Hybrid Roof Assembly – Location of Interior Insulation
I’m planning a low slope roof in climate zone 6 which I’d like to use a hybrid roof assembly for. I’m stumped on where/how to locate some of the interior materials though. Above the roof deck will look like this:
Torched on base sheet (air & vapor barrier)
6″ Polyiso Insulation
2-ply modified bitumen roof system
This gets me about R-30 above the roof deck which allows me to install up to R-30 within the roof cavity, but my trusses are ~26″ deep so that space is more than needed. I read the article “Insulating Low Slope Residential Roofs” (https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs), which makes reference to the interior insulation being in direct contact with the bottom of the roof sheathing. Any suggestions on how to accomplish this with blown-cellulose? I had originally planned to install Intello Plus membrane to the bottom of the trusses, then blow cellulose in the cavity to the desired thickness. This would leave an unvented airspace between the top of the cellulose and the bottom of the sheathing though.
Lastly, I’m also wondering if the Intello on the ceiling is even necessary since there is already an air/vapour control layer above the roof sheathing. I am located in Canada so I suspect there may be some push back from the building inspector if this were to be omitted, but if it would be doing more harm than good I would certainly try to leave it out.
Thanks so much for any input you might have!
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Replies
If you have 26" of space, why are you going with exterior insulation?
Venting a flat roof doesn't work as well as a pitched roof but it still works. As long as you leave plenty of space above the fluffy, have at least code min vent area at the perimiter and a well sealed ceiling they work just fine.
If you must have the unvented roof with foam, you need to use batts and hold them to the underside of the roof deck with straps. Something like this:
https://www.jlconline.com/how-to/insulation/why-and-how-to-build-an-insulated-overroof_o
Picture is in the slideshow.
In a hybrid roof assembly, all of the insulation materials need to be as close together as possible. You can carefully net your trusses with Insulweb and fill the cavity with any depth of cellulose your want; it's annoying to work around the truss members but it's possible. Adding an interior variable permeance membrane is always good insurance but as long as your ratios are good, painted drywall is probably enough of a vapor retarder.
I avoid foam when possible, for various reasons, but this is a case where open-cell foam might be your best option; it sticks well and expands a lot, though it can still leave small gaps around the truss members.
Thanks the responses! I suppose I decided to go with the hybrid assembly after researching it and finding most of the advice to suggest against a vented assembly with this setup. There will be a rooftop deck there so I'm not super keen on having cupola style vents running through the centre of it. I think the hybrid approach will strike a good balance of cost vs functionality.
Michael, that's a good point about weaving a membrane through the trusses and then insulating that space. It's not a huge area so I might see if I can get this detailed out nicely. That would leave me a nice large service cavity beneath it as well. Thanks again for the input!