Question about whether venting is needed on this roof.
I’m a fairly accomplished do-it-yourself builder, having just completed a second house. (If you’re tryly interested, I have a blog at www.beatricedohrn.blogspot.com) But this question is in aid of some friends of mine. I’m never clear on the situations in which a roof need not be vented:
The house in question is a poorly insulated log home in the pacific northwest (not that cold, lots of rain). Years ago the owners had rigid insulation put on top of the tongue and groove roof/ceiling deck, then a new layer of ply and then shingles. The owner put vents into the soffits, thinking they were needed, and there is a ridge vent, but no vent space below (or above) the rigid, so not likely that the vents are moving much air. THey are currently doing some work (including possibly replacing the shingles with metal roofing) and wondering whether they should stretch to creat maybe a 1/2″ vent space under the rigid — which the contractor feels he might be able to create. There is some evidence of moisture at the soffit but it happens to be where there’s a bathroom nearby, with a fan…. so really not clear what’s the cause or whether it’s major. That same side of the roof is very shaded by trees, while the other sidse stays dryer, when it is dry. No evidence of rot, and no moisture seeps through the pine tongue and groove deck, which is also the ceiling inside.
Adivce? THanks in advance.
Replies
Clearly, the answer is, "It
Clearly, the answer is, "It depends..."
The soffit and ridge vents are now doing nothing, in theory at least.
I'd be curious to know how thick the riigid foam is, what kind it is, and what their degree day heating zone is.
Also whether there is an air abrrier under th e foam and over the T&G. And if, when installing the foam, they were careful to have perfectly fitted and sealed joints and used spray faom to seal/glue them together.
But as a general rule, there would be benefit to adding furring over the foam and under a metal roof to create a cold vented roof
As much as I know about what's there
Thanks for helping out. I think of you whenever I use a sheetrock screw!
I think the foam is 2", and there is tar paper under it, over the t & G. I'm nearly sure it would have been layed , perhaps with overlap, but without other sealing. The proposal of the GC now is to put strips under the plywood that would hold the metal roof. Is that what you mean when you say :under metal roof" or did you mean directly under the metal?
Go too the Energy Star web site and find out exactly which climate zone you are in, and what amount of insulation is called for in that climate.
Then add the appropriate amount of new foam board OVER the existing foam board to get to the desired R value. If you want to still vent, tape the foam joints, put 2x4 furring over the last layer of foam, put a deck on that. Use the air space above the foam as the continuous vent channel and put in ridge and soffit vents. This may add 6" to the height of your roof deck, but you will have a completely air sealed insulation layer (no worries about air leaks in the attic anymore!) AND you have yet another waterproof layer protecting your house.