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Is an air space between a ceiling and SIPs a problem?

Driven_5EM | Posted in Green Building Techniques on

I have a timber roof system that goes like this.
timber rafters
plywood with a flooring glued to it and fastened down on top of the timber rafters
suredeck membrane (to protect the plywood) and double as vapour barrier
2×6 strapping on top of the plywood to facilitate electrical
8 1/8″ sips
synthetic membrane and peel stick for eaves protection
laminated shingles
Should I be concerned about the 1.5″ space between the top of plywood and bottom of sips.
We are going to be extra ornery about air sealing the sips.

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  1. GBA Editor
    Martin Holladay | | #1

    The best location for a ventilated air space is above the SIPs, not under the SIPs. That said, the type of cavity you are proposing -- a service cavity for electrical wiring -- is not problematic. (For more information on service cavities, see Service Cavities for Wiring and Plumbing.)

    There are some problems and oddities in your planned stack-up, however.

    1. What type of flooring do you plan to glue to the plywood? What is the purpose of this flooring?

    2. With SIPs, you don't need an interior vapor barrier. But you do need an interior air barrier.

    3. It's important to install high-quality tape on the interior side of the SIP seams, which raises buildability issues for your stack-up. Don't omit the tape.

    4. If the synthetic roofing underlayment that you have selected is not vapor-permeable, the manufacturer of the synthetic underlayment probably forbids the use of this product above unvented roof assemblies. As described, your roof assembly is unvented.

  2. Driven_5EM | | #2

    The flooring is vinyl plank that I've ok'd for use from the rep. This is what my clients wanted for a look. I do understand the ventilation on the top side is better but for ease and quality of assembly, we want to lay the plywood/flooring first and build up on that. With the need for electrical, this makes a cavity a requirement. Thus like you mentioned, we would assemble the sips and seal the underside with utmost attention, then flip over and fasten on the roof build up. (sounds like a hassle but we have done it before). My big concern is that we aren't creating a vapour trap by the vinyl plank ceiling, synthetic membrane, cavity then sips. The main purpose of the synthetic membrane is to protect the ply/vinyl plank from rain. Still sound ok to you? or would I be better to fill the cavity with 1.5" rigid insulation and if so what type? If the assembly I am proposing sounds good, I would rather not install the extra insulation as it is a 14.3/12 pitch so that presents some install challenges.

  3. GBA Editor
    Martin Holladay | | #3

    As long as the SIPs stay dry until the roofing is installed -- or as long as the SIPs are allowed to dry thoroughly once the roofing is installed -- you shouldn't have any worries about creating a "moisture trap." Just make sure that the interior facing of the SIPs is dry.

    Your order of assembly is still a mystery to me -- but apparently not to you.

    Your roof assembly will be much more robust if you include a ventilation gap between the top of the SIPs and the roofing. For more information, see Air Sealing SIP seams.

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