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Wall details

oldhousesam | Posted in Energy Efficiency and Durability on

A few part question, trying to get the details ironed out before I get to the insulation part of my remodel. Its a 1900’s house in climate zone 6, the siding is in decent shape and original so I don’t want to remove it, there for im insulating the inside. And i have no weather barrier or resin paper between sheathing and siding. 
My idea so far is ridged foam spaced half inch off from my sheathing fill the rest with Rockwool insulation( vent the half inch space to the un conditioned attic and through the sill plate). Then 6mil poly for a air barrier over that continuing over the ceiling and down the wall and over the sub floor. And on top of that run 2×4 studs 24 on center horizontally so my outlets switches and what ever wont be air leaks plus i can put fiberglass insulation in to increase the r value a little more. Then cover with dry wall.
Will i gain anything by sealing the ridged foam with some  foam like poormans  spray foam?
Should i use something like CertainTee’s MemBrain instead of the poly?( we don’t use A/C in the summer, we just open windows and turn a fan on)
Thank you

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Replies

  1. Expert Member
    Michael Maines | | #1

    Oldhousesam, this is always a tough situation. One problem with your approach: leaving the stud bays open to the attic to allow the wall to dry creates (or leaves) a significant fire hazard. For life safety reasons you should have solid blocking anywhere a vertical cavity meets a horizontal cavity. Life safety should come before building durability or environmental considerations, in my opinion anyway. As a compromise, I would vent the bottom of the cavity. Just include something to keep pests out, such as metal screening or Coravent baffles.

    I'm not sure that the exterior foam is doing much for you. Mineral wool is dense and has a relatively good R-value; there would be some wind-washing but not much.

    In zone 6 it's not a good idea to include poly sheeting in the assembly. As the climate warms we will be using air conditioning more often and poly prohibits drying to the interior, which happens when air conditioning runs (or when the interior is otherwise dryer and cooler than the exterior). I have not installed Membrain myself but I have heard that it is difficult to use as an air barrier. I have installed a lot of Pro Clima Intello and some Siga Majrex, and can confirm that they make excellent air control layers as well as allowing drying to the interior.

    Between the interior battens, you could use fiberglass batts, though you'd have to strip thicker material to fit the space. Same with mineral wool, unless you got rigid material (Comfortboard, i.e.) but that would cost more with little thermal advantage. Foam would also work, though the assembly detail may change with different foam products.

    1. oldhousesam | | #2

      Michael, thank you for taking your time to read and respond. With fire blocking installed to code(which I forgot to look at before thinking up the design) will venting just the bottom of the wall be enough?
      I also forgot to mention my wall assembly is rough cut 2x5, so after the inch I lose from spacing and ridged foam I will have 4inches to fill. Or would I be better ahead of using a 6 inch batt and not worry about the air space
      I have read a little more about MemBrain vs Intello, and a lot of people are saying the same and out its easier to work with. There is also more information on it to be a air barrier.
      Thank you for spreading some knowledge to someone that is trying to learn

  2. brian_wiley | | #3

    Hi OldhouseSam, while I can’t comment specifically about your details, I’m in a similar (albeit a little more complicated) situation regarding an insulation retrofit, and found this really helpful in explaining both concepts and details: https://www.buildingscience.com/documents/building-science-insights/bsi-120-understanding-walls

    Also, Brian Pontolilo’s Walls That Work Webinar has a brief section on retrofits that was really helpful: https://www.finehomebuilding.com/2020/03/27/walls-that-work-an-intro-to-the-four-control-layers

    I’ve also been comparing Intello versus Majrex, and while Majrex is a bit more expensive it seems like the details options may be a little more DIY-friendly. Maybe one of the more experienced members can weigh in from a practical application standpoint, but it may be worth a second look at that product.

    Maybe you explored all those options, so forgive me if they’re old news. Just thought I’d share on the off chance they were helpful.

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