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Roof underlayment for vented roof with standing-seam metal roofing

Westhawk39 | Posted in Energy Efficiency and Durability on

Im nearing the end of construction of a high efficiency timber frame hybrid house I’m building for my self and have a low slope roof question that I would like some opinions on. Roof pitch is 1.5/12 (7 degrees)

Our main roof structure consists of timber rafters, 20″ I-joists, dense pack cellulose, 1/2″ plywood sheathing taped to the walls as air barrier. We drilled holes all over the roof to dense pack the cellulose from above (as our interior finish was installed on top of the timber beams as soon as they were installed.

We sealed the holes by installing Blueskin VP100 across the entire roof. Now to the tricky part. Blueskin VP100 is not rated as low slope underlayment. I figured we would either negate needing a low slope underlay with our vented over approach or install a proper underlayment when building the upper roof assembly.

So our shell is air tight with no overhangs, VP100 on the roof (lapped over #15 felt on walls). Next we install 4×6 timbers @5′ O/C across the entire roof. Then 2×4 “periins” at 2′ O/C above, then 7/16″ OSB on there.

So in conclusion, the roof material will be installed on the OSB, which is a total of 9.5″ above the VP100, with 6″ of continuous vent space between my secondary timber beams.

Roof material will be a New Tech SS150 standing seam profile (1.5″ tall) , single fold. The roofing company said they would prefer to install a Titanium PSU30 underlay. It appears to be self adhered, but only rated so 2/12 slope according to manufacturer website. Installer wants $1300 for this underlay on my 1500 square foot roof.

My question is, with our vented over approach, will the VP100 suffice as underlay? How much water will actually get through the standing seam roof with the fact that its vented above 20″ DP cellulose? I can’t see any heat loss occurring, especially enough to cause melt and ice buildup that would compromise the roofing. And if any water gets through occasionally, will the VP100 work any better since there is 6″ of vent space above and no capillary action keeping the water pushed against the underlay?

Perhaps its an issue of capillary action with the low slope?

Any advice would be appreciated.


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  1. Westhawk39 | | #1

    See PDF sketch of the assembly attached to my original post. Note the structural has been engineered. We have no inspections or code officials mandating what to do

  2. Expert Member


    I'm finding this a challenging roof to understand all the implications of all the separate parts.

    The interior finish that sits below the I joists. Is it continuous over the beams , and what is it?

  3. Westhawk39 | | #3

    Thats correct Malcolm,

    7/16"X3" cedar is continuous and was installed on the beams (from above) This was too flimsy to dense pack against so we installed 1x8 rough pine directly above that (see photo "cedar pine decking")

    On the upper timber layer there is also 7/16x3 cedar just where visible from below at overhangs (soffit). See image (timber layer 2)

    The final layer of 2x4 perlins and 7/16" OSB has not been completed yet.

    Last image shows venting just below the soffit decking

  4. Expert Member
    Michael Maines | | #4

    Scott, assuming the roofing layers above the Blueskin are open to the air at eave and ridge, there is absolutely no worry about moisture from inside causing any problems, from what I can tell from your photos and sketch. (The sketch is very helpful, by the way.) I don't think Blueskin VP100 is intended for this application, but it looks pretty well protected by other layers.

    One problem with metal roofs over purlins is that night sky radiation can cause the air below the roofing to condense, where it will then drip onto the roofing underlayment below. The layer of OSB below your roofing will probably provide enough insulating value that this won't be a major problem for you. But it may be something your roofer is concerned about.

    The other problem is potential leaking at such a low pitch. 1 1/2" tall seams are good, but I would want a double fold to better resist leaks. Even then, with such a shallow slope, assuming you're in a zone 7 location where it snows, a deep layer of snow is likely to accumulate; a sudden warm streak could melt the snow at the bottom of the pack and overwhelm even 1 1/2 double-lock seams. If you had board sheathing instead of OSB, and a fully waterproof membrane instead of Blueskin, it might not be necessary (boards can wet and dry safely; most OSB eventually turns to oatmeal). But based on the assembly you show, I would err on the side of safety, listen to your roofer, and include a waterproof underlayment.

    Edit to add: I did not realize there was no interior vapor retarder. I believe that the lack of an interior vapor retarder makes this a risky to very risky assembly, in danger of moisture accumulation at the plywood sheathing.

  5. Expert Member

    Michael and others,

    I have a hard time understanding what is at play with these low-slope roofs with venting above the sheathing. Perhaps someone can offer an explanation..

    Breaking it down:

    - There is no interior side air barrier or vapour retarder.
    - The cavities between the rafters are completely filled with cellulose.
    - The sheathing is in contact with the insulation.
    - The sheathing can dry to the exterior or interior.
    - The sheathing is not protected from becoming cold by attentional insulation.
    - Continuous ventilation is provided from soffit to ridge above the sheathing.
    - The roof pitch is 1.5/12
    - The upper-part of the ventilation path is broken by horizontal purlins.

