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Community and Q&A

Roof Underlayment Product Recommendation

jbmoyer | Posted in Energy Efficiency and Durability on

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?

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  1. GBA Editor
    Martin Holladay | | #1

    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.

  2. Brent Eubanks | | #2


    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?

  3. GBA Editor
    Martin Holladay | | #3

    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.

    1. Laura62584 | | #23

      Quick question about this.. You mention a gap between the sheathing and the metal roof to help drying. If the ply is attached to original shiplap would the small gap between the ply and the shiplap help at all with drying toward the outside? Im sealing up my non vented cathedral ceilings (as air tight as possible) and if any moisture were to get through to the ply Itd be nice to know it could possibly dry due to the gap. Im not sure if the peel n stick under my metal roof is vapor permeable or not. TIA

  4. Brian A Evans | | #4

    Why not use a closed cell foam? Won't that eliminate/reduce the vapor diffusion issue?

  5. Danny Kelly | | #5

    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

  6. Chungee Mom | | #6

    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.

  7. Alex J. Knox, AIA | | #7

    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

  8. Steve Dawson | | #8


    I work for Cosella-Dorken and we produce several "breathable" roof underlayments. Please check out 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 [email protected]

  9. Riversong | | #9

    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.

  10. GBA Editor
    Martin Holladay | | #10

    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.

  11. Riversong | | #11

    Yes, and for the same reason that the optimum siding "underlayment" or WRB is 5-20 perms, but not higher.

  12. carpeverde | | #12

    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.

  13. Riversong | | #13


    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.

  14. roofrins | | #14

    My understanding is, laminated asphalt shingles, once sealed together, are a pretty low-perm layer (about 0.15 - 0.30 perms, as per this article by some Ph.D's - ). Therefore, what good does it do in the case of vented shingle roofs to have a "breathable" layer of synthetic? In other words, if your synthetic is, say, 16 perms, but then the moisture gets stopped cold at the underside of the shingles, and has to go back through the synthetic, and then back through the plywood, what good was it for the synthetic to be "breathable"? In any event, if you have a vented roof assembly, isn't your humid air supposed to be naturally transported out of the attic, as air intakes through the soffit vents, and outtakes through the ridge vents?

  15. roofrins | | #15

    Brett, I'm sorry I didn't read your question very well. If you have a conditioned attic, and you want your synthetic to breathe, you could go with GAF Deck Armor, which has a perm rating of 16. I assume you're using a metal roof or something on top? Otherwise, the asphalt shingle layer would stop any good that your breathable synthetic was trying to do, as it is not permeable (0.15 - 0.3 perms).

  16. jackofalltrades777 | | #16

    As mentioned by Martin and others. It makes no sense to install a "breathable" membrane and install a metal roof, asphalt roof, etc, that will not breathe to the exterior. If water gets past your standing seam metal roof and hits the membrane, you have this "breathable" membrane that lets moisture get past it and hit the OSB sheathing. It saturates the sheathing and when it attempts to dry to the exterior, your breathable membrane lets the moisture through but when it hits the metal roof, it is vapor impermeable so all drying stops.

    Basically your breathable membrane did more harm than good because it let water in one way but it won't help when it comes time to dry because your finished roof material (asphalt, standing seam) won't let the vapor out.

    Best thing to do is install a synthetic peel & stick membrane like Grace that is vapor impermeable. This way any water getting past your roofing material will not pass through the membrane.

    The only other solution is a tar paper on the roof and using a Spanish tile. Since the tile has all these voids and channels it allows for vapor drying. What is bad about this build up is that in 10-15 years you will spend about $30k having the crew come out and remove ALL that heavy yet brittle Spanish tile and install a new layer of building paper and then put ALL that heavy tile back into place.

    I think the consensus is that using a closed spray foam is better since it will not allow vapor drive from the interior to hit the sheathing and condense. Then install a synthetic membrane on the roof and make sure all penetrations are double sealed.

    Are you using SIPs or trusses?

