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Vapor retarder needed with mineral wool?

Zephyr7 | Posted in General Questions on

I have a small renovation project about to start where we’re replacing old fiberglass in two exterior walls of a corner room with new mineral wool (among other things). The house has 1/2″ foil faced polyiso (old late 70s vintage) on the exterior, labeled as R5 but I don’t trust it to be much better than R3. A future project will be to reside the house at which time 3″ of polyiso will be installed, but it will be a few years before that happens. This is in the northern part of zone 5. The polyiso is directly over the studs with T1-11 over that as the siding, so there is no wooden sheathing in the stud bays.

My question is will a vapor retarder be needed on the interior, or will the usual latex paint over drywall suffice? The mineral wool is the usual unfaced batt stuff.


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  1. Zephyr7 | | #1

    Anyone have any thoughts about this?


  2. Aedi | | #2

    *If* the polyiso was truly R5 (and provided these are 2x4 walls), the IRC allows you to use latex paint as your sole vapor retarder. But as you say, that is very unlikely to be the case. You could probably get away with it for a couple years, but the assembly is a little risky.

    Otherwise, the IRC requires a class I or II vapor retarder on the interior side of the wall. This is also risky, since the foil-faced polyiso is a class I vapor retarder itself; by following code you could restrict the ability of the wall to dry. However, this wall can be perfectly safe provided that the interior vapor barrier is coupled with an interior air barrier (see Using one of those smart membranes would be even safer.

    If you were planning on having your air barrier be on the interior anyway, then go ahead an throw in a vapor barrier. If you were planning on putting an air barrier on the exterior, then I'd just roll the dice ignore the vapor barrier requirement -- just don't wait too long to put up that polyiso.

    1. User avatar
      Dana Dorsett | | #3

      >"Using one of those smart membranes would be even safer.

      If you were planning on having your air barrier be on the interior anyway, then go ahead an throw in a vapor barrier."

      A true vapor barrier with foil faced polyiso on the exterior would create a true moisture TRAP. Any errors or failures of bulk water moisture measures then rapidly become high-risk, even after the 3" of exterior polyiso goes up.

      The variable permeance membrane type smart vapor retarders, detailed as an air barrier would be a far better choice. Even relatively inexpensive 2-mil nylon (Certainteed MemBrain) behaves as a Class-II vapor retarder during outdoor temperatures where it matters, but becomes more vapor open than latex paint on wallboard when the humidity inside the wall cavity rises to levels that would support mold. More rugged and more vapor tight versions such as Intello Plus would also get you there.

      Yes, the material cost of 2 mil nylon is 2-3x more expensive than 4-6 mil polyethylene, but it's buying resilience. Consider it cheap insurance.

      1. Aedi | | #4

        I agree the smart membranes are a cheap insurance, but the moisture trap isn't as bad as it seems at first glance. I'm willing to bet there is a lot of air getting by that old polyiso. This won't really lead to more condensation in the winter (the outside air is dry), but it will allow the wall to dry when spring rolls around. So long as the interior air barrier prevents moisture-laden air from infiltrating the assembly in the winter, it should be fine.

        I think the risk actually increases when the three inches of polyiso goes up, at least when it comes to bulk water management failures. Then again, bulk water is almost always high-risk in modern assemblies anyways, so I'm not sure how much worse off the wall actually is.

        I'd err on the side of caution and go with the smart membrane, but polyethylene is still a perfectly reasonable option.

  3. User avatar
    Dana Dorsett | | #5

    >"I think the risk actually increases when the three inches of polyiso goes up, at least when it comes to bulk water management failures."

    That's exactly right, and why building in a moisture trap would be a mistake, given that the cost-adder of 2-mil nylon would be a few hundred dollars at most.

  4. Zephyr7 | | #6

    Thanks for the info guys. I was kinda thinking this would be a good application for a smart vapor retarder like MemBrain. I don't trust the existing 1/2" polyiso to keep temperatures high enough to avoid moisture issues in the exterior walls. I do intend to do some air sealing work while the walls are open on the interior. The corner room has wallpaper that appears to have been applied to unprimed drywall, so it won't come off without damaging the drywall. Instead of stripping and skim coating, we're just replacing the drywall to save on labor. While the walls are open, some insulating and air sealing work is going to be done.

    Once the 3" polyiso goes up I won't worry about vapor barriers in those areas (all the exterior fiberglass is getting replaced with mineral wool since the old fiberglass has had a lot of issues with critters nesting in it). The eventual plan is to put 3" of polyiso on all exterior walls, new mineral wool in the stud cavities, and a rainscreen finished off with fiber cement siding (woodpecker proof :-)


  5. User avatar
    Jon R | | #7

    It's a risky wall - as others say, use MemBrain or Intello and air seal it well.

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