    Is the sheathing acting much as vent channels do in conventional vented roofs? How does having the venting above rather than below the sheathing change the way it performs?

    Is it somehow immunized by the venting above from moisture problems typically associated with sheathing in unvented roofs? How problematic is it that the sheathing will remain cold?

    Is there something about locating the ventilation above the sheathing that makes venting a low-slope roof effective, when it isn't if the vents are below the sheathing?

    Any help would be appreciated.

  6. Westhawk39 | | #6

    Thanks for your input Michael

    Will go ahead with a low slope underlay.

    I did not have any building science or energy modellers involved and designed the assemblies based on knowledge gained here and other places. We used the VP100 to air seal the sheathing after drilling holes all Over to dense pack through. I used VP100 specifically due to its high perm rating that any approved low slope underlay (that im aware of) did not To facilitate outwards drying of the roof sheathing.

  7. Expert Member


    That I'm uncomfortable with your roof assembly doesn't mean I think it is problematic, just that I don't understand it.

  8. Expert Member
    Michael Maines | | #8

    Scott, it's too late in your case, but for anyone else who finds this thread, some European membranes that are water-tight and vapor-open, and would have been perfect in place of Henry Blueskin. If you use another underlayment under your roofing I don't see a problem with using Blueskin, but I don't hold the warranty. is expensive and requires field-welding, but is vapor-permeable and waterproof. does not appear to have slope limitations, and is vapor-open (34 perms) and waterproof (16' water column).

    Malcolm, Scott has a large airspace under his sheathing--at least 5 1/2" of clear air, plus 3 1/2" between the purlins. There is essentially no restriction to airflow, even if the stack effect won't do a lot in this case. Lstiburek and others would prefer a central, cupula-style vent ( My gut feeling is that with Scott's slight slope, cold climate, and generous airspace, that he will have enough airflow to clear any moist air. But I don't have data to support that position.

    Venting below the sheathing will keep the sheathing drier than venting above the sheathing, but if there is not a lot of moisture flow and the sheathing is vapor-permeable to some degree and resilient against moisture damage, either approach can work. Venting above the sheathing on more conventionally-sloped roofs is not uncommon (though not particularly common either).

    One thing I didn't catch is what is happening below the cellulose. I assumed there was painted drywall, at minimum, to retard vapor diffusion and to provide fire resistance. If the only thing between the cellulose and the interior are wood boards, that's not good at all--the cellulose is going to take on a lot of water over the course of the winter and spring. Building codes always require some sort of interior vapor retarder, painted drywall at least. With the vapor-open Blueskin on the exterior it may not be a problem, but for the amount of material, layers and money invested in this roof, it's not a very resilient assembly, and would not meet any building codes that I'm aware of.

  9. BPetersPries | | #9

    On the matter of the interior vapour retarder, I would be interested to know what wood finish options might offer similar permeance, or lack thereof, to latex paint over GWB. Somewhere I heard that shellac is a preferred finish for woodworking jigs, minimizing wood movement due to its low vapour transmittance. I haven’t seen any numbers, and don’t know if wood finish manufacturers include these ratings in their Tech Data sheets, but maybe there is an option available that you could still apply at this stage in construction.

  10. Expert Member
    Michael Maines | | #10

    Benjamin, I don't know for sure, but imagine that any film-forming finish slows vapor movement to some degree. Oils that polymerize within the wood cells probably do as well. The problem in a wood ceiling is the gaps between boards, which get bigger in cold weather due to drier air, when you most want to slow vapor movement. In a painted wood ceiling you could caulk the gaps but based on personal experience that doesn't usually hold up well over time.

  11. BPetersPries | | #11

    Obviously the gaps between the boards is a less than ideal condition, but considering that vapour diffusion is the concern, and not air leakage, it seems like a vapour retarding finish would still be mostly effective (I’m thinking of Joe Lstiburek’s “golf-cleats on subslab poly” analogy here).

  12. Expert Member
    Michael Maines | | #12

    Benjamin, the difference is that the pressure differential is much higher on either side of a roof in cold weather than it is across a floor slab. The warm, moist indoor air is pushing to get outside with a pressure of up to 0.2 inches of mercury, equivalent to 0.1 psi or 14.4 pounds per square foot. That's assuming indoor conditions of 70°F and 40% RH, outside conditions around 0°F. That force is concentrated at the seams. The pressure across a slab is insignificant in comparison.

