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Community and Q&A

Ensuring drying on flash and batt unvented standing seam roof

ekettenburg | Posted in Energy Efficiency and Durability on

I am currently building a 2600 sq ft home – with beyond code insulation, ducted mini splits, HRV for ventilation, and high efficiency windows – and the framing is nearly complete.

This is located on the line between Marine 4 and 5 (Clark & Skamania County line in SW Washington State). We are located at 1000 ft and see everything from 5 degrees to 100 degrees, about 60 inches of rain, and as much as several feet of snow (county design snow load is 70 psf roof).

From the start we planned a unvented cathedral roof at 10/12 pitch, using flash and bat, 2×12 rafters (stick framed room, all ceilings in the house are vaulted), and a standing seam metal roof. Thanks to many articles I’ve read here, and the community here, I felt I had a good understanding of what was required to do this (spray foam under the sheeting, then batts, then well sealed drywall with no can lights) and why it was best (the roof is a simple gable but has skylights and dormers in it).

Now as we get close to putting the roof on, with quotes from roofers in hand and a few selected based on their experience with standing seam – I realize that one thing I did not fully understand is ensuring the sheeting can dry in at least one direction.

Our roof stackup is currently planned as:
1. Standing seam metal (ASC Metal Skyline 16 in a medium gray color “Old Zinc Gray”)
2. Synthetic Underlayment (Titanium 50 is what the roofer wants to use)
3. 5/8 Plywood (already in place)
4. 3″ of closed cell spray foam (exceeding the R value requirements for flash an batt in Marine 4 since we are next to zone 5)
5. Remainder of 2×12 rafter bay filled with blown in fiberglass or cellulose
6. Drywall ceiling with all seams taped.

I’ve seen several suggestions here and elsewhere to ensure the plywood sheeting can dry, however some don’t seem ideal for our construction:

1. Rigid foam on the exterior – both cost and aesthetics prevent this from being a good option. The metal roofing requires a solid substrate so we’d have to put down more plywood over the foam. The resulting 4-5″ sandwich would also look rather strange because the roof already has 2×10 fascia and exposed tails – I think a 2×16 fascia would look too strange.

2. A vent channel under the metal combined with #30 felt or breathable underlayment – again this would require another layer of plywood which would be expensive, and would make the deck look thicker. If #1 and this are the only options I guess we would pick this.

3. I’ve seen some mention of using Cedar Breather between the underlayment and the roofing – this seems doable and affordable but I’m not sure how effective – has anyone done this? what are the opinions of whether this provides enough drying in a stackup like we want to use?

4. I’ve seen some mention of using #30 felt or a breathable underlayment and just putting the metal over it, but it isn’t clear to me if this has any benefit – I don’t think it does since the metal cannot breathe. Would this work? Can it dry to the inside with 3″ of closed cell spray foam?

Any help understanding this fully and how best to build this roof without having a thicker roof deck would be most appreciated!

Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    Most flash-and-batt roofs don't have any path to allow the roof sheathing to dry. This is also true for roofs insulated with 100% closed-cell spray foam. (They exist.)

    Here's the idea: You make sure that the roof sheathing is dry on the day that the spray foam contractor installs the spray foam, and then you don't worry.

    As long as the roof sheathing starts out dry, it shouldn't take on any new moisture until you get a roof leak. If you've got a roof leak, you've got wet sheathing -- but that's the case with any roof. In that case, you repair the roof leak or install new roofing (and you make sheathing repairs if necessary).

    If you're the type of GBA reader who doesn't like this approach, because you want the roof sheathing to enjoy the benefits of a drying mechanism, you can either (a) choose a different type of roofing like concrete tiles (which are vapor-permeable), or (b) install a ventilation channel above your roof sheathing, along with a second layer of roof sheathing (as you noted in point #2 of your comment). Most people wouldn't do this -- but it's your house, and you get to spend your money how you want.

  2. John Clark | | #2


    Q: What about installing the metal roof on top of 2x4 purlins and doing away with the 2nd layer of sheathing altogether?

  3. GBA Editor
    Martin Holladay | | #3

    The approach you suggest is allowed by some, but not all, manufacturers of standing-seam metal roofing. Check with the roofing manufacturer to determine whether solid decking is required before proceeding.

  4. Expert Member
    Malcolm Taylor | | #4


    My knowledge of Washington State seismic geography is a bit sketchy, so I don't know if this is an issue for you or not, but our code up here in coastal BC no longer allows unsheathed roofs. They want the roof to act as a structural diaphragm.

  5. Jon R | | #5

    > Cedar Breather between the underlayment and the roofing

    With perhaps .7 perms and good air sealing below it, the sheathing sees some but not much wetting. In my opinion, a CB vent channel plus breathable underlayment would work very well to remove this amount of moisture.

