Durable, yet breathable roofing underlayment.
I am building a home with a conditioned attic. I will be spraying 11-12 inches of open cell foam underneath the roof sheathing. I need to allow the sheathing to breathe to the exterior, for it will not breathe to the interior with the 12 inches of foam. Thus, I was wondering if anyone was aware of a roof underlayment (other than building paper) that is breathable, durable, and that has a high exposure rate. I know that there are synthetics out there, but none has an appropriate perm rating (most are around 0.3 perms which would not breathe.) Ideally I would prefer a synthetic underlayment that has a perm rating of around 5.
Anyone? Anyone?
Asked by Brett Moyer
Posted Wed, 07/22/2009 - 11:44
Posted Wed, 07/22/2009 - 11:44
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James,
There are several issues arising from your question.
1. Twelve inches of open-cell spray foam has a permeance of about 5 perms, so the spray foam will be semi-permeable. Some drying to the interior will occur.
2. Regardless of your roofing underlayment choice, your insulated roof assembly will not dry to the exterior unless the roofing is vapor permeable. Many types of roofing, including EPDM and standing-seam steel roofing, are virtually impermeable to water vapor. Other types of roofing, including cedar shingles and concrete tile roofing, are fairly permeable. Asphalt shingles are a little bit permeable in windy conditions -- but not very.
3. If you are set on using a permeable roofing underlayment, the obvious choice is asphalt felt, which has been used successfully for that purpose for decades.
Posted Wed, 07/22/2009 - 12:05
Martin,
You seem to be implying (with your wording on #3) that you think the best option would be to use an impermeable roofing material (and underlayment as well) and then rely on the interior drying capacity to deal with the relatively small amount of water that gets in. Is that correct?
Posted Thu, 07/23/2009 - 19:36
Brent,
If you want to build a roofing assembly that dries to the exterior, the choice of roofing underlayment is a small part of the equation. For robust drying, the system should include a generously sized ventilation channel from eave to ridge, directly under — or, in the case of metal roofing, perhaps over — the top layer of roof sheathing.
Many, perhaps most, roofs have fairly impermeable top layers, with underlayment directly underneath. That's why the permeability of the underlayment rarely matters much.
I really don't want to wade into the debate over whether a thick layer of foam, sprayed directly against the roof sheathing, is likely to be a good long-lasting way to build a roof assembly. The jury is still out on two questions:
1. How much water leaks into such assemblies from above?
2. How long does it take for the average homeowner to notice and repair roof leaks when spray foam disguises them?
Finally, in cold climates, sloped ceilings insulated with open-cell spray foam are at some risk of developing diffusion-related moisture problems. If interior humidity is high enough, vapor can diffuse through the open-cell foam and raise moisture levels of the roof sheathing. This problem is best addressed by including an interior vapor retarder — for example, by spraying the cured foam with vapor-retarder paint.
Posted Fri, 07/24/2009 - 03:00
Why not use a closed cell foam? Won't that eliminate/reduce the vapor diffusion issue?
Posted Wed, 07/29/2009 - 12:59
You did not tell us where you are building - the 2009 code requires a 1 perm or less vapor retarder installed on top of the roof deck in WARM HUMID locations
Posted Thu, 08/13/2009 - 00:45
GAF Elk Deck Armor is 16 perms. You should check out the online spec sheet. Sounds like what you're looking for if you haven't been convinced to go a different route. I'm interested to know what you decided and also what climate zone you're in.
Posted Thu, 10/15/2009 - 15:52
I'm in the camp with the impermiable in this case. The foam is your barrier and, I think, you want to protect the adhesive connection between your foam and your roof substrate. If that adhesion fails let it be do to years of freeze thay in lieu of condensed water vapor. I'd also also ventilate the attic side of the faom so any of the house moisture has a chance to escape. One thing, where you are the most vulnerable is at the penetration points of your impermeable to your substrate, use a cap or a washer to protect your penetration point. Just my (2) cents, first time I've done this. Thanks for the tip on the Gaf Elk Deck for my problem
Posted Sun, 02/21/2010 - 21:36
Hello,
I work for Cosella-Dorken and we produce several "breathable" roof underlayments. Please check out www.deltaroofingunderlayments.com. Delta-Vent S has a perm rating of 120 perms, water tight, air barrier, excellent UV resistance, and very strong. Delta-Foxx has a perm rating of 550 perms and is even more durable than Vent S. If you would like to discuss any of our Delta Roofing underlayments in more detail, please feel free to email me at sdawson@cosella-dorken.com
Thanks,
Steve
Posted Fri, 04/09/2010 - 14:44
Sorry Steve, but those perm ratings are astronomical and far in excess of what is safe to protect roof sheathing from moisture accumulation. The OP is correct that a roofing membrane should be about 5 perms.
Posted Fri, 04/09/2010 - 17:14
Robert,
Are you worried about the phenomenon of solar vapor drive through asphalt shingles leading to moisture accumulation in insulated cathedral ceilings? If so, a very low perm roof underlayment is what you want -- much lower than 5 perms. In all other situations, I can't think of any disadvantages to a high-perm underlayment — assuming, of course, that the underlayment sheds water.
Posted Sat, 04/10/2010 - 04:29
Yes, and for the same reason that the optimum siding "underlayment" or WRB is 5-20 perms, but not higher.
Posted Sun, 04/11/2010 - 23:48
Okay, now I'm confused. I had thought a highly permeable underlayment was advantageous under a metal roof because condensation trapped UNDER the underlayment could rot the roof deck over time. Because this would be a winter condition, it would likely only occur in a vented attic assembly where the warmer attic temperatures can more easily reach the top surface of the deck. Can I assume that a permeable underlayment is desired for ventilated attic assemblies and that a low-perm underlayment is preferred for unvented attic assemblies (with open or closed-cell foam applied to the underside of the decking)? For hot-humid climates, I'm leaning toward Martin's comment in answer #10.
Posted Wed, 04/21/2010 - 11:16
Stephen,
You have it backwards. A breatheable underlayment is necessary if a vapor impermeable layer (spray foam) is under the sheathing. All wood products need to be able to dry in at least one direction and preferably two. A breatheable underlayment is preferable in all circumstances.
A 10-year computer simulation study, using hourly weather data for a hot climate (Miami) and a cold climate (Boston) demonstrated that a roof deck with vapor impermeable layers on both sides was highly vulnerable to moisture damage in either climate in the event of a roof leak, which has to be assumed as a high probability during the life of a structure.
A "perfectly" sealed element is durable only as long as perfection can be guaranteed. Since lifetime perfection cannot be guaranteed even with the most sophisticated technologies, a "fail-safe" mechanism must be incorporated. In building envelopes, that mechanism is high drying potential. Breatheable underlayment and vented roofs create high drying potential. Anything else undermines long-term durability.
Posted Wed, 04/21/2010 - 11:35