0 Helpful?

XPS or EPS Foam on the outside of SIP

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?

Asked by eyremountllc
Posted Nov 3, 2010 7:07 PM ET


3 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.

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.

Answered by Riversong
Posted Nov 3, 2010 8:14 PM ET


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.

Answered by J99aAMQzYo
Posted Nov 3, 2010 9:08 PM ET


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.

Answered by Jim Bannon
Posted Nov 4, 2010 8:55 PM ET

Other Questions in PassivHaus

how to insulate a shipping container home

In Energy efficiency and durability | Asked by N/A N/A | Feb 24, 17

How to insulate a cathedral ceiling from within the rafter bays

In Energy efficiency and durability | Asked by Terry Sharpe | Feb 24, 17

How to get notifications from comments

In General questions | Asked by Gerald Pehl | Feb 19, 17

Follow up question re: basement subfloor (climate zone 7a)

In Energy efficiency and durability | Asked by N/A N/A | Feb 24, 17

2x4 vs. 2x6 Exterior Framing

In Energy efficiency and durability | Asked by Brad Burch | Feb 24, 17
Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!