Over vented SIPs low slope application
Hello,
We are designing a house in Wisconsin, climate zone 6A, 50lb snow load. The roof and walls will be prefabricated and installed on site. The structure is mortise and tenon Timber Frame.
The roof is a low slope roof (1/2″ per foot). The client really liked this low slope style. We talked with the SIP manufacturer and they highly recommended a vent cavity above. We will be foaming the joints and installing poly under all the joints per the manuf details.
I have attached the current assembly detail (have not done full CDs yet so dont have fascia details). Thinking of a fascia vent to connect to the over vent.
I was hoping for some feedback on experience with venting on low slope roofs and if it actually is less risky than no vent cavity or more risky due to maybe a lack of stack effect. Anyone know of any wingnut type field testing with low slope ventilation? I’ve seen the build show guys do it, but it was on a 2/12 I believe.
Thanks!
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Replies
How committed to your SIP plan are you?
Sip were a great new idea in about 1980. In the next 40 years or so they have failed to become the market leader. The market place has decided that they simply cost too much for whatever benefits they offer.
In short you can build walls and roofs with the same or higher R values for less money than you can using SIP with less risk.
Note the same logic applies to your framing choice. If you want a few decorative beams that is great.
In the real world making large air tight joint turns out to be next to impossible. Air and moisture will find your mistakes and mold and rot are likely to damage the almost unrepairable structural deck.
Before you buy a ticket on this train, please do yourself a favor and read this article.
https://buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks
https://www.greenbuildingadvisor.com/article/how-to-make-a-sip-roof-better
Walta
Since it is an over-vent only (not part of an vented roof), your detail should work fine.
Important bit is that permeable underlayment over the SIP which your drawing shows, just make sure it gets built with it. You show a warm side weather resistant membrane, not sure how that would be applied. Might be simpler to tape the seams of the SIP. The important bit is preventing warm air from getting into the gaps between panels, the panels themselves are vapor closed.
Metal is possible at 0.5 slope but you have to be very careful because water moves uphill at that slope. All your flashing details need to be perfect and need to be done with sealants to stop this. If you bump up the slope to 2:12, lot of those issues disappear plus you can use a simple snaplock roof panel.