A low slope, cathedral roof in Zone A-4: builder says he can vent it
Hi! I've been following a variety of discussions about ventend/unvented assemblies in cathedral ceilings, in low slope roofs, in different climates, on FHB, JLC for close to two years. I’ve read Martin, I’ve read Lstiburek, I’ve talked to the architects (hopeless) and structural engineers. I thought I had this nailed down, and now the builder (who also happens to be a SE - long story), comes at me with a new idea.
So I’m finally going to “vent” my worries and seek some advice specific to our situation.
This is new construction with a big, simple shed roof, slope of 1.5":12" (7.13 degrees, or 12.5%) There will be 3 skylights, which cut across the slope/plane of the roof, so they will block/interrupt some 15-20% of the rafter bays. With overhangs, the roof is about 44 1/2’ long and about 47’ wide. I’m attaching a sketch which shows the shape and proposed framing.
Currently we are planning for the entire second floor to be cathedral ceiling, because there is little sense in framing in an attic floor due to the low slope of the roof. There is no need for ceiling penetrations except for bathroom exhaust fans and the skylights (one of which is in a shower with a cathedral ceiling, just so you know). We’ve found flush light fixtures we can live with, so no recessed cans.
We are just outside of Philadelphia, in climate zone A-4. Roof system will be modified bitumen. We are not using spray foam in the house, so don’t recommend that. Walls will be insulated with cavity batt with 1” continuous ISO exterior.
I thought that the best way to handle this roof would be an unvented, "hot" assembly with 3" of ISO above the deck (code requires R-15 here), and fiberglass cavity insulation. No vapor barrier to the inside so that the assembly could dry to the interior. I assumed GWB with proper sealing around the few penetrations would be a sufficient air barrier (would it?)
Our builder/SE is now encouraging us to consider venting the roof assembly. I think most of his reasoning relates to the cost of the ISO. What he wants to do is to place 2x furring crosswise against the roof framing to create a ventilation channel above the (R-38 batt) insulation and under the roof decking. If I understand it properly, the idea is that those channels are not exactly continuous, so they create a pathway for air to circulate and - I guess we hope - move around enough to actually vent the roof. He’s trying to save us money (he is a construction manager as agent working for a flat fee, so the savings will be ours, not his).
The only thing I've seen that is close is Lstiburek's vented-unvented hybrid assembly which adds a vented air space above the exterior rigid insulation (so still uses the ISO in any event) in high snow load areas to avoid dams. I think that system is prohibitively expensive for us.
We’re going to put this out to roofing contractors with both options to compare the prices. Our builder is sure that it will be far, far less expensive to do it the way he suggests, given the price of 3” of ISO. I’m not attached to one particular solution. I just want it to work and, yes, money is a definite concern. Can the system he is proposing work, ever? Is there any amount or configuration of above-rafter furring that can effectively vent a roof of this slope? Alternately, if we use the unvented assembly I'd settled on initially, what ABOUT ice dams in this climate?
Any advice is greatly appreciated.
Posted Mon, 04/07/2014 - 14:07
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