GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Future Wall Assembly Upgrades

agm413 | Posted in Energy Efficiency and Durability on

I live in a townhouse with 18 feet of exterior facing walls (front/back) that have interior facing foil faced batt insulation.

If in the future, if I want to do any kind of retrofitting of the water resistive barrier or add exterior foam, am I out of luck if I want to install something that is vapor impermeable like exterior foam – without opening the wall to take out the foil faced batts?

Not sure if it matters for this situation, but the installation isn’t great on the batts from what I can tell, which would lead to some interior drying potential through gaps on the side of the stud bays etc.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Michael Maines | | #1

    You can use a vapor-open exterior insulation such as EPS, GPS, mineral wool or wood fiber.

    1. agm413 | | #4

      Thanks for the input, that's along the lines of what I was thinking.

      Is rigid mineral wool a good air barrier?

  2. Expert Member
    BILL WICHERS | | #2

    Rigid mineral wool (Comfortboard) is probably your best bet here, since it's the most vapor open rigid insulation available. It's not as rigid as rigid foam though, and it's pretty expensive. EPS/GPS would be a distant second in terms of vapor permeability, and I'd be reluctant to go over about an inch or so with your assembly.

    Check if that foil facer isn't perforated. Sometimes foil facers are perforated to make them vapor open, and if that's what you have, then you don't have anything to worry about.


    1. agm413 | | #3

      I'll have to double check, but I think the only perforations are the ones I accidentally make when cutting into the drywall (before I tape them back up ).

      Would the mineral wool semi rigid board be a sufficient air barrier by itself?

      Thanks for the input. On a separate note - cold I do something like dense pack blown in from the outside behind the batts, then have a vapor open WRB, rain screen, and siding re-installed?

      1. Andrew_C | | #5

        Mineral wool is neither an air barrier nor a WRB.

        1. agm413 | | #9

          Thanks for the clarification.

      2. Expert Member
        BILL WICHERS | | #6

        Dense pack cellulose would typically be used INSTEAD OF batts, but I suppose if you had some extra space to fill, dense pack could work. The batt might complicate the "dense packing" part of the installation process though.

        Mineral wool is no an air barrier, as AGM413 mentioned. Mineral wool is insulation only, and not a vapor or air barrier.

        A vapor open WRB would be something like Tyvek. The entire purpose of Tyvek is to act as WRB and still allow vapor to get through for drying purposes.


  3. Expert Member
    Akos | | #7

    Rigid mineral wool gets spendy pretty quick. Besides the material costs it is a bit of a pain to work with as it is still somewhat squishy. You have to take a fair bit of care to make sure the strapping is flat to avoid wavy siding.

    I'm in the land (zone 5) of interior poly combined with R5 exterior rigid insulation. These have been built for a long time here, the walls work just fine provided there are no bulk water leaks. I've opened up 30year old walls built like this and they were pristine on the inside, just like the day it was built.

    If the windows and doors are re-flashed properly and a decent WRB is installed on the exterior, I would not worry about rigid foam insulation. You can always use one of the more permeable foam products as suggested above. There is also permeable polyiso (ie EnerAir) you can get that has higher R value/inch if you are tight on space.

    1. agm413 | | #8

      Thanks for the reply. So something like zip r could work as long as all the other details are done well so they work properly (i.e no leaks )?

      1. Expert Member
        Akos | | #10

        Zip R should be fine.

        I would check what is locally available, even if a bit more expensive, in terms of install labor, there is no extra install cost for thicker Zip R. I would just avoid the R12 version as it needs a special framing nailer to hang. Even that is not that big of an add as the nailer is ~$500, so noise in terms of renovation costs but you might get some pushback from the contractor.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |