Unvented roof design in hot/humid climate
I’ve been doing a lot of research but still have some questions. Hopefully some of the experts can chime in.
Im in climate zone 2. Hot and humid. A few years ago we converted the attic to living space. We converted the traditional vented roof to an unvented roof by installing open cell spray foam in the 2×6 rafter bays. The attic is now fully conditioned by connection to the central AC system.
We now plan on reroofing the house and installing an additional layer of 3” polyiso on the exterior since the current 5.5” of open cell spray foam does not offer the required R38.
So here is what I haven’t been able to figure out yet:
1.) should I put an air and or vapor barrier like some peel and stick membrane on the exterior side of the roof sheathing to keep humid outdoor air from entering through the roof? And if so, would I put that membrane on the existing sheathing underneath the polyiso or on the layer of osb that goes on top of the polyiso?
2.) I read that ventilation channels should be added with 2x4s between the polyiso and the shingles to prevent ice dams in cold climates. Since I’m in the hot humid zone, is there still another advantage those ventilation channels would provide? Would they keep my shingles cooler and therefore extend their life?
3.) the open cell spray foam is already in place so I can’t really change it anymore. I now read that article about open cell foam being risky in all climate zones because moist vapor can still travel through from the interior and reach the sheathing. How big of an issue is that really in my climate zone where the heat is on maybe three weeks a year? Am I in the clear if I put the 3” polyiso on the exterior and therefore keep the roof sheathing warm. Plus my attic is fully conditioned. So do I still have to worry about mold issues down the line because of the open cell foam?
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Replies
Hi Quantum Girl.
The short answer is that you don't need to do much, except for add the insulation and reroof.
If the spray foam was installed well, it should be taking care of the air sealing. If you want to take a belt-and-suspenders approach, and it may be good cheap insurance, you could tape the sheathing joints and use any underlayment you'd like. If you are planning to use foil faced polyiso, you don't need a separate vapor retarder above the sheathing. If you are using polyiso with a different facing, it may be helpful to have a vapor-impermeable membrane in your climate and since the interior space will be air conditioned for much of the year.
To address the open-cell against sheathing risks, pay attention to the relative humidity in the attic space when you are heating. I don't think you should use an interior vapor retarder; your assembly needs to be able to dry inward. Despite the research that points to real risks, I do know an experienced architect and builder who regularly uses open-cell in unvented roofs in a hot and humid climate and has never has a problem.
Also, keep in mind that most high-performance builders prefer two or more layers of rigid foam with staggered, taped seams to one layer.
I don't know too much about the benefits of building a vented roof deck above the insulation for asphalt shingles, but I don't think it is necessary. Maybe someone else will weigh in with some experience.
Thanks for the input Brian! It’s much appreciated!
Brian I have a follow up question and hopefully u see this.
U said it’s a good idea to put a vapor barrier on the outside of the roof assembly When not using foil faced Iso . Would that vapor barrier go between existing sheathing and Iso. Or would it go between shingles and new osb on top of the Iso? Does it matter?
Generally, that layer would be the roofing membrane. I have seen builder use peel-and-stick ice-and-water membranes on the roof sheathing before installing the rigid foam. They do this for waterproofing before covering the roof with many more materials. These membranes are also impermeable. I have also see builders put the foam right on the roof sheathing and not deal with waterproofing until the next layer of OSB is installed. I don't know enough to have an opinion on these methods.
With OSB, a few inches of rigid foam, and asphalt shingles though, you probably don't need to be concerned about this; you already have a lot of vapor retarding materials on the exterior of your roof and the only problem would occur if there was enough inward solar vapor drive to push vapor into your assembly where it found a cool air conditioned surface on which to condense. This can happen, but I think it is unlike given how your roof will soon be assembled.