    >metal cannot breathe.

    But the seams can. So you don't have to move moisture very far. A guess is that even a crinkle wrap breathable roof underlayment would suffice - but no idea if one exists.

    Replacing/re-sealing sheathing directly sprayed with foam sounds like an adventure.

  6. Expert Member
    Michael Maines | | #6

    I don't think much drying is happening through the seams on a metal roof. Maybe on a conventional screw-down rib/panel, agricultural-style roofing, but certainly not on double-lock standing seam, and unlikely on the snap-lock standing seam the OP noted. (Jon R., I'm really not out to get you--this is a topic I discuss often with clients and builders.)

    One note to add to Martin's advice--if you install metal roofing with any air space below, be sure to install a WATERPROOF roofing underlayment, because you have created a condensation machine. Many roofing underlayments are water-resistant but not waterproof.

  7. Expert Member
    Dana Dorsett | | #7

    Standing seam metal roofs definitely offer reasonable drying capacity, especially when mounted on purlins above the structural roof deck. With proper underlayments there is still some outward drying, but in often-rainy sometimes windy Skamania County purlins would be a safer bet.

    Designing for seismic loads in that location is prudent, given that Skamania is the home county of Mt. Saint Helens, and the western slopes of Mt. Adams.

    Using 3" of rigid polyiso above the roof deck and a full cavity fill of cellulose or fiberglass in the 2x12 bays would be higher performance, more resilient and probably comparable or cheaper than a 3" closed cell flash 'n'fill, even using virgin stock polyiso. This outfit in Portland always seems to be advertising 2"-4" reclaimed polyiso at a fraction of the cost of virgin stock goods, which would make it cheaper still:

  8. GBA Editor
    Martin Holladay | | #8

    Just an example -- from the web site of Clicklock Premium Standing Seam from Classic Metal Roofing Systems: The manufacturer explains, "our roofs require solid decking."

    The requirement can be found on this page:

    Other manufacturers of standing seam have different requirements.

  9. Jon R | | #9

    Maybe there isn't much air movement through snap-lock seams - but air movement generally moves much more moisture than diffusion. And in this case, there is so little diffusion that no ventilation is an option. Hard data is needed....

  10. ekettenburg | | #10

    First of all - thank you everyone and Martin for all the extremely helpful responses!

    Solid decking is required under the standing seam metal - both by manufacturer and by the engineer - so the only option we have to create a true air gap would be purlins and more sheathing - something we would like to avoid for cost and aesthetics.

    Jon R expanded on my cedar breather question and that seems like a promising compromise - but what is not clear to me even after reading everything on "cedar breather unvented metal" I can find is -

    Would adding cedar breather between the metal and the underlayment create as Michael Maines said an air gap would cause "a condensation machine"?

    So if I went the cedar breather route would I want to use non-breathable underlayment? or breathable? And would I be inviting more problems by putting all that condensation on the underlayment instead of the roof surface?

    Thank you all again,

  11. Expert Member
    Michael Maines | | #11

    Erik, I doubt installers would want to put metal roofing over cedar breather. It makes a good capillary break but is flexible and will result in oil canning, the bane of metal roof installations. And it's not thick enough for significant air flow. That's good, because air flow (on cold nights following sunny days) is when the bottom of skip-sheathed metal roofs produce condensation. But it also means that you won't get significant drying. At least that's my semi-educated opinion on the matter. (I've only used cedar breather and similar products under cedar shingles; even clapboards and solid board sheathing is difficult to install well over it.)

    If you want a more resilient assembly, you need to vent the roof properly either above or below the current sheathing plane. Code requires a 1" vent but 1 1/2" to 2" is better. Furring the entire roof with 2x material and installing a second layer of sheathing will cost money, as would installing spray-foam-proof vent baffles on the interior, but if you want a vented roof, those are your options unless something else changes. There are metal roofing companies who will install their panels over horizontal 2xs, but you might have to search for them. Or you could accept an unvented roof that is a slightly riskier assembly than a vented roof, but that should be fine as long as everything is done well.

  12. Expert Member
    Malcolm Taylor | | #12

    I don't know how you would install a metal roof over a flexible substrate like Cedarbreather. All the Flashing and penetrations in the roof rely on gasketted fasteners, which in turn rely on everything staying tight, with no flexibility between the panels and what is underneath.

  13. Expert Member
    Michael Maines | | #13

    Jon, my only issue with cedar breather is its compressibility. 100 psf is a lot less than what you get at a nail or screw, which is probably well over 1000 psf. Having dealt with a lot of metal roofs and metal roofers, and installed cedar breather and screw-down metal roofing, I do not think they are compatible. But I agree that it will certainly do no harm from a building science point of view, and may even help. The problem is not between fasteners, where the metal panel may or may not be able to span (I've done a lot of skip-sheathed metal roofing). It's that at the point of fastening, the metal will deform, likely all the way down to the roof deck. Spun-bonded polyolefin is just not that resistant to compression.

