Venting a hip roof
I’m hoping someone can tell me if I’m asking for the impossible. We’re renovating an 1860’s building with a hip roof. Our intention is to insulate the roof with 6″ of rigid foam and leave the rafters exposed on the interior (meaning there will only be insulation above the existing roof deck and no sheetrock, insulation or paneling installed inside). The very narrow (approx. 5′) of ridge at the “peak” is broken up by skylights. So far only one of the 5 roofers I’ve spoken to has included any venting in his plan and he calls for prefabricated metal vents every other rafter bay.
Is there no other option? Are products such as Hipmaster, SmartRidge II, and others ineffective? Or are they not appropriate for this situation? What about a product like DCI’s RafterVent? Could this be used to allow air to flow towards a hip vent?
I’m open to any option that will work in this situation or will stop my search if you can give me a definitive, “sorry, no way, no how”.
Thanks.
Kim Dolce
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Replies
Kim,
The roof you describe doesn't sound like a candidate for a vented approach. I would go with an unvented roof.
I don't know what climate zone you are in, but 6 inches of rigid foam isn't much. If you're using EPS, that's about R-24. If you are using XPS, that's about R-30. And if you are using polyiso, it's about R-30 or (at most) R-39, depending on whether you have the de-rate the polyiso performance due to its performance at cold temperatures.
Martin,
Would that mean insulation above the existing roof deck followed by new layer of plywood then whatever underlayment and shingles? Or am I missing something?
As for the R-value, we know we'll be on the shy side.
Kim
Kim,
You have three basic choices:
1. Install one or more layers of rigid foam, followed by a second layer of roof sheahting (usually plywood or OSB, although 1x4 or 2x4 purlins, 24 inches o.c., are possible for some types of metal roofing).
2. SIPs.
3. Nailbase.
For more information, see How to Install Rigid Foam On Top of Roof Sheathing.
In any climate zone 6" of continuous polyiso will meet IRC 2015 code on a U-factor basis if using the labeled R value. Derated to R5/inch it might be just shy of code, or it might make it, depending on the thickness of the old roof decking, and the choice of nailer deck & finish roof materials.
Code for zones 4-8 require U-0.026 , max, which is a "whole assembly R" of (1/0.026 =) R38.5.
An inch of full-dimension pine decking is good for ~R1.2, the additional R of the rafters bump that up to maybe R1.3, asphalt shingls + #30 felt is another R0.46, and the interior + exterior air films add another R0.7, a half-inch OSB or CDX nailer deck would add another R0.6. So you're looking at about another R3 of other materials + air films, worst-case, which means you only need at most R35.5 of rigid insulation to meet code on a U-factor basis in zones 4 & higher.
Most foil faced polyiso is labeled R6/inch or higher, which would be R36 @ 6". Most fiber faced roofing polyiso is labeled R5.5/inch or higher, so you'd already be at R33.
Zones 2 & 3 calls out < U-0.030, which is ~R33.3.
Zone 1 only needs < U-0.035, which is ~R28.6.