I was hoping someone could help me out with a grey area we’re facing with a home renovation. We’re remodeling a home from 1938 on the coast near San Francisco. During the renovation, we’ve made the ceiling in the master bedroom suite a vaulted ceiling. At first we were trying to decide how to vent the roof with this new configuration, but now we’re trying to decide if venting is even a good idea in this case.
The house, since it’s on the pacific coast, gets a lot of cold and damp air thrown at it by the wind, so the argument against venting the roof would be that it would bring in more moisture than it would prevent. The rest of the house will have a vented attic space.
The roof is spanish clay tile over roofing paper and 3/4″ sheathing, so the weathering surface is already vented to an extent. We were originally going to cut lengths of PVC pipe in half and fit them between the trusses, running from the eaves to a new ridge vent, but the eave detail will make this difficult as well.
Any advice on this conflict would help greatly.
Replies
I work on log and timber homes. 90% of the roofs we do are "unvented."
We commonly use the term "hot roof" when referring to an unvented roof system.
There can be disastrous results if this type of roof is put together incorrectly -- most especially in a cold wet climate!
The SIP industry has been facing this problem for many years now.
The main concern is moisture migration. Interior moisture migrating into the roof system can lead to roof system degradation very rapidly. This problem is exacerbated in a cold and/or wet climate where the roof system never gets a chance to thoroughly dry after experiencing a moisture problem.
The most important consideration is to mitigate this moisture migration through the use of vapor retarders. We use unbroken (no seams) 6 mil plastic between the conditioned space and the insulation. This prevents moisture ladened air from escaping into the roof system.
Eliminating all areas of moisture egress prevents any potential problems with the unvented system.
The other main problem, which you won't have to be concerned with, is finding shingle manufacturers who will warranty their products on an unvented roof system. Although several manufacturers are starting to come around.
I have recently noticed that many engineer / architects are beginning to agree that venting may not be as beneficial as we think it is!
The NRCA has a manual online with some excellent information on moisture control.
http://www.nrca.net/rp/technical/manual/manual.aspx
Good luck with your roof.
Thanks, that definitely helps. I'll bring this up and see if it settles the arguement!
Also look into the resources at buildingscience.com, a very informative site re: roof venting.