Is it prudent to apply exterior foam insulation on a SIP wall to minimize thermal bridging?
user-965067
| Posted in Energy Efficiency and Durability on
I am building a home in northern Indiana with SIP walls on an ICF foundation. To minimize thermal bridging I was wondering if there is a downside to adding 1″ exterior foam before applying fiber cement cladding?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Sarah,
Answering your question requires some complicated hygrothermal analysis, some head-scratching, and some judgment. If you decide to go ahead with your plan, I would use EPS foam -- the most vapor-permeable of the three types of rigid foam. You might also want to include bumpy housewrap between the SIP and the rigid foam. (For more on that suggestion, you might want to read "Mind the Gap, Eh!" by Joe Lstiburek.)
A while ago, there was a long Q&A thread here on the GBA site about installing rigid foam above a SIP roof. Read about it here: How do you outsulate a SIP roof?
The more people discuss attempts to try to solve the thermal weakness of SIPs, the more people conclude that maybe you don't want to choose SIPs in the first place.
If you do go ahead with your idea of using SIPs with 1 inch of exterior foam, I strongly suggest that you install vertical furring strips between the rigid foam and the siding.
SIP walls won't have any thermal bridging if you don't use solid framing. That means splines for the panel joints. Around the windows, you can use 2x2s instead of solid 2x6s.
But 6" EPS SIPs don't get you to R40, which is where you'd like to be.
Sarah - I have built several SIPs homes and studied this issue in some detail. In addition to window and door framing, the thermal bridging at the top and bottom plate can be significant in SIPs wall systems. However - because the OSB skins are an integral part of the structure (not just racking resistance, but linear compression too), I wouldn't do anything that would decrease the drying potential of the exterior OSB skin, like adding foam board insulation. EPS is more vapour open than XPS or foil face polyiso, but will still reduce drying potential. The only exterior insulation I would ever use on a SIP is rockwool sheathing. As Martin mentioned, I would also always use a ventilated cladding system to increase drying potential.
Also - make sure you have a balanced mechanical ventilation system to control indoor humidity during cold weather.
Trevor
Thanks for your advice and input. I will continue to research the pros and cons of SIP construction and which additional details need to be incorporated.