  13. BPetersPries | | #13

    That may be, but it seems like the VP100 which has been detailed as an air barrier is tackling that issue. If air is not exfiltrating appreciably from the exterior of the assembly, and additionally the cavity is dense-packed with cellulose, there should be virtually no air pressure difference across the ceiling boards, which brings it back to being a question of vapour diffusion through the mass of the boards, and not between the cracks. I interpreted the perforated poly analogy to mean that if 99% of the surface area of the vapour retarder is intact, 99% of the moisture *diffusion* should be retarded, driven by a humidity gradient across the material or assembly, and not by an air pressure differential.

  14. Expert Member
    Michael Maines | | #14

    Benjamin, I'm still learning the fine points of hygrothermal analysis, so you might be right, but it seems to me that pressure is pressure. Granted air moves a lot more moisture than diffusion, but diffusion still moves moisture, even if only across 1% of the surface.

    I have data loggers in a cathedral-ceiling house in zone 6, with taped Zip sheathing on the interior as a vapor retarder (dry cup 5 perms), and steeply pitched metal roofing installed over a vent space. Even with an interior vapor retarder and a very airtight house (0.25 ACH50), the roof accumulates moisture around this time of year. Granted the exterior could be more vapor-open, like Scott's, but otherwise my assembly should perform better than his, yet moisture still gets into the cavity.

    The questions in Scott's case are will the drying ability stay ahead of the continual wetting from the interior, and does anything bad happen due to continual wetting? My guess is yes to the first and no to the second, but they are just educated guesses.

  15. BPetersPries | | #15

    I feel like this discussion is a great illustration of the air barrier vs. vapour retarder confusion. I’m certainly not qualified to weigh in with any authority (the pyrometric chart confounds) but I’m not sure that ‘pressure is pressure’. When considering a balloon, it seems clear that air pressure differentials will concentrate air flow at interruptions, but my impression is that diffusion is much like thermal conduction, with the energy (or in this case water vapour) migrating more or less directly across the gradient without the fluid properties of air. In that case the analogy would be less like a balloon, and more like a bag of chips left partially open in a humid space. That might be good news for Scott, but in either case, his outward drying potential should be excellent and if his roof rots and falls on his head, he can just point at his low-slope roof and blame it on the roofers...

  16. charlie_sullivan | | #16

    Michael, I agree that with any pressure difference, the air flow through cracks and holes will be dominant over vapor diffusion. But I do want to correct your numbers. 50 Pa, which is kind of a reasonable upper limit for pressure differences experienced across a residential envelope, corresponds to 0.2 inches of water, not mercury. That's 0.007 psi, or 1 lb per square foot. That's plenty to create large airflow.

    In the ideal case, holes in the vapor barrier don't matter. But we rarely have the ideal case. For example, a cathedral ceiling with 100% sealed roof deck and perimeter of the rafter cavities might seem like it's got the air barrier taken care of and a wood ceiling is fine with just a vapor barrier. But if the cracks in the wood ceiling are generous, and the cavity insulation is somewhat air permeable, there can be air that goes into the rafter cavity near the peak, gets cooled by the roof deck, flows downward through the insulation to the eaves, and goes back into the room near the eaves through cracks in the wood ceiling.

    In these cases, it's not that the vapor barrier isn't working. It's really just that there are real advantages to having both the interior and exterior detailed as an air barrier. Maybe one is only truly meticulously sealed, but the other should be sealed as much as can be done easily.

    I think I've seen old building science publications that have vapor permeability for shellac and the like, but I don't remember whether they were high or low.

  17. Expert Member
    MALCOLM TAYLOR | | #17

    I'm sorry I'm so thick, but I'm still to getting it.

    If this assembly had the air-space and venting under the first layer of plywood sheathing (That is, it was built like most cathedral ceilings), and no interior air-barrier of vapour retarder, it would be quite risky, because of the low slope. What changes when the air-space is above this sheathing?

    Does it mitigate the problems of night-sky cooling? Does it allow the lower layer of sheathing more of a chance to dry? how does the location of the ventilation diminish the risk, or does it?

  18. Expert Member
    Michael Maines | | #18

    Charlie, thanks for the clarification. I'm self-taught when it comes to the psychrometric chart and still learning some of the finer points. As I read the chart, 70° air at 40% RH has a vapor pressure of about 0.25 inches of mercury. 0° air fully saturated has a vapor pressure of about 0.05 inches of mercury. The difference between the two is 0.2 inches of mercury; using Google's unit converter, that is 0.098 force-pounds per square inch. Round that to 0.1, multiple by 144 to convert to pounds per square foot, and you get 14.4 pounds per square foot. I'm not saying I'm right, I'm just curious what is wrong with my approach, if anything.

