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Vapor permeability of polyiso

dirkgently | Posted in Green Products and Materials on

Attached is a picture of foil-faced polyiso which as a bunch of holes in it. The holes were made with a wall paper removal tool which puts holes in the paper to allow liquid stripper to get behind the paper. I welcome comments on this approach to see of foil faced polyiso could be modified to allow vapor permeability thru the sheet if holes were done on both sides.

I was given 22 brand new sheets. In there current impermeable form I cannot use them. I need a product which will allow drying of a cathedral ceiling to the inside of the home . I can hang onto them for a future project but I would rather use them up now if possible.


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  1. wjrobinson | | #1

    Dirk, IMO you are miss using the foam and not improving the perm of the product.

    Why does your ceiling have to dry to the interior. Build it so it can't gain moisture. What climate? What details for the ceiling assembly?

    Dana... next... the amazing perm calculating bro.

  2. wjrobinson | | #2

    I build cathedral ceilings with a layer of the same foam with all seams taped to keep interior heat and moisture in the home. This assembly has worked here for decades.

  3. davidmeiland | | #3

    That won't get you any improvement in vapor permeability. The foil is still at least 99% intact, so it's at least 99% effective as a vapor barrier. It's also still 100% effective as a radiant barrier.

    I would sell that foam on craigslist or something like that, and get mineral wool panels if you need rigid that passes vapor.

    1. BuildingFun | | #14

      I know this is an old thread, but simply updating for future searches.

      I think this logic is flawed. SilveRboard Graphite XP has FAR FEWER perforations in the foil facers and claims perm rating of 2.7. Either they are lying or tiny peforations allow more vapor permeance than one might think?

      1. Expert Member
        MALCOLM TAYLOR | | #15


        They are not incorrect. Unlike an air-barrier, a vapour-barrier works in proportion to the area it covers. If the foam is covered by an impermeable surface, a few pin pricks will have a negligible effect on its perm rating.

        1. BuildingFun | | #16

          How do you explain the perm rating of over 2 on perforated SilveRboard graphite XS then?

          1. Expert Member
            MALCOLM TAYLOR | | #17


            I haven't seen the product's perforations. I suspect you have misunderstood the amount of the surface they cover. What mechanism do you suggest could allow a few tiny holes to move any significant amount of water vapour though the foam?

            But let's be clear: Whatever the explanation for SilveRboard's perm rating, there is no debate that Martin and David were entirely correct in their answers to the Op in this thread, and their description of how vapour-barriers work.

            I'm curious - do you know who Martin and David are? What do you think the odds are that their understanding of fundamental building science is wrong based on the perm rating of a product you found on a manufacturer's site?

          2. Deleted | | #20


          3. Expert Member
            MALCOLM TAYLOR | | #21


            "How does this product defy this concept?"

            I don't know. I wish someone would chime in with a theory.

        2. BuildingFun | | #18

          MalcomTaylor- I respect their opinions very much which is why I happily pay to read their articles on this website.

          Re: "misunderstanding the amount of surface the perforations cover".. That I have not. The perforations are exactly as they appear on the manufacturer's website. I have many of the panels on site right now as well and can assure, it is merely a small hole roughly every inch, in every direction. IE: One square inch would have a tiny hole in each corner of it.
          Maybe I should have re-phrased my comment to....

          IF that statement is true, is this manufacturer completely lying and making false claims of a perm rating of 2.7 perms? Or maybe I completely mis-read the spec sheet?

          Not trying to ruffle any feathers here, honestly curious where the discrepancy, misinformation or simple misunderstanding is.

          Now that I'm thinking about it more....If I drill a 1/2" hole in a 48sqft panel, that hole is far less than 1% of the surface area. To say that panel is still over 99% effective as a vapor barrier simply wouldn't be true, would it? So I guess I still think that statement is flawed. If a tiny hole in an air barrier can allow a bucket-full of moisture in your ceiling cavity, why cant a tiny hole in a vapor barrier allow more than its equivalent percentage-to-size of vapor through? I'll admit, I have no where near as much experience as either of these guys, but I don't think that means we shouldn't question something that seems incorrect. Again, not trying to offend anyone here, just seeking clarification.

          1. Expert Member
            MALCOLM TAYLOR | | #19


            "If I drill a 1/2" hole in a 48sqft panel, that hole is far less than 1% of the surface area. To say that panel is still over 99% effective as a vapor barrier simply wouldn't be true, would it?"

