All my roofers are saying vent, but I don’t want to.
I’m having a hard time getting roofers in my area to agree that exterior insulation is a good idea and when i bring up the idea of unvented construction I get a look like you wouldn’t believe.
so here is my question: do I need to vent this roof?
I’m going to combine interior and exterior insulation in a cape home from 1922. I’m starting with the roof, which will be resheathed and r20 laminated polyiso/osb panels installed above that and underpayment on that then Shingles. In between the rafters I’m going to sacrifice a few inches and sister onto the 2×6 rafters to accommodate r30 fiberglass batting. Vapor retarder beneath the batting and then drywall
with r50ish between my inside and outside, I shouldn’t need to vent to avoid ice dams. Correct? Anything else I’m forgetting? Many people have suggested spray foam, which I’m open to but skiddish about the actual safety of the products as they get installed.
no. I’m not putting in any canister Lights
am i losing anything by not venting?
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Hey there,
It would be great to get your name...
Your assembly will work if you are in climate zone 5 (or lower) where R-20 is the minimum amount of exterior insulation you need for this assembly. What are you planning to use for a vapor retarder? This roof needs to be able to dry inward.
There two articles should be helpful:
https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation
My name is Paul and yep I’m in climate zone 5. Buffalo ny.
I haven’t decide on a specific product yet.
I suppose what I should say is “vapor permeable air barrier”
Good to know that I don’t have to vent.
If the assembly must dry inward, am I better off with just drywall beneath the batting?
Hey Paul,
You definitely want to air seal and drywall can be an excellent air barrier with a little extra detailing around the perimeter and at penetrations. You just don't want poly sheeting on the inside of the assembly, which some builders and building inspectors still specify as standard practice in cold climates.
I’ve gone back over the articles detailing unvented cathedral ceilings and combination rigid and fluffy insulation. I take away two big questions:
The Polyiso panels I’ve selected are laminated with osb, which gets screwed to the existing deck. Do I seal the boards already there with a vapor permeable membrane, and then put my underlayment above the osb lamination on my polyiso panels? Or simply use an impermeable membrane on the osb laminated to the polyiso?
Second question:
There seems to be some equivocation about ventilation and ice damming. I’m in climate zone 5. I’d like to do this without ventilation for cost reasons. Will I be able to avoid ice damming that way or am I better off building venting with an additional layer above the poly iso?
I assume that you are talking about nailbase panels, with OSB on one side. I believe that it would be best to air seal the existing sheathing first. You can use a self-adhering membrane like this: https://foursevenfive.com/adhero/
Ideally, you'd have an adequate thermal layer and an airtight assembly and that would mitigate ice dams which are the result of heat loss from the interior. However, in cold and snowy climates like Buffalo, that's not always enough to do the trick. You might consider adding furring and an additional layer of OSB to minimize the potential of ice dams.
Talk to a general or framing contractor about the project. The insulation aspect may be too much for a residential roofing contractor.
> and underpayment on that then Shingles
You need to make sure that air and vapor from the interior is blocked before it gets through the foam. Otherwise this underlayment layer may rot.
>"...I’m going to sacrifice a few inches and sister onto the 2×6 rafters to accommodate r30 fiberglass batting. "
Cutting a bunch of 1" foil faced polyiso into 2" wide strips for the rafter edges would give an even greater thermal break on the rafters, and would allow the use of R30 rock wool (performing at R29 when compressed to 7.0" ). The drywall screws would have to be an inch longer, but it should be a lot easier than adding on a milled 2x4 to make a 9.5" cavity for an ill-fitting full-depth fiberglass R30 (nominally 9.25" thick) , or a 2x3 for an 8.5" cavity to accommodate a "cathedral ceiling " fiberglass batt (nominally 8.25" thick.) The batts may still need to be trimmed for width, but if the polyiso strips are the same width as the rafters there won't be a step, creating convective thermal bypass channels. The additional R6 of 1" polyiso edge strips would outperform the R3- R4.2 of a milled 2x3 or 2x4 too.
To cut polyiso edge strips cleanly without a lot of mess try this:
https://www.finehomebuilding.com/2009/01/29/theres-a-better-way-cutting-rigid-insulation
Or, set up a table saw and rip-away, if you don't might flakes of foil facer and polyiso crumbs following you around for a few days. :-)
The ceiling gypsum still needs to be air tight and somewhat vapor retardent for this to work from a moisture point of view. Standard latex primer and a couple coats of ceiling paint is enough, but at the cold edge of zone 5 "vapor barrier latex" (about 0.5 perms) would be preferable.
Dana,
That’s a good idea. I’m interested in losing less space.