Unvented low slope roof assembly vapor retarder/barrier location
I’ve perused quite a few articles, listened to podcasts and YouTube, etc. regarding unvented low slope roofs. A lot of common themes exist but details are seemingly light on the most important (arguably) detail of the assembly.
I’m building a 2/12 pitch roof with an approximate R80 roof assembly in climate zone 6 (UP Michigan). My want of a Mid-Century Modern trumps the inherent complications of the low slope roof. I plan to be very careful with sealing details, especially with 70 PSF snow loads.
That roof assembly is, inside to out as follows: Paint/primer achieving vapor retarder perms, 1/2″ lightweight drywall which is foam sealed, taped and with zero ceiling penetrations for can lights, etc., un-faced R37 fiberglass batt in 2×12 16″ OC rafter cavity (insulation is in contact with both sheathing and drywall), 7/16 commodity OSB. On top of this is what I’ll call “Vapor Location A” in which current plan would be a self adhesive vapor barrier like Grace, etc. (Lstiburek seems to suggest that a continuous self adhesive vapor barrier belongs at this location in the assembly. )
R47 of rigid installation @9″ thick is installed over the OSB lower roof deck (a mixture of EPS, XPS and PolyIso used/new foams). On top of this would be a synthetic roof felt (or something else) at “Vapor Location B”. This layer may not be needed .
Then installed are eave to ridge 2×4 flat purlins to allow limited ventilation of the assembly from eave to ridge vent, on which another layer of 7/16″ commodity OSB is installed (or thicker if purlin spacing dictates) and then a layer “Vapor Location C” of synthetic roof felt on rows 3+ and the first two? rows being grace again for ice damming due to low slope.
Vapor Location A – self-adhesive roofing ice shield barrier
Vapor Location B – synthetic roof felt or nothing at all
Vapor Location C – synthetic roof felt except ice damming lower locations
Does this sound correct?
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That assembly seems like it should work. Be aware that XPS ages to around R-4.3/in and polyiso varies with time and temperature to as little as R-5/in (or less), so be conservative in your ratio calculations to achieve at least 50% of the R-value on the exterior. The roof won't vent very well due to the slope but it won't be getting any moisture from the interior so it should be ok.
Thank you for the reply. I'm planning on putting the polyiso on the bottom of the roof assembly to mitigate some of the R-value loss at low temperatures. Some seem to suggest that thicker polyiso assemblies aren't really affected as much, something about the R-value in the middle, but I get lost in how that same scenario doesn't apply to XPS or EPS. That or I'm totally misunderstanding why some say polyiso is great in climate zone 6-7.
The temperature inside the assembly varies, from matching the outdoor temperature at the outer face to matching the interior temperature at the inner face. If the insulation is homogenous, the temperature change is linear--i.e., if it's 20° outside and 70° inside, the center of the wall will be 45°. The math is a little trickier with non-homogenous assemblies like yours, but that's the principle. So depending on the outdoor temperature, it's only the outer few inches of polyiso at most that's affected by the R-value drift.
Polyiso is unique among insulation products in that the blowing agents filling the foam cells, butane-like compounds, which provides the insulation ability, loses R-value as the temperature drops. The blowing agents in other foams increase in R-value as the temperature drops. The insulating agent in fluffy insulation is plain old air, which also increases in R/in as the temperature drops.
Over time, the blowing agents in foams is replaced by air, but it takes a long time and varies a lot. Despite the R-value drift, polyiso still has the highest R/in of any readily available rigid insulation product and its global warming potential is not as bad as some of the other foams.
i recall an old FHB article on cold roofs where the author's experience was that the flat framed 2x wasn't enough air space to take full advantage of a cool roof. The author recommended a full 3-1/2" tall air space.
You don't talk about the method for attaching the 9" of insulation. your sleepers will hold the roof down evenually, but it's hard to hit 2x framing on edge that deep. even though it's a shallow pitch, it doesn't seem fun to work on top of insulation that's not fully secure. I'd consider running flat 2x sleepers over the OSB. Your R-value would go down because the insulation would notch around these, but you'd have something wider to aim at when attaching the high sleepers.
I did intend on placing flat 2x4 sleepers on the roof deck for a larger target, nailed to roof rafters. I have a multiple span section where the offset of the rafters would be nullified by the sleeper riding up the centerline of the joist offset (with 1/4" overhang on one rafter side, 1-3/4" on the other, flipping the opposite way at the joist offset). I've been working on an image to show this complicated assembly and will try to post soon. I think the r-value reduction from the sleepers in the assembly is negligible.
I would also lean towards a full 3-1/2" tall air space. The early work done by CRREL engineers gets into trying to provide guidelines for roof venting. In particular, the article linked below is one I lean on when I have unusual roof conditions such as low slopes and/or long spans.
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
Excellent resource, first time I've seen this. The cathedral ceilings on page 4, figure 7 using R40 and 3/12 pitch shows that 1.25" would be acceptable with up to a 30' roof plane. I'll be working with just about 30' on the longest shed plane. I'm using 2/12 as the ideal pitch and will need to do the math but with the assembly a higher R value than R40, I think that will get me to where 1.5" is sufficient.
Having a deep air space is important when there is risk of moisture migrating from interior to exterior. That's not the case here; the foam and membranes will eliminate all interior moisture concerns, so the vent space is mostly superfluous. It will keep the roof surface near ambient temperatures and the deeper the better, but I don't see why it would need to be more than 1 1/2".
More info here: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs.
Given the amount of insulation, you may be right. It would help to know the roof span (eave to ridge vent distance).
ClimateZone8,
Is there any research into the required depths for ventilation channels mainly designed to avoid ice damming, rather than to remove moisture, as this one is?
The link to the CRREL research I posted above is the one I've used several times. The example tables and graphs cover a pretty good spread of scenarios. Page 4 starts the "Design aids for cathedral ceilings section". Where I have found it useful was for longer runs like a 40' run at a given pitch.
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
More information on the research can be found here in these two articles:
https://www.poa.usace.army.mil/Portals/34/docs/engineering/MP5420,%20Ventilating%20Cathedral%20Ceiling%20to%20Prevent%20Problematic%20Icings%20at%20Their%20Eaves.pdf
https://www.aivc.org/sites/default/files/airbase_11883.pdf