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Will poking small holes in 2 inches of exterior polyisocyanurate allow my wood studs to breathe?

JLBbc37ET4 | Posted in Energy Efficiency and Durability on

I have a continuous layer of 1in polyiso on the exterior of the studs, then 2×3 strapping, and another layer of 1in polyiso inbetween the strapping. The stud cavities are rock wool. I have a 6mil poly vapour barrier and then a 2in layer of XPS before strapping and drywall on the inside of the studs.

In my attempt to get a high R value, I didnt fully understand the implications of creating a vapour barrier on either side of the studs. I am now concerned about moisture being trapped in the walls and the rot and mould that goes along with it.

My siding will eventually be board and batten but I only have the boards installed so far. I was thinking that a quick and easy way to fix this is to poke small holes though both layes of polyiso by pushing 6 guage wire in between the boards right before I put the battens on.

I consulted a local home energy professional before building the house and he said the wall construction detail wasnt ideal but if the blowerdoor test reveals that the house has a 1 or less air changes per hour (@ 50kPa) it should be fine.

I live south of Ottawa Ontario (similar climate to upstate NY).

I’m open to some creative suggestions! … but realize that to solve the problem I may have to remove the polyiso and replace it with XPS.

Thanks for your time and energy

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  1. GBA Editor
    Martin Holladay | | #1

    Poking holes in the polyiso won't help.

    Switching to XPS won't help.

    I hope you didn't nail the board siding directly against the rigid foam without an intervening airspace. (Board and batten siding should never be installed directly against rigid foam.)

    Your wall assembly is poorly thought out and has several problems. You will need to either disassemble your wall from the exterior or from the interior, so that one layer of rigid foam can be removed. Which side do you prefer to disassemble?

  2. JLBbc37ET4 | | #2

    definitely the exterior! Do you recommend that no exterior insulation be used at all?

    Their is a 1/2 in space between the boards and the polyiso.

    You must be concerned about the moisture condensing on the exterior foamboard regardless of the type because I have so much insulation to the interior.

    Do you have any comments on the notion that a 1 or lower ACH will prevent the issue in the first place? This was actually endorsed by both the local professional and the building inspector.


  3. GBA Editor
    Martin Holladay | | #3

    Well, your wall has interior polyethylene, which is unfortunate. (I know why... It's a Canadian thing... and we all know you have to keep your local building inspector happy.)

    That means that your wall assembly has to be able to dry to the exterior, not the interior. So you shouldn't install any rigid foam on the exterior of your wall.

    If you want additional exterior insulation -- and I admire your goal of creating a well insulated wall -- then you should install rigid mineral wool panels (like Roxul) on the exterior of your wall.

    Here's more information on that approach: Installing Mineral Wool Insulation Over Exterior Wall Sheathing.

  4. GBA Editor
    Martin Holladay | | #4

    Thinking a little more about your dilemma: the advice I gave in my last posting is correct, but if you have already installed your board siding, the expense of disassembling your wall and changing the exterior foam will be significant.

    You may prefer to leave everything the way it is and to accept the risk of a "foam sandwich." I wouldn't recommend it, but it is possible that your wall will last for years -- especially if you pay close attention to air sealing and do an excellent job of flashing your exterior penetrations.

  5. JLBbc37ET4 | | #5

    I've definitely done the best possible job I could wrt air sealing on the poly. The dozens of empty tubes of acoustical and rolls of tuck tape is a testament to my dedication there!

    I have taken my time with every aspect of the house and havent made any compromises. In fact I've improved on the initial design along the way.

    If removing the siding and replacing the foamboard is the only way to ensure that this place will last, then that's what I will do. I'm building it myself, so its not a cost, just time.

    I didnt mention it earlier, but does it go without saying that I have an HRV and you've already taken that into account?

    Lets say that I do a good job of air sealing and flashing, what do you think the life expectancy of a foam sandwich wall like mine is? I just really dont know what kind of risk we're talking about.


  6. wjrobinson | | #6

    Bryan. Leave all alone at this point. If you run a normal home and do not humidify you should be fine. Next time you might want to backprime siding if you didn't along with board ends and have a small rainscreen gap behind it.

    Martin has said the same. Leave it.

  7. JLBbc37ET4 | | #7

    Well, that's definitely good news. Thanks guys


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