Rigid foam around SIPs in climate zone 4a?
I’ve been a passive (not pun intended) participant of these boards and this site for a while. It seems hard to catch up on it all, so hopefully my question here isn’t terrible 🙂
I’m thinking of scraping my house off the land and building new. My goal is zero net energy. I’ve extensively remodeled one home including an addition myself, but I’m no professional. I do want to build and subcontract most of this home myself as well. I’m just an avid building science lover and construction enthusiast.
I’m looking to build a SIPS home, likely on a poured concrete, exterior insulated (or ICF) basement. In researching all the details and joins in a SIPS home, while obviously substantially better than a stick framed home, it seems a stick framed home with exterior rigid foam and attention to detail would outperform a sips home.
I like the speed, simplicity, and seemingly reduced environmental impact of SIPS building, but I’m wondering if adding a thin layer of foil faced polyisocyanurate to ensure a thermal break in all the joints that don’t have them like window/door header, lumber splines, and some other stuff would be useful or if I could use that money to buy better windows or something else. (for windows looking into http://www.litezone.ca/ , any experiences?). I plan to also probably add a UV stable WRB, PT furring strips before siding and roofing so maybe that is enough?
Thanks.
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Replies
One definitely can build a stick frame home that outperforms R-Value wise a SIP home. The advantages of SIPs is speed of build (what takes months can be knocked out in less than a week with a pro crew), air sealing, less gaps to seal than stick frame, and ease of assembly (SIPs are designed, cut and labeled at the factory to assemble quickly at the site).
If you want MAX R-Value, then stick frame is the way to go, If you want speed, ease of assembly, and easier to air seal, SIPS are a viable option. ICF also provides speed and the strength is night & day vs stick frame.
Michael,
There are pluses and minuses to your idea of installing a layer of rigid foam on the exterior side of a SIP wall or a SIP roof.
The pluses are that the rigid foam should reduce air leakage at the SIP joints, and that the rigid foam should help keep the OSB warm and dry.
The minuses are that the rigid foam will prevent the exterior OSB facing on the SIPs from drying to the exterior and may violate the SIP manufacturer's instructions and warranty.
On balance, I'd vote that you should skip the exterior foam layer.
If you do go ahead with your plan to build a SIP home, two steps are essential:
1. You must install high-quality SIP tape on all interior seams (in addition to spray foam at the seams).
2. You must install a ventilation gap between the roof SIPs and the roofing. (For walls, a ventilated rainscreen gap is also a good idea.)
Also take a look at Ecocor. They offer a panelized system made completely differently that I like much better.