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Community and Q&A

Failing SIPs in climate zone 6

Parkermacdonald | Posted in General Questions on

Hello, I am currently working on a punch list project on a home another builder built and sold to my clients.

The house is a kit timber frame with SIPs as roof panels. We are located in Ithaca NY, climate zone 6. The roof panels measure 4 5/8” thick, which is completely under spec’d for this climate zone.

Is there something about SIPs performing differently than conventional framed roofs that could have allowed for such thin roof panels?

Now that the house is being occupied and winter has set in, it is clear that the roof is leaking heat and condensation has began forming on the outside walls at the SIP seams. I wonder how much is forming on the underside of the roof panels throughout the house, it’s just a matter of time until the pine t&g ceiling starts to show signs of moisture.

I am mainly looking for advise and also for recommendations for specialists who would be able to do a through assessment of the building and advise on how to proceed.


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  1. Expert Member
    Dana Dorsett | | #1

    SIPs have far less thermal bridging than framed assemblies, and code compliance would be done on U-factor basis, not center-cavity-R. An IRC 2018 code-compliant roof in zone 6 would be R49, or U0.026, which would be R38.5 "whole assembly". Some polyurethane SIPs would be north of R25 whole-assembly at 4-5/8", which isn't bad (performance-wise, lifecycle environmental impact would be something else. It might be under U0.035 (R26.6 whole-assembly), the equivalent of R30-ish cellulose in a vented attic), even though it doesn't meet current code north of US climate zone 1.

    If it's an EPS core SIP it doesn't even quite make code as a wall in zone 6.

    SIP seams need to be sealed with a high quality tape, not just some goop. The damage from roof seam leakage in a cold climate would more likely to show up on the exterior, not the interior, as warm humid air from indoors will rise through the seam depositing it's moisture load in the exterior skin of the SIP. This is a known (and all too common problem) with SIP roofs in cool climates:

  2. Jon_R | | #2

    +1 on sealing the SIP seams with tape, preferably including the interior side.

    If this isn't possible, you might look into depressurization (which also reduces airflow).

    The moisture problem is not related to the low R value.

  3. GBA Editor
    Martin Holladay | | #3

    As Dana noted, the R-value of the roof panels depends on the type of foam used in their manufacture. According to one web site, a 4.5-inch SIP manufactured with EPS has an R-value of R-15, while a 4 5/8-inch SIP manufactured with polyurethane foam has an R-value of R-27. Either product would be less than minimum code requirements for a roof in Climate Zone 6.

    If you can see condensation at the SIP seams, you have a problem -- either air leakage or some type of void at the seams that should have been filled with spray foam. An infrared camera inspection might reveal what's going on.

  4. JC72 | | #4

    How does a structure with these specs in that climate zone get built?

    Homeowner was unaware.
    Builder was unaware or didn't care.
    Permitting office missed it.
    Building inspector missed it.

  5. GBA Editor
    Martin Holladay | | #5

    You forgot, "House was built in a jurisdiction without code requirements or without code enforcement."

    1. JC72 | | #7


  6. Expert Member
    BILL WICHERS | | #6

    Parker, I was reading your question and remembered an article written by the Building Science Corp guys:

    It was your “leaking at the seams” comment that made me remember that article. Even with less insulation than required, sealing air leaks may make a big improvement for you.

    Perhaps that article will provide some insights into what you’re seeing.


  7. ohioandy | | #8

    Parker, this situation will only go from bad to worse. Even if the ONLY problem here is an egregiously undersized and leaky SIP envelope, it could be the homeowner will quickly--and rightfully--make life difficult for a contractor who takes responsibility for it. If I were you, I'd encourage the homeowner to document the deficiencies ASAP. Start with an energy audit that includes a blower door with infrared camera study, which may help show the extent of seam leakage. Additionally, remove trim or finish materials to take pictures of SIP seams and SIP thicknesses where possible. Is it still possible to get the original builder to accept responsibility for what might be a VERY expensive fix?

    I built (and am living in) a SIP house. The SIP manufacturer I worked with refused to sell their product without full engineer design services, and even though I also live in a jurisdiction without code enforcement, the company actually sent someone to check my work. Given all the bad press, it's really disappointing that there are still clueless contractors who can source panels and assemble such a disaster.

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