Extra precautions on low-slope vented roof
I’m building a shop-over-garage building in climate 6B, with 90 psf ground snow load. Due to some design constraints on height and cost, I’m not doing a fancy hot roof with vented overroof. Instead, it’s basic cathedral ceiling construction: sawn rafters with batt insulation; vapor membrane between finished ceiling and rafters; plywood deck, underlayment, and standing seam roof above rafters.
The roof is lower slope (mostly 2.25:12), but I’ve spent enough time with the CRREL papers/equations to convince myself I have sufficient venting. (https://www.poa.usace.army.mil/Portals/34/docs/engineering/MP5420,%20Ventilating%20Cathedral%20Ceiling%20to%20Prevent%20Problematic%20Icings%20at%20Their%20Eaves.pdf, https://www.poa.usace.army.mil/Portals/34/docs/engineering/MP-02-5778,%20Guidelines%20for%20Ventilating%20Attics%20and%20Cathedral%20Ceilings%20to%20Avoid%20Icings%20at%20Their%20Eaves.pdf)
I’m committed to doing a thorough air-sealing on the ceiling, and my interior design makes this fairly easy: simple roof with big open area, membrane will go up prior to interior wall construction, and very few penetrations. Of course I’m still worried, both about moisture control (even if I get the vapor barrier perfect, the roof could leak) and ice dams due to insufficient venting, and I have two lingering questions:
1. Should I use a smart barrier like Intello Plus? The biggest selling point (vapor-open in a humid summer) is irrelevant in my area where we have very arid summers (dew point 40-45F). I only worry about moisture in the winter. If I have moisture issues in the winter, will drying to the inside (where inside might have a dew point of 45F) even be possible? For $1000 is it a good backup plan, or just a waste of money in my climate?
2. I’ll be using snow brakes and I expect the main roof ridge vent to be covered in snow for 3 months a year. I read https://www.greenbuildingadvisor.com/question/ridge-vents-being-blocked-by-snow, which suggests that it will still vent, but just perhaps not as much. The CRREL math indicates I need 7 in^2 per foot NFA of vent (per side), which isn’t insignificant. The one thing that came to mind is that the longitudinal open area of the ridge vent is somewhere in the neighborhood of 40 in^2, and I could open up downward vents on the underside of the gable overhangs at the ridge. This would add 80 in^2 of venting at the ridge, which is only a fraction of the 490 needed (35 feet of ridge over the conditioned space) and biased towards the sides of the roof. Is this worthwhile, or a waste of time?
-Mark
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Mark,
I don't see any problems with your plan. 2.5/12 roofs perform fine if build diligently.
1. I agree. Given your climate, a variable-perm vapour retarder doesn't add much that is useful. In a heating dominated climate the vapour drive is from inside to out.
2. I would leave well enough alone, and rely on the eaves to ridge venting.
Taking into account your desire not to complicate things too much, one option you might consider is to use a continuous membrane (like house-wrap) above your rafters instead on making baffles in each bay:
https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
https://www.finehomebuilding.com/2021/04/28/a-new-take-on-insulating-a-roof
Thanks, Malcom. While most of my roof is 2.25:12, a smaller portion is approaching flat roof (0.6:12). Because the eave-to-vent distance is small (12 ft), the CRREL math says I'm still in the green on that portion of the roof, but part of me is worried that diligence wouldn't be enough. In the chance that water does manifest inside the roof assembly, I imagine it's possible to reverse the vapor drive, but the question being if it reverses strongly enough to do any good. Based on your response, I'm leaning towards no, and using poly sheet.
I do enjoy the elegance of the assemblies you linked, but I'm planning on using mineral wool batt insulation. Due to building height concerns, I didn't want to furr up another 1-1/2". While unfortunately expensive, mineral wool has the thermal efficiency to let me downsize the rafters while maintaining a 1.75" airway. And AFAICT mineral wool batts are dimensionally rigid enough to eliminate the need for baffles (I live in a WUI, and I can't seem to find a suitable baffle material that is cheap, vapor-open and flame-resistant).
Mark,
I'd make the same choices.
What is the roof surface going to be? The back of my current house is 2-2-1/2 and I went with EPDM because shingles had leaked for 40 years.
gusfhb,
From his original question:
" it’s basic cathedral ceiling construction: sawn rafters with batt insulation; vapor membrane between finished ceiling and rafters; plywood deck, underlayment, and standing seam roof "