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Community and Q&A

XPS or EPS Foam on the outside of SIP

Roger Lin | Posted in PassivHaus on

We are considering adding a layer of either XPS or EPS foam on the outside of a SIP panel house. This allows us to break the thermal bridges resulted from the wood around windows and the corners where SIPs join. Also it adds a little more R-value to our structure.

The question is about the vapor layer. One of the potential wall assembly from inside to outside is Gypsum, SIP, XPS, Furring, then cement siding. The thought is to use the XPS as insulation as well as a vapor barrier.

Another possible assembly is Gypsum, SIP, EPS, House wrap, furring, then cement siding.

Which one would you do?

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Replies

  1. Riversong | | #1

    If you're in a cold climate, any vapor retarder must be on the inside. It is always a mistake to sandwich something as moisture-vulnerable as the OSB on SIPs between two relatively impermeable layers.

    EPS would allow some minimal drying to the exterior, but I would advise against any foam/wood/foam sandwich.

    If you wanted more insulation you should have used thicker SIPS or one of the several much better superinsulation systems that don't require petrochemical foam. A primary selling point of SIPs systems is the lack of thermal bridging, but they often fail to mention the additional framing inserts required at openings and panel joints.

    I would advise you to live with your choice and install a WRB, rainscreen and siding. If you want to improve the thermal envelope, then purchase better doors and windows since they are the weak link, and pay careful attention to air sealing, particularly at the ceiling/roof plane.

  2. Andy Ault, CLC | | #2

    You should definitely check with your SIPS provider before making any modifications to the envelope profile. Robert's right, you run some pretty ugly risks if you sandwich the OSB layer(s) (interior or exterior). Up to, and including, the possibility of completely voiding your panel warranty. The original provider should have some existing wall assembly profiles based on their product R&D vs. you trying to reinvent the wheel.

  3. Jim Bannon | | #3

    If you are wanting to add R-value to the wall, you might consider adding a 2x4 curtain wall to the interior side and insulating that w/ high density cellulose. This configuration would also provide a place for your wiring/plumbing without having to run them through the sips.

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