    Lstiburek says you need at least 3/8" for air flow. The building code says you need 1". Lstiburek says more than 1" is better, and that less than 3/8" is a good capillary break, but not enough space for significant airflow. But it would certainly provide more drying than no space at all.

  14. Expert Member
    Malcolm Taylor | | #14

    I install my own metal roofing, so for me this isn't a theoretical problem I'm speculating about. Fasteners need to be attached to a solid substrate. Purlins are a solid substrate, you don't fasten in between them into air. Screw down a subfloor over foam and you will see the compression at the perimeter of each sheet at the fasteners. it isn't a problem because unlike gasketted fasteners, the compression isn't being relied on to maintain the integrity of a roof.

    Given that the drying potential of a 1/4" gap with no openings to the outside except perhaps at the peak is negligible, and no manufacturer I've seen allows installation over Cedarbreather, why would anyone want to do so?

  15. Jon R | | #15

    I suggest that compared to the amount of wetting from below in Erik's roof, moisture movement via a 1/4" vent gap is very significant, not negligible, and more resilient than no gap. Larger would be wasted expense. On a roof designed to unvented specs, 1/4" *is* code compliant.

    There is a question of outside airflow under metal roofing being a net plus or minus, but that's true at 1/4" or 2" and arguably true with no intentional gap. A little bit of moist outside air gets under metal roofing even with no gap (and the moisture it sometimes deposits has a harder time getting out).

    I find it unlikely that for moisture in Erik's design, a 0" vent is fine, 1+" is fine and somehow 1/4" is a negative.

  16. Expert Member
    Malcolm Taylor | | #16


    It makes the discussion a lot more difficult to follow when you go back and significantly edit your posts.

  17. Jon R | | #17

    Not my area, but it has a spec - it changes from .27" to .26" under 100 psf load. I expect that most gaskets can tolerate far more movement. But if not, replace the Cedar Breather with 1/4" plywood in those areas. Somehow people build metal roofs over super compressible air (ie, purlins and no additional sheathing).

    Odd that Benjamin Obdyke would sell CB for an application that didn't work:

    And they aren't the only ones. For example "Tyvek Metal".

  18. Jon R | | #18

    You mean linking to the installation instructions *again* (they were linked to earlier but apparently ignored). We will just have to disagree about the significance of that.

  19. ekettenburg | | #19

    Thank you all again for the detailed thoughts about using cedar breather - I'm going to explore that further with the two roofers I am selecting from and we're also exploring doing a 1x or 2xs and more sheathing on top of them - except the show stopper there seems to be that we have 2x12 Fascia already (2x12 rafters with exposed tails) - adding any more height would look pretty strange and require a second Fascia board.

    A few questions just to ensure I have this straight from a building science point of view:

    1. Unvented, no air gap - would I still want to use a breathable underlayment like I would with an air gap, for any ever so slight drying between the seams, etc? Or would I want to use the most waterproof underlayment I can get because the sheathing can't dry?

    2. Unvented with air gap - this was touched on above - but it's not completely clear to me still - will an air gap cause MORE condensation under the roof and on top of the underlayment because the dew point occurs in that gap versus - with no air gap - on top of the surface of the metal?

    Thank you again!

  20. GBA Editor
    Martin Holladay | | #20

    Q. "Unvented, no air gap - would I still want to use a breathable underlayment like I would with an air gap, for any ever so slight drying between the seams, etc? Or would I want to use the most waterproof underlayment I can get because the sheathing can't dry?"

    A. You are overthinking this question. There won't be any outward drying if you install a standing-seam metal roof above your underlayment with no air gap, so the vapor-permeance of your roofing underalyment is irrelevant. Use whatever you want.

    Q. "Unvented with air gap - this was touched on above - but it's not completely clear to me still - will an air gap cause MORE condensation under the roof and on top of the underlayment because the dew point occurs in that gap versus - with no air gap - on top of the surface of the metal?"

    A. I'm not going to speculate on the Cedar Breather approach, because it isn't a standard approach for standing-seam roofing. I would certainly contact the standing-seam roofing manufacturer to ask about installing their roofing over Cedar Breather before deciding whether it's a good idea or not.

    If I had to speculate, I'd say that real ventilation drying through the Cedar Breather is unlikely, because (a) the gap is too small to allow for significant air flow, and (b) it's not clear to me what type of soffit vent or ridge vent you'd use with this approach. The lack of air flow means that condensation in the springtime (when warm weather follows snowfall) is less likely than it would be with a metal roof that has a more generous ventilation channel.

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