            Yes it would. That's exactly the point they are making - and it's an important one. Back in the early 2000s it was suggested that one way of helping walls dry to the outside was to drill a hole in the sheathing or each stud cavity. Joe L. and John Straub pointed out that while the holes would allow large amounts of air-leakage, because vapour diffusion was proportionate to area, it added a negligible amount of dry capacity through diffusion - and the practice was abandoned.

            That's the fundamental difference between moisture that moves into building assemblies in air, and through diffusion - and is why we spend so much time trying to air-seal, and so little on vapour control.

            Edit: Imagine you had a sheet of OSB, which is about 2 perms, and you covered it with 6 mil poly, of (roughly) 0 perms. If you cut a hole in the poly equivalent to 10% of the area, it would then have a 0 perms rating over 90% and one of 2 over the 10%. The same logic applies whether it is one hole or hundreds of small ones. It's the percentage of area covered my each material that matters - and there is nothing m0re complicated to it.

            That's different than air-movement, which is much more reliant on pressure difference. A hole right through both the sheathing and poly equivalent to 10% of the area of the OSB can yield various amounts of air-movement depending on the difference in pressure on each side - but in almost every situation allows what we would think of as a disproportionate amount of air to get through relative to the area of the hole.

          2. Joe_Garvilla | | #25

            "If I drill a 1/2" hole in a 48sqft panel, that hole is far less than 1% of the surface area. To say that panel is still over 99% effective as a vapor barrier simply wouldn't be true, would it? "

            You switch methods of transfer with this analogy; Moisture traveling through the hole is carried by air (Air Control), where diffusion (Vapor Control) is through a medium. I should also point out the difference between a vapor barrier and a vapor control layer. A vapor barrier is a permeance of .1 perm or less, where a vapor control layer can be any combination of class 1, 2, or 3 vapor retarders. the non-perf SilvRBoard is a class 2 and the perforated version is a class 3.

            The perforations aren't exactly air barrier holes because the underlaying eps isn't perforated (to my knowledge). The Graphite product you're listing is classified as an ASTM 578 Type XI. Type XI EPS (generally) has a vapor permeability of 5 per inch @ 75 degrees. The ASTM E96 is the testing standard for vapor permeance; however, there is more than one test method and its up to the manufacturer to recommend which test method for the designed application of the product. It is entirely possible that they could use a different methodology to test the perforated panels vs the non-perforated which may influence the seemingly wide disparity of the Perm ratings. Also, in the submittals section of the manufacturer website, I only see a submittal for the perforated panel and not the non-perforated panel.

            To your point BuildingFun, the perforation alone is less than .2% of a square inch. So why would diffusion at 5 perms have such a disproportionate influence on the whole panel permeability when the rest is at .77 perms.

            Well... people smarter than me will have to answer if it isn't related to different testing methodologies. Either that or the perforation is much deeper than we suspect. Its a mystery, and like you, I'm interested in an answer because digging through the technical stuff didn't give me anything satisfactory. I don't think the manufacturer would lie, although the submittal document does not specify the testing standard even thought the data sheet does.

            Of course, like always, maybe I'm way overthinking it lol.

        3. BuildingFun | | #22

          Interesting, thanks for typing that out, although I still have a little trouble following that theory as vapor is much more free flowing than water, hence water barriers not always being vapor barriers. Probably just a result of my own ignorance. I'll try to find some articles on drilling holes in each stud bay for vapor diffusion. Sounds like an interesting read.

          If what you're saying is true, how does the below linked perforated panel with far less than 1% area of perforations defy this rule?

          Again, thanks for your replies. I'm interested to hear your take on the above links for SilveRboard's perm rating of 2.7 with mildly perforated dual faced foil GPS.

          1. user-7951015 | | #26

            I want to use foam board insulation on outside of 2x4 stud wall surrounding a bathroom. I plan to use Kerdi-Board, which of course is a vapor barrier. I therefore need a foam board that breaths. Lots of references here to products but no one has any of them in stock. Some good "breathable" EPS panels, (SilveRboard Graphite XS; Johns Manville AP™ Breathing Sheathing Polyiso Continuous Insulation board; and my fav, IKO Ener-Air. But again, no one seems to actually sell them. Any idea where to buy? Thanks.

          2. gusfhb | | #27

            I suggest starting a new thread, took me a few minutes to even locate your question in this thread

      2. maine_tyler | | #23

        It seems to me (but I don't actually know) that permeance calculations should be treated like U-factor thermal calculations. In other words, a weighted average. This seems to be what Dana is saying below as well.

