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Alternative to SIP roof?

user-7070878 | Posted in Green Building Techniques on

Hi. I’m building a cabin in Wa State and the designer has spec’d a SIP roof. I have two concerns…

– First, how to finish the visible underside of the SIPs? The SIP company (Premier) apparently only offers a secondary ACX plywood lamination, which I don’t care for. Is there a common method for producing a finished ceiling with SIPs?

– Second, the SIPs are pretty pricey. I’m looking at $20K for a 1600 sqft roof. The manager of a building supply place in the area said that a cheaper alternative to SIPs is to lay t&g car decking first, then poly iso foam, then sheathing. Material cost would be less I’m sure but I’m also thinking about labor and time. This is an owner-built cabin (with professional help) and I’m concerned about spending weeks building the roof if I don’t use SIPs. I need to get this dried in before the snow hits in the fall.

Thanks in advance…


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  1. GBA Editor
    Martin Holladay | | #1

    The interior side of the SIPs can be finished with drywall or T&G boards. Some SIPs are engineered to be load-bearing from the exterior wall to the structural ridge beam; other roofs require rafters. Talk to you SIP company. It's certainly easier to drywall the ceiling with fewer rafters or purlins getting in the way.

    Pay close attention to air sealing if you install SIPs. I advise you to read this GBA article: "How to Protect Structural Insulated Panels from Decay."

    Clearly, SIPs are more expense than the other option you mention (assembling the roof sandwich yourself using rigid foam). But SIPs are faster. The choice is yours.

    For more information on Option B, see this article: How to Install Rigid Foam On Top of Roof Sheathing.

  2. user-6623302 | | #2

    Tell us more about the design. I did a major SIP addition to my house so I have some experience.

  3. user-7070878 | | #3

    Thanks for the links above. I'll review the info.

    Sorry, I should have included more about the roof structure. It's a 9:12 gable with glulam ridge beam and glulam rafters 4' OC. The steep pitch makes me want to minimize being on the roof. Also, we have an open-to-above living room so finishing the ceiling from underneath after the SIPs are installed isn't that attractive either, hence my desire to have a solution incorporated into the SIP panels.

    A drywall ceiling is not my preference. My first choice is something like baltic birch plywood skin but Premier doesn't offer this, nor do they offer t&g, as I've seen with other SIP manufacturers. Installing car decking first is an option, but adds substantial cost as well as the labor to install.


  4. user-6623302 | | #4

    Have you shopped around for other SIP providers? Maybe you need to rethink your design. Sounds expensive. Going to be a challenge to heat.

  5. user-7070878 | | #5

    Heating shouldn't be a problem. We'll have significant passive gain + radiant concrete floors on first level + freestanding propane stove + standalone electric wall units in the upstairs bedroom and bath. This is a small place, only 1200 sqft.

    Yes, there are some expensive elements, that was understood. Low cost was never the primary driver of the design. But the SIPs have somewhat exceeded what I had considered "expensive" plus there is the cosmetic consideration of how to deal with the underside of the panels.


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