    Malcolm, imagine a roof surface ten feet above the rest of the roof assembly. Concerns about stormwater intrusion aside, would you see any concerns about water vapor? What if the roof surface was two feet above the rest of the roof? One foot? As Lsiburek (and others) have said, dilution is one solution to pollution. In this case the pollution is water vapor. There is not enough stack effect in a low-slope roof with a typical, small vent space to move enough air to keep the assembly dry, but at some point the air space is large enough that the moist air can diffuse throughout the dry air surrounding it.

    As for night sky cooling causing condensation, I've only heard of it being a problem when there is exposed air directly below the metal. When there is sheathing directly below the metal roofing, its lower face becomes the condensing plane, since no relatively moist air is reaching the metal. The insulating value of the sheathing, though slight, should keep its surface temperature closer to ambient conditions, preventing condensation. If the temperature differential between the top surface of the metal and the ambient air was large, or sustained for a long time, I imagine condensation would occur. But the conditions we're talking about are just a few degrees, for just a few hours at a time, so a small difference has a big effect. At least that's how I understand it; I have not seen data or studies on this topic.

  19. Expert Member
    MALCOLM TAYLOR | | #19


    Thanks you. your explanation was very helpful. And I don't need a reply. I've banged on about this for too long already.

    Two things still worry me.

    I'm not sure whether the dilution argument works with roof air-spaces, no matter how big. It is common, at least here, to have problems with trussed-roofs with huge airspaces that are insulated at the ceiling. We still sometimes get significant mold growth on the sheathing, if the attic space is inadequately vented. We also experience small amounts of mold growth on the sheathing in these attics even when properly vented, which BSC has attributed to night-sky cooling. If these two mechanisms are in play in these conventional large attics, they might well be problems in the (relatively) small air-space above a low-slope cathedral roof.

    Even with a high-perm lower sheathing and underlayment, I'm not convinced that the drying will exceed welling in the plywood in contact with the cellulose. This in the absence of an interior vapour-retarder or air-barrier. I know Dan Kolbert has built roofs dense-packed against the sheathing which seem to work, but that was with air-tight drywall.

    I'd be interested in Martin's view on this once he gets back and confronts the enormous number of discussions awaiting him.

  20. charlie_sullivan | | #20


    Your interpretation of the psychrometric chart and use of it to find the difference in vapor pressure is correct. I assumed your 0.2" number was coming from something else--stack effect and wind and such--not from the vapor pressure. So I misinterpreted that original comment.

    But I do still disagree with the original comment. There's 0.2" of mercury, or if we switch units to avoid avoiding toxic materials for green building, there's a 0.1 psi difference in vapor pressure across the vapor barrier at the roof. But that doesn't mean there's that much force against the barrier, concentrated at the seams. Outside the barrier, the pressure is about 15 psi, which is mostly the pressure of dry air--call it 14.975 psi dry air and 0.025 psi water vapor. Inside, the water vapor pressure is higher, about 0.125 psi. But the dry air pressure is lower by 0.1 psi: 14.875 psi. Or, because of stack effect, 14.88 psi (very strong stack effect. The total is 15.01 psi inside and 15 psi outside. That's the actual mechanical pressure against a combined air/vapor barrier.

    So it's true that there's 0.1 psi higher vapor pressure inside than out, but there's a 0.09 psi lower dry-air pressure to compensate, with the net result that the mechanical pressure against the membrane--what concentrates at a seams--of only 0.01 psi.

    I fear I may have just made that way more complicated than it needs to be, and we are certainly far from the original question but I wanted to confirm that you are right about the vapor pressure difference, but clarify that that doesn't result in a mechanical force against the membrane.

  21. Expert Member
    Michael Maines | | #21

    Charlie, I appreciate the lesson, and think it's worthwhile, as it's a central component of the OP's question, and my answer was not fully correct. I'm afraid we may have lost the OP (or he's frantically sealing seams) but maybe he'll come back.

  22. Matt_Michaud | | #22

    Michael, way back in comment 8 you said that the Blueskin was fine as an underlayment (ignoring the warranty), so why not recommend eliminating the OSB and the second underlayment and just mount the metal straight to the 2x4 battens?

  23. Jon_R | | #23

    > concentrates at a seams--of only 0.01 psi

    Which is a very significant (compared to stack effect) 69 pascals. But a pressure that acts evenly everywhere (unlike stack effect) usually equilibrates out. Would be interesting to see more about this.

  24. Expert Member
    Michael Maines | | #24

    Matthew, my intent with that comment was just that the Blueskin should allow the CDX to dry readily to the exterior. It is sold as an airtight, water-RESISTIVE barrier, not waterproof. They list only that it passes ICC-ES AC38 for water-resistive barriers. The metal roofing is going to drip water onto whatever is below it. I would want a waterproof surface to catch the drips, not a water-resistant surface.

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