        Uaverage = (Acavity · Ucavity) + (Aframing · Uframing)

        Replace U for Perms, and let's use 0.05 perms for foil facing and 5 perms for EPS at 1%:

        Pavg = (0.01 · 5) + (0.99 · 0.05) = 0.0995
        Or about 0.1 perms

        If the area goes up to something like 5% and the perm value of that 5% to something more like 10, and maybe the foil up to 0.1 instead of 0.05, we get:

        0.595 perms.

        So I am not sure how 1% or less of perforated foil would ever yield such high perm ratings. Note that I am using 5 perms for the perforations area and not a number representing the permeance of air (a really high number) because the vapor would still need to pass through the foam in those areas.

        Whether or not this is the math that should be used for permeance calculations, I'm not sure, but it makes logical sense to me.

  4. GBA Editor
    Martin Holladay | | #4

    David Meiland is right. The foil facing on the sheet of polyiso shown in the photo is still a vapor barrier.

    AJ Builder is also right (if I understand his point correctly): Depending on the details you plan to use, it's possible that your cathedral ceiling assembly will still work, even if you include foil-faced foam. For more information on this topic, see How to Build an Insulated Cathedral Ceiling.

    1. BuildingFun | | #13

      I believe this post is incorrect. Old, but incorrect. Just wanted to update for future searches.

      How would you explain SilveRboard Graphite XP board that has silver foil on both sides with tiny perforations, roughly 1 inch apart, with a factory claimed perm raiting of 2.7 or so per inch? The tiny perforation are roughly needle size and are literally only 1 inch apart, evenly spread throughout the facers. FAR FAR less than 1 % of the total area, yet still 2.7 perms. It seems like perforating foil faces CAN greatly improve perm rating if using this example. Or maybe I'm misreading the spec sheet? Or the manufacturer is posting incorrect info? Curious if you have any speculation?

  5. dirkgently | | #5

    Thanks David that pretty much answered my question.
    I was thinking if the same number of holes were in an normal vapor would be considered useless.
    The reason for the drying to the interior is that much of the cathedral ceiling is low slope dormers which have are covered 100% with Ice shield and asphalt shingles. The open cell foam will need to dry to the interior. Zone 5.
    Because of the 3/12 slope and also a valley, venting is not really an option for this remodel.
    Looks like I will be getting reclaimed felt facers to boost my r-value/reduce thermal bridging.

  6. GBA Editor
    Martin Holladay | | #6

    You wrote, "I was thinking if the same number of holes were in an normal vapor would be considered useless."

    I think that you are confusing an air barrier with a vapor barrier. For more information on the difference, see Do I Need a Vapor Retarder?

    If a piece of polyethylene has a great many holes -- say, holes amounting to 5% of the area of the sheet -- it is still a perfectly acceptable vapor retarder. But it isn't an air barrier.

  7. Expert Member
    Dana Dorsett | | #7

    Vapor diffusion is a surface area x permeance deal, but with enough perforations the foil isn't much of a vapor barrier.

    Vapor permeable radiant barriers with grids of tiny holes on roughly a half-inch grid have a measured permeance greater than 10 perms (eg: 14 perms, not even a class-III vapor retarder!) To get above 1-perm with two facers you'd probably have to use a 2" grid on both facers with that tool, but it would get there. The polyiso itself is greater than 25 perms @ 1".

    Polyethylene sheeting punctured on a grid that constitutes even 1% (let alone 5%) of the total area it would also probably be greater than 10 perms if done on a tight grid as in the perforated foil RB. Air is EXTREMELY vapor permeable.

  8. wjrobinson | | #8


    1" unfaced polyiso perm is 2.5-4. with the punctured foil that would drop to much less than 1 IMO


    other basic roof polyiso foams that are faced with other than foil, glassed felt etc... perm .5

    EPS 1" 5 perm, 2" 2.5 perm, 3" 1.67 perm, 4" 1.25 perm

    You need a perfect air barrier. And low indoor moisture. Or you need to closed cell spray foam which needs to be done right during a mild day with no burning sun, overcast perfect. You need the roof to be all the same temperature so the spray temperatures are similar. Less chance of problems. Not a fan of spray foam but done right is great.

    No spray foam option; I would trust myself to install your foil foam perfect enough to be your air barrier. For me the ceiling would have to include a vent channel and high density fiberglass batts. Then all is pretty much bullet proof.


  9. dirkgently | | #9

    I appreciate your input. Do you use a 1" vent channel or a 2" channel?
    Your point about even roof temperature is also a great one.
    I see the permeability of 1" poliso in your link....I am not sure where Dana got his number from. I will have to do some more research into that to see if there is data to back his claim.

