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Community and Q&A

2×4 wall construction insulation retrofit

mk10 | Posted in General Questions on


I am re-insulating my walls from the inside as this is the only option for me. It seems like any time i do any kind of rehabbing there is a lot on info available but never spot on for my plan.

Structure built 1968
Zone 6, Vermont
2 x 4 wall on slab
Exterior: 1/2″ sheeting (no house wrap), 1″ board and batten cedar, painted (not stained)

Here is what i have done so far
1. Air sealed with caulk and foam all wood to wood joints, foamed small cavities.
2. Air sealed electrical boxes
3. Installed Roxul r15

The question:
I want to increase the r-value and create a thermal break from the wood framing, i want to keep the finished thickness to the minimum, I plan on using JM 1″ polyiso foil faced.
1. Is this approach ok? And is this the correct rigid foam.
2. Is the r6.5 of this enough, i think I saw in my zone it should be at least R7.5.
3. I plan on caulking the perimeter of foam to the studs, ceiling and slab to air seal, will also tape seams. Does this make sense?
4. I was planning on tacking the foam to the studs with a few roofing nails with the assumption that when I screw the drywall it will be more than enough, is this the best way?
5. I think I read somewhere that the screw heads (from question #4) have a greater chance of popping thru the drywall due the the warmer wall and the expansion/contraction of the wood between Summer/Winter, seems plausible – what do you think? If so is there a way to prevent it?

Oh yea, I plan on doing this today – nothing like waiting for the last minute…

Thanks for your help in advanced! Mark

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  1. GBA Editor
    Martin Holladay | | #1

    You can install rigid foam on the interior of your wall if you want. It will definitely improve the thermal performance of the wall. When rigid foam is installed on the interior of a wall, there are no minimum or maximum R-value requirements.

    You can attach the foam to the studs with construction adhesive if you want to minimize the number of metal fasteners. The drywall and drywall screws will hold everything in place.

    Foam under drywall is slightly squishy, but I have done exactly what you are doing without major problems. Use long screws when attaching the drywall, and fastener pops should be minimal. Don't forget to caulk the cracks between the electrical boxes and the drywall.

  2. davidmeiland | | #2

    I have also done the same thing, but used polyiso with a felt face, maybe it's a little more vapor-open. I nailed the foam to the wall with a few long roofing nails here and there, and then screwed the drywall in place with 2-1/2" screws. You probably want to pay more attention to pushing the drywall tight with your hand as you set the screw, and you may use more screws than normal. Before you start, give some thought to the inside corner fastening.

  3. mk10 | | #3


    1. I won't have any vapor/moisture issues? This was my main concern, as a lot of the reading I have done says not to have a vapor barrier in the north, cold. That's what I am essentially doing with the foil face foam. right?
    2. You say "You can install rigid foam on the interior of your wall if you want" does it make sense? Is it worth the extra expense, I am mostly doing it because I Have convinced my self that r15 isn't enough and if anything it will increase the comfort level of the room.


  4. GBA Editor
    Martin Holladay | | #4

    You're right that foil-faced foam is a vapor barrier. But it also has considerable R-value.

    The danger with polyethylene is that it will become a condensing surface during the summer, when the home may be air conditioned. That won't happen with the polyiso, because its R-value is high enough to keep the foil that faces the stud cavity fairly warm. That foil will never get as cold as the polyethylene in an air-conditioned house.

    Of course, it makes sense to design your wall assembly to be able to dry to the exterior.

    Q. "Does it make sense? Is it worth the extra expense?"

    A. In my opinion, yes and yes.

  5. Expert Member
    Dana Dorsett | | #5

    Is there even so much as #15 felt or something between the 1/2" CDX and the 1x board & batten, or some sort of lath or furring to back-vent the siding?

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