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Thinnest interior insulation with high R value

orange_cat | Posted in General Questions on

Due to some unforeseen changes, need to add interior insulation to the foundation wall that is now an above ground wall. (1×3 feet). Elsewhere using Rockwool, but there are no studs in that spot. What is the thinnest high R-value option? It is a narrow hallway which I loath to make more narrow by using rockwool

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  1. jollygreenshortguy | | #1

    Realistically the maximum you can expect to get is R5 per inch. Some foam manufacturers of polyiso and XPS foams quote higher, but these foams less value over time and R5 is a more realistic figure. Type IX EPS will give you permanent value close to R5 and may be cheaper. Other materials are in the R3-4 range.

    There may be issues regarding vapor transmission that need to be considered. Without more information it's hard to know but basement applications can get rather complex. There are others here who can address those issues with more expertise than me.

  2. aaron_p | | #2

    You didn't say it had to be practical... this is R32 per inch and no that isn't a typo.

    edited to add: that was the center R-value, because it is like glass there is an edge effect that makes the overall of a panel ONLY R28 per inch. :)

    1. orange_cat | | #5

      That might actually work. I only need R-22. Anyone know any other crazy product (that the building inspector in Ontario will accept)?

  3. orange_cat | | #3

    There is not much in terms of detail. Exterior walls are R-24 + R5 CI (solid brick, ventilated cavity, XPS R-5, WRB, Plywood sheathing, 2x6 wood frame, mineral wool batt R-24, vapor barrier, drywall).

    There is a segment where concrete was poured around staircase landing higher than the finished landing, so we have a portion of the wall that now needs to be insulated on the inside (on the outside that section has the same XPS and brick) - it is not framing, so nowhere to put the rockwool.

    To add insulation on the inside would narrow the landing. The area is really small - so looking for a really high value products where 1x3 feet section is "within budget".
    The panel above might work or something that the inspector here in Ontario will accept.

  4. Expert Member
    BILL WICHERS | | #4

    Polyiso gets you the most R per inch of any of the readily available insulating materials. The only downside is that polyiso can't be exposed to water, so it can't be used in some areas where XPS and EPS can. On the interior of a foundation wall, polyiso is usually fine though. I would use polyiso in your application, which will get you around R6 per inch or so, slightly outperforming XPS and EPS in most cases.


    1. orange_cat | | #6

      Thank you. That would translate into about 3.5-4 inches which is a bit more than I would like. Any novelty products?

      1. Expert Member
        MALCOLM TAYLOR | | #7


        No chance of saw-cutting a bit off the stem-wall to reduce the width?

        1. orange_cat | | #11

          Well, it is of course at the base of the lower floor, so there are two stories worth of 2x6 framing above it and the brick cladding already installed on the outside. It looks impossible (they already saw cut a portion to install a sliding door near this problem - but that one was caught early.

      2. Expert Member
        BILL WICHERS | | #10

        There are things like vacuum panels, but you might as well rebuild your wall -- it would be cheaper than buying the vacuum panels! I'd go with polyiso here, as much as you can reasonably fit without messing up the architectural details of your wall.


        1. orange_cat | | #12

          Even if it is just 1 foot by 3 feet?

  5. Expert Member
    Akos | | #8

    For a small area like that I would glue on 1" of polyiso and glue drywall right over it. I can't see an inspector making a big fuss about it (or even notice) especially if you are above code min elsewhere.

    1. orange_cat | | #13

      But would not I feel the cold (or think I feel the cold)?

      (I am a bit disappointed over this. Trying to build a small footprint place only works when blunders like this do not happen. Because then there is too little room for errors).

      1. Expert Member
        DCcontrarian | | #16

        Plenty of errors in larger houses too.

        With 1" of polyiso you'd have R6, plus it's air sealed. That's enough that it wouldn't be noticeably cooler.

        Our code allows you to calculate R-value on a "whole-assembly" basis. The whole wall doesn't have to meet the minimum, it just has to average out.

        1. Expert Member
          Akos | | #17

          Assuming 2F outdoor design temp, 70F indoor.

          With R5CI +1" polyiso, you have about R11 there over 2x3 area.

          Heat loss over that area:
          -(70F-2F)*(2 x 3)/R11=37BTU

          If built like the rest of your walls (R24+R5) this drops to 18BTU.

          Overall your delta is 19BTU, well bellow anything you could measure even with the most sensitive instruments.

          I would loose no seep over this.

          The problem would be if there is zero insulation over this area and all you have is R0.1 concrete between the indoors and outdoors.

          1. orange_cat | | #18

            Thank you both - these are very helpful.

            Does anyone know what Ontario SB-12 says (I just checked it but it is so damn dense) about "whole assembly" basis? Because the rest of the wall is R-24 and I need R-22 by code, I may be able to legally get away with 1 inch or so if i am allowed a weighted average over the surface of that wall. But my uneducated read of SB-12 does not see this language.

            There is also some language about "foundation wall" . This is literally a piece of foundation that just went up a bit higher than intended. Can that portion be still treated as "foundation wall" piece even if it is above (the groun) floor?

          2. Expert Member
            Akos | | #19

            Which compliance package are you building to?

            Good point, just because above grade, that is still foundation.

            Most have the foundation at R20 (or R12+R5CI), so not much less than a wall.

            For U factor based compliance you are looking at the Max U value in the table (ie Table (IP), A1, foundation U is 0.047 imperial).

            You than take an average of the whole foundation including that small area. Such a small area with R11 (as I suggested above), you would barely budget the whole foundation U factor.

          3. orange_cat | | #23

            Well, with 3 mechanical engineers "which compliance package" is a surprisingly tough question (when this is over, I will describe. But it has been very frustrating).

            . But I noticed that there is a note for all 3.1.1. compliance packages regarding effective R value of the entire assembly, and that if "entire" passes, the rest does not have to comply.
            - item 13
            So I think we can go the R6 for this area only.

            thank you!

          4. johngfc | | #25

            Just think of this as a 1x3 ft window. 1" of polyiso + the wall, sheet rock and whatever and you've got the equivalent of an exceptionally good triple-pane window.

  6. kyle_r | | #9

    I would google aerogels or kooltherm

    1. orange_cat | | #14

      I did thank you- I just do not know if they are recognized by the building inspector in Ontario (assuming I can even get it delivered to Niagara falls, NY).

      1. Expert Member
        Michael Maines | | #15

        Kooltherm is frustratingly hard to get. I talked with a rep a few months ago, and I've been checking in with them for years, but they are not distributing any at this time from what I understand.

        Aerogel is a legitimate, if expensive, product. There's no reason your inspector shouldn't accept their documentation. That's not to say they might be leery if they haven't seen it before, but it's been on the market for well over a decade.

  7. gusfhb | | #20

    No way to insulate outside?

    1. orange_cat | | #21

      Unfortunately not. Exterior insulation and brick cladding already installed on the outside side of the wall.

      1. gusfhb | | #22

        Well, if you are considering very expensive insulation, perhaps tearing off cladding and adding to the outside insulation is not unreasonable.

        1. orange_cat | | #24

          The vacuum panels are like $500 for the area I need (from google). Adding a continuous 1 inch for entire outside wall (otherwise I have an unsightly bump?) and redoing the brickwork is a lot more.

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