  17. user-1041981 | | #17

    My builder and I are going through this exact discussion right now and need guidance ASAP.

    Is a roof design of (inside-to-outside):
    closed-cell spray foam insulation
    Huber Zip System sheathing
    breathable underlayment
    Roxul Comfortboard IS/CIS
    metal roofing (some standing seam, some shingle

    desireable / acceptable? Does the 1" of ComfortBoard provide the desired drying capability?

    I want to make sure that my roof sheathing is properly protected and ventilated to last for 40-50 years.


    (ps. we want to build this roof next week)

  18. GBA Editor
    Martin Holladay | | #18

    Your proposed roof assembly sounds fine from a moisture management perspective.

    However, the use of mineral wool insulation above the roof sheathing is very rare except on low-slope (flat) roofs. That doesn't mean that your plan won't work. It only means that you will be blazing a new trail when it comes to attachment details for your roof purlins or furring strips. Before proceeding, you might want to check with the technical help people at Roxul to make sure that they support your planned details for installing Roxul above the roof sheathing on a sloped roof.

  19. user-1041981 | | #19

    Thanks for the quick feedback Martin!

    Yes, as of this morning I'm in contact with the technical folks at both Huber and Roxul, but we aren't talking attachments - just moisture management since I really, really don't want my spray foamed, metal clad roof to fail for a long, LONG time.

    I think the roofer was just going to nail the metal shingles down through the Roxul Comfortboard into the sheathing, but I'll bring it up now that you've made me aware of the possible issue.

  20. severaltypesofnerd | | #20

    I found this technical white paper interesting on this topic:

    Owens-Corning 2011

    Basically saying that asphalt singles are so impermeable that the underlay might as well be also.

    This of course does not apply to any corrugated metal roof, which has airflow, nor would it apply to most of the "100 year" tile or slate type roofs. Under those roofs an underlay like the Delta Foxx would perform very different from, say, Ice & Water guard.

  21. user-7061227 | | #21

    So, what was the advice? Permeable or non permeable underlayment where we have closed cell insulation below and asphalt shingles above? Thanks

  22. Markitect54 | | #22

    I am in the same conundrum right now building in Western MA. Sips panel roof on a timber frame and asphalt roof (just can't swing the metal $). The Sips company says a vapor permeable underlayment or 30 lb felt (which is about 5 perms?) to prevent moisture from being trapped. I have been looking at deck Armor (16 perms), FT Synthetics Hydra (30/150 perms), USP AirOut Shield (119 perms). To add to the confusion the test methods varies inconsistently among the technical spec. between ASTM E96 A, B, and BW so there are a lot of apples and oranges to deal with.

    To add to the problem, the roofer is saying his Certainteed warranty is only good with their underlayments which all seem to have low permeability in the 0.2 range.

    It seems that even with an asphalt roof on top, some permeability is better than none when it comes to letting moisture out of the sips, and including a ridge vent makes sense since it seems moisture moves towards and can rot the top per the Alaska studies.

    I know this all just sends the conversation in more circles, but like C.B. above it is go time and I need to get something on top of the sips to protect them until the roof goes on. It is now 6 years later, so maybe there are some new insights! Thanks everyone for the discussion so far.

    1. Expert Member
      Akos | | #24

      I recently pulled off an a pretty old shingled roof that had 30lb felt. Even after a couple of decades of some ice dams, the felt was solid and so was the roof underneath.

      If your SIP folks say 30lb felt, I would have no issues with putting it on. The roofer might prefer deckarmor as it is easier to walk on and harder to tear, it should work just as well.

      As for ridge rot, the most important part is air sealing under the SIPs. So make sure you handle that before the SIPs go on. If plywood deck, tape the seams. If T&G/boards, fully cover in peel and stick.

      1. jacques_nj | | #25

        "If T&G boards, fully cover in peel and stick."
        Can you please clarify what type of peel and stick you have in mind in this case? Is this some kind of clear, transparent peel and stick?



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