Need help with my SIP roof assembly
mapnerd
| Posted in Green Building Techniques on
We are planning to use SIP’s for our roof. Right now we have specified the following assembly (inside to outside):
-8″ polyurethane SIP
-1″ XPS foam insulation
-2×4 sleepers
-1/2″ plywood sheathing
-underlayment
-standing seam steel panels
We are in Omaha, NE (climate zone 5). Some of this is based on Martin’s advice from another question on this site (https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/18127/most-cost-effective-system-energy-efficient-h).
Any thoughts on this assembly?
Thanks
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Replies
Michael,
Just be sure to stagger the seams of your roof layers, so that the 1-in. XPS foam covers the joints of the SIPs.
Michael,
Why do you think you need an extra layer of foam board on top of highly insulative 8" SIPs? 1" ov XPS will be insufficient to keep the outer layer of OSB above dew point temperatures and it will trap any moisture that might accumulate.
What you need, instead, is to thoroughly seal all SIPS panel joints, cover with a breatheable roofing underlayment, then your standing seam roofing. The rest is redundant and wasteful.
Michael,
You'll have gyp board as inside finish? There are code requirements for interior finishes. If the inside face of the SIP's OSB is to be your air barrier, you'll need to avoid ceiling penetrations (conduits, lighting, fans) or plan and execute very carefully.
If you're looking for more insulation than the 8.25" SIP provides, you could use 2x2 furring @24" o.c. on the interior face of the SIP (after taping the seams and blower door testing) and fill that 1.5" cavity with foam or cellulose, for instance, and then you could use that cavity for conduits if needed. (With gyp, board finish to the inside of course.) That's similar to a a Passive House detail we use.
I agree with the above comment about furring below the panels for conduits, and about not adding 1" XPS above the SIPs. That's putting the upper OSB surface in jeopardy by limiting drying into the vent space. A better way to vent the roof is to cross-hatch 1x4 furring. First install 1x4s parallel to the slope over roofing felt, then a second course perpendicular to that. If you use a steel roofing panel that can span across a 24" furring interval then there's no need for plywood sheathing. This arrangement allows venting for a cold roof and also lets any water in the vent space run out over the roofing felt.
Many owners of SIP homes report ghost lines on their roofs when there is a thin layer of frost, indicating SIP seams. Seams will leak more heat than the center of the panel.
You can ignore the issue, or handle it above the SIPs or below the SIPs.
I am planning to either use gypsum board or T&G planking as the interior side finish. We will also use structural 6-3/4" x 12" glulam beams @ 6' oc to support the SIP's (and as an architectural feature). We plan to make very few roof penetrations, if possible. Do I take Robert's advice and simply add breathable underlayment over the well-taped SIP's, then install the roof panels or do I need to create a vent space between the roof panels and the SIP's? Either way, it sounds like I should make sure my builder is extra vigilant about sealing the SIP seams.
Sealing the panel joints is the critical area and weak point of SIPS, and the reason for dramatic failures in Alaska.
If properly sealed, there will be no "ghost lines" on the roof, as that is an indication of air leakage.
A vented roof makes OSB sheathing far more durable. The only reason I suggested a non-vented roof is that standing seam metal typically requires a solid substrate and installing a secondary roof deck is wasteful. But if the metal can span between purlins (24g rather than 26g), then a furred or cross-hatched cavity would be preferable.
Hi Michael,
This is Scott with Earthcore SIPS.
I agree with the majority of posts regarding sealing the panels for air infiltration on the seams.
In the event that you want a vented roof application, that is something we can provide with Polyurethane foam vs. xps and it comes fully assembled so you are not having to install the wood and then cut foam to go between your fur strips. OSB can be 7/16-5/8-3/4
We are starting to offer an FSC version as well.
Talk soon.
Scott Anderson
Earthcore SIPS llc
[email protected]
303.618.1834