Am I throwing my money away by venting a vaulted ceiling with a SIP shed roof topped with 3-layer peel-and-stick roofing?
I have a shed roof with vaulted ceiling covered by 8″ SIP panels. I originally intended to lay reflective foil membrane over my SIPs followed by 2x sleepers and 1/2″ sheathing to form 1-1/2″ air channels between my SIPs and roofing layer so it vents passively. I plan to cover it with Certainteed’s Flintastic SA CAP 3 layer granular rolled roofing in their “white” color which is like a light grey. My dealer insists I’ll be wasting my money adding a vented space and that there is no need to vent SIP roofs. I understand that the white-ish rolled roofing will reflect more heat than conventional roofing or black epdm, but I find it hard to believe that venting would not be beneficial. I am in Portland, OR where half the year it is cool and wet, and for 4 months it is warm to hot and dry. Clearly we are not as hot as California where Certainteed’s extra reflective Coolstar product is common. None of the roofing suppliers up here stock that product. Am I overthinking this?
What do others think?
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The most important detail on the SIPs install is taping the interior seam to prevent interior humidity from damaging the panel. After that, the vent channel serves a second purpose and becomes your wear layer. If you put Ice and Water directly on the SIPs panel and need to replace the roof you can't pull up the top osb layer of the SIPs panel and replace it like you can in a cold roof system. Typical cold roof SIPs detail is 15# felt, 2x4s flat 24" OC w/ 5/8" OSB, and then your typical roofing underlayment/exterior finish.
Greg,
If it were my SIP roof then I would definitely add an air gap above the upper OSB layer.
I would lose the radiant barrier as I suspect it is a class I vapor retarder and will defeat any benefits of the air gap. (Warm, moist air will condense on the under side of the cold radiant barrier and ultimately lead to the OSB rotting- especially at the ridge.)
I would add 30# felt or another vapor open membrane over the OSB Then add your strapping, underoofing and then roofing.
Like Zdesign stated, I would also meticulously tape the interior SIP seams with a high quality European tape such as Siga Rissan.
What slope is this roof? seems flatish if using rolled roofing. venting is sensitive to roof pitch.
Don't worry about low slope with your well air sealed, over-sized vent, "vent optional" roof. For example, a 1.5" vent in a 1:12 roof will move about the same CFM due to stack effect as a 1" vent in a 3:12 roof. This is way more than you need for properly air sealed SIPs.
+1 on not blocking the moisture coming from inside with a vapor impermeable membrane if you want the vent to remove moisture.
Thanks all for chiming in...
Zdesign-
I applied mastic at the base of every spline/seam and do plan to tape interior sip seams as well.. I wasn't too concerned about having a divorcement/wear layer under my peel n stick (3 ply flintastic sa cap White) because Certainteed said I could just add new layer on top of old in future. But 50 years down the road I or next owner would have 7 layers if I recapped it every 10 years. How many layers are too many? I'm starting to see the value in a divorcement layer. If I add a divorcement layer of osb I might as well puts sleepers in and create air channels too, right?
Rick,
Good point, however The radiant barrier I plan to use is perforated so condensation should not be an issue. It's about $150 for the roll. Do you think it is worth using if I create an 1-1/2" vent layer? Seams are sealed with mastic & tape provided by the sip manufacturer. There is no ridge. It is a one plane shed roof.
Tim,
Slope is about 1:12. Do you think I'll get enough convective air movement for vents to be useful on a slope that low?
I see the value of having a divorcement layer. I'm still not convinced I need venting with my light colored roof. However vented channels would provide ability to fry out if a leak ever developed.
Hmmm? Any more thoughts out there?
Make sure you search this site for how to vent a low slope roof. It has been asked many times. Low slope does not vent thru convection well. But look at the articles on this site so that you understand the issues.
Greg,
A 1-in-12 roof is a low-slope roof, and venting doesn't work in a low-slope roof. For more information, see "Insulating Low-Slope Residential Roofs."
The most important thing you can do is to spend a great deal of time ensuring that you have a bulletproof interior air barrier. I also recommend that you avoid putting any peel-and-stick product on top of your SIPs -- because peel-and-stick products complicate future repairs. Remember: when SIPs rot, it's always the exterior OSB that rots first, not the interior OSB.
Thorough air sealing under sips - check.
Ok it sounds like even though I need to add a divorcement layer between my sips and peel n stick to ease future repairs, adding venting channels via 2x strips would be useless. If my divorcement layer is attached directly to top layer of sip's OSB what material and at what thickness do you recommend I use for the divorcement layer?
Should I put any type of membrane or felt between the sip and divorcement layer to help protect sips from rot in the event of leaking deteriorating roofing?
Thanks all!
Hi Greg.
How did your roof install go?
What did you finally decide?
I plan to build a SIP house in zone 7 this summer and am searching for options.
Thanks.