Timberframe SIP roof (yet again)
Zone 3A – Middle GA.
I know this topic has been discussed many times, but I would like feedback on the following possible detail. I am aware that one of the major concerns with SIP cathedral roofs is the possibility of moisture/condensation due to failing SIP seams over time. The general theme I have read here many times is to pay great attention to the taping/sealing of seams from the inside.
So consider a timberframe roof structure with 1.5″ T&G decking. The underside of the SIPs will not be accessible to seal. That said, why would the following detail not address the concern?
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If a well chosen and well applied barrier is applied on the exterior of the roof decking (WRB 2), before the SIPs are set, would that not prevent warm humid air from interior in winter to make its way to the cooler exterior face of SIP even with failed SIP seams.
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If a well chosen and well applied barrier is applied on the exterior of the SIP (WRB 1), after the SIPs are set, would that not prevent warm humid air from exterior in summer to make its way to the cooler interior face of SIP even with failed SIP seams.
Note the standing seam roofing is raised off the SIP (and WRB) by 3/4″ strapping allowing venting above SIP.
If this IS a reasonable approach, I would love to hear opinions on the best WRB/vapor barrier/membrane choices for each location.
(note..i forgot to draw soffit/undercladding for the underside SIP..Ii will not leave SIP osb exposed as in the drawing)
Thanks!
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I guess my question is why sip? It’s big draw is the strength you get and the spans it can achieve. Why not just do exterior insulation, and if you need a nail base product? I would still apply a membrane to the tg and I would use pro clima adhero.
Good point. I am not familiar with how exterior insulation on roofs is (well) done. Might you be able to direct me to a reference detail? Curious how overhangs are dealt with.
But yes.. maybe 3 layers of 2" Kingspan R-10 XPS ($56/sheet) = $5.25.sqft for R30, or 4 for 8" = $7.00 sq.ft for R40, on top of Mento. Then 3/4" stapping, then standing seam.
Your thoughts?
https://www.greenbuildingadvisor.com/article/installing-rigid-foam-above-roof-sheathing
https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-the-top-of-the-house
https://www.greenbuildingadvisor.com/article/how-to-talk-to-contractors-about-exterior-insulation
https://www.greenbuildingadvisor.com/article/low-carbon-roof-retrofit
https://buildingscience.com/documents/building-science-insights-high-r-value-roofs-high-r-value-foundation-assemblies-building
https://buildingscience.com/documents/building-science-insights/bsi-141-shakespeare-does-roofs
There are many more resources. Use the search feature on GBA and building science corp.
Thank you. I will dig more, but one issue I have is that so far, many of the articles I have read cover the idea, but lack on the detail. For example, in the below image from
https://buildingscience.com/documents/building-science-insights/bsi-141-shakespeare-does-roofs
, there is no discussion of how all that foam and upper sheathing is fastened. Are we running long screws all the way through all the foam? How are those penetrations (through decking/strapping, foam, membrane!) dealt with, etc.
Again, thanks, and I will continue to dig to find more details.
Yes many people use sip screws or similar. It is the same as if you put shingles, metal, or any other material over you waterproof layer. Almost all membranes seal around screw or nail penetrations.
Fair enough. Makes sense. I appreciate your input.
Won't the SIP Panel manufacturer have this detail in their installation instructions (it should if they are reputable)?
No assembly is ever perfectly vapor tight. In recognition of this fact the usual practice is to make assemblies vapor tight on one side and vapor open on the other. If you try to make it vapor tight on both sides you run the risk of trapping moisture inside the assembly with no way to escape.
Agreed. And after more thought, I think perhaps ditching the SIPs and going exterior insulation on top of membrane may be a smarter approach. Allowing anything above the membrane to dry outward (via vented channel under metal roofing). I even wonder if anything beyond a typical vapor open roof underlayment would be needed on top of the exterior insulation.
The way many people deal with the specific "seams issue" of one thick SIP panel - beyond in your case of a WRB adhered to the roof deck prior to setting the SIPs - is to do more than one sheet of foam (rather than a SIP panel) & just make sure you stagger the joints of each of the sheets ...
You can also go with a simple over roof. Install rafters over the T&G and insulate with fluffy. User taller rafters than the insulation batts to for the vent gap. Roof deck over the rafters followed by underlayment and metal.
You now have a simple vented roof built with cheap materials. Bonus is that all the overhangs are now fully supported by these rafters, you can even trim the bottom of the rafters to have thinner overhangs.
The membrane over the T&G would still be needed to form your main air barrier. In your climate, the membrane should be permeable so something like VP100 or VaporDry SA would work.
Yes. This approach makes sense. That said I wonder if the rafters are needed in the case of rigid insulation, like Rockwool Comfortboard, or XPS, etc... where strapping could be used directly on top, and provide the vent space. That would also reduce the thermal bridging.
You are in warm climate, thermal bridging doesn't matter as much as there is much smaller temperature difference and no issues with melting snow on the roof. Wood is also still a decent insulator, so as long as the assembly R value is decent, thermal bridging doesn't matter.
If you really want to do better, you can do a layer of purlins and half the insulation, rafters over this for the rest of the insulation. This would give you near continuous exterior insulation value as the assembly has almost no thermal bridges.
Rafters only makes sense with batt or loose fill insulation, for anything else use strapping.
The nice part about rafters is framers know exactly how to build it, an over-roof like this with batts will go up in no time. Rigid with strapping will take much longer plus you would still have to figure out how to support your overhangs (not as easy as it sounds once you dig into the details).
Certainly agree regarding the overhangs! And I hear ya regarding the framers. And yes, I know my climate situation is different than many of the example articles, which seem to focus on colder northern climates where heat loss is a bigger deal.
Thanks, and I appreciate your input.