What options for unvented, low-slope roof?
FixitAgainSam
| Posted in General Questions on
I am trying to determine what kind of roofing material to use on a low-slope (2.5/12), unventilated, shed roof. The roof will have closed cell insulation below the decking, so it must dry outward. But the typical materials for low-slope roofs, such as EPDM, are impermeable. I cannot take any approach that would add more than an inch total to the height of the roof, because it is a shed off a gable end of the house, and the roofline actually intersects the gable end soffit on one side. Other things that may matter: Zone 6, SE Exposure
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Fix it Again,
Unvented low-slope roofs are never designed to dry to the exterior (because low-slope roofing membranes are vapor barriers).
While this fact adds some risk to these assemblies (compared, for example, to a traditional vented unconditioned attic with asphalt shingles), this type of roof is very common. Every supermarket and WalMart in America has an unvented low-slope roof assembly that cannot possibly dry to the exterior.
For more information on these issues, see Insulating Low-Slope Residential Roofs.
Thanks, Martin. I am hearing similar opinions to yours, from a number of sources, but have also discovered some ideas on this forum and elsewhere. One approach is to use metal roofing over notched battens (see Rainscreen Roof?). I don't think that would work in this situation because the roof will be walked on occasionally, and the metal panels would deform. Another approach uses 2x4s on their sides as purlins over the sheathing and a second layer of sheathing. That adds too much height for this roof, but maybe I could use thinner purlins, and put EPDM on the upper layer. Huberwood.com's "zip system" and GAF Deck Armor are also interesting and could be used under metal, seeming to imply that the ridges in the metal panels supply sufficient ventilation for drying. Am I on the right track?
Fit it Again,
Of course it's possible to create ventilation channels above your roof sheathing with 2x4s. The problem is the low slope -- without a significant difference in elevation between the soffit vents and the ridge vent, you get hardly any air flow. All of this is explained in my article, Insulating Low-Slope Residential Roofs.
Venting vs. Drying
I do want to thank you for the articles on insulating low-slope roofs and the unvented cathedral ceilings. Those were my starting point, but I don't have adequate space either above or below this particular repair situation to really follow those articles in full. Maybe I am misunderstanding drying vs. venting, but I was basically thinking that the roof decking has to be able to dry (however slowly) either outward or inward. Since I am thinking closed cell foam on the inside, it can't dry to the inside, so what's the workaround, especially when I can't add much height on top? Is an air channel, however small, and open only at the eave end of a shed roof (behind the drip edge, I guess), worthwhile, useless, or worse than useless?
It dawns on me now that slotted battens perpendicular to the rafters could be open both at the rakes and the eave, so maybe that would be best? However, I would steer away from metal then, and use a second later of decking. I'd have to restrict the battens to about 1/2 inch thickness.
Thank you, sorry if I am not communicating clearly or seem to have a head made of XPS ...
Fix it Again,
In most cases, builders who install closed-cell spray foam on the underside of roof sheathing don't worry about drying. Here's why: the closed-cell spray foam is an air barrier as well as a vapor barrier. It prevents moisture movement from the interior to the cold sheathing by vapor diffusion (since it is a vapor barrier), and it also prevents moisture movement due to air leaks (since it is an air barrier).
The only remaining mechanism to get the sheathing wet is a roof leaks. Once you get a roof leak, it's time for a new roof, no matter what venting strategy you employ.
All of that said... you can install a ventilation channel above your roof sheathing if you want. (a) It won't hurt, and (b) It may allow very minor amounts of moisture from a small roof leak to dry outward, and will provide your roof with a few more months of life, until the roof leak gets worse.