    The breathabilty of the perforated foil is the main goal. I remember using some type of paper/foil which was perforated under vinyl siding back in the 80's. and that is what gave me the idea.

    My original plan was 1" channel then dense pack cellulouse AND "cut n cobble" rigid foam into the area where the valley is (it is not very big). I was to follow that with 2" rigid foam on the underside of rafters to reduce thermal bridging. BUT the more I learned about lower slope roofs (a large portion of my project is 3/12 and 2/12 shed dormers) The less I felt comfortable with vented route. Both of those roofs are E or W facing.
    ALL of the insulation contractors I had look at the project wanted to use open cell foam on the one advised dense pack (indeed they talked me out of it) OR closed cell foam (CCF).
    I may be open to CCF. I am a believer of Riversongs advice on not trapping sheathing between 2 impermeable surfaces (ice shield and CCF). Is Riversongs line of thought still valid?

    A note about insulation contractors: I have a small amount of open cell on underside of roof deck (about 7 linear feet @8" thick) in my cape/knee wall areas. NO drywall over it. It was painted with fire retardant paint and I was told that was all I needed by the contractor in 2010. I know of 2 others who have that same setup (more disasters waiting to happen). Given what I know now about OC foam needing drywall and paint.....NONE of those contractors have much credibility with me now....

  10. Expert Member
    Dana Dorsett | | #10

    AJ: OOPS! It Looks like missed the decimal point- I need an editor. I know from reading way too many specs that 2lb & 3lb per cubic foot roofing polyiso can be as low as 1.5-2 perms @ 1" without facers, and you can actually buy it that way (eg: ) But nearly all foil-faced goods sold these days is 1.5lbs which is quite a bit more permeable. It's hard to find hard data or specs on unfaced 1.5lb polyiso is never actually sold that way. I was trying to be conservative with 2.5 perms. The holes made by that stripper tool are far larger than those in vapor-permeable radiant barrier, my unstated WAG on it was that if you run a 2" grid of holes you'd get the permeance up to about 5, and if you put two ~5-perm facers on a 2.5 perm core the total permance is north of 1. If you put two 10 perm facers on a 2.5 perm core it's in the 2 perm range. At the size of those holes it might get there too, even if you assumed 2.5 perms for the foam. If it's actually 4-perm foam (within the range of the BSC estimates) you'd be over 2.

    Your EPS permeance estimates are only applicable to 1.0lb nominal density "Type-I" EPS. At higher density it has lower permeance. At 1.5 density (Type-II) EPS is typically ~3 perms @ 1" range, Type IX 2lbs nominal density goods run around 2 perms @ 1". Here are the specs across a range of densities from one manufacturer:

    Nick: In-re exposed open cell foam- it's worth spray-applying "vapor-barrier" latex on it, which according to BSC data comes in at about 5 perms in that application (not the ~0.5 perms it reaches when applied to wallboard.) At 5 perms it's the rough equivalent of standard latex on gypsum, and though you'll have some moisture cycling in the roof deck it's not a problem as long as the interior humidity is kept to 35%RH @ 70F (it can be a higher RH at lower indoor temps- consult a pschrometric chart.)

  11. dirkgently | | #11

    I just wanted to update this topic.
    This portion of the project was on hold for the busy summer season and just getting back to it now.
    I purchased a different wall paper stripper tool which had 2x the blades 1/2" apart at Lowes. I did both sides of my first sheet in 5-6 min. It has a gazillion holes per side now.

    I am confident that the 8-10" 3/4# foam filled rafter bays will be able to dry to the interior as they need to once this is installed to underside of rafters.

    I was disappointed this summer in the amount of heat gain the room received, but I am thrilled to be able to work in there in the winter with no heat source other than halogen work lights.

  12. user-7568882 | | #12

    Hey Dirk, stumbled upon this thread in search for a solution to a similar predicament. I have an unvented shed roof which has been insulated between rafters bays with 4" of foil faced polyiso, mistakingly creating a moisture trap so the ceiling structure cannot dry to the inside. Wondering a few years out if your perforation (porosity) experiment has worked. I was also thinking of doing this in tandem with a smart vapor barrier on the inside to help mitigate diffusion from the inside air going up.

  13. dirkgently | | #24

    just to update this topic of mine I stumbled upon again. I am 8-9 years into this and have no evidence of a problem.
    Still, I do not think I would do this again. That room is great and costs nothing to heat and cool...but....I still wonder about the system.
    fwiw those foil facer sheets on the polyiso were abosolutely totally shreaded....I mean shreaded.

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