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Community and Q&A

Is an ICF built house going to work for me?

bobhol | Posted in Energy Efficiency and Durability on

I have posted various times about the house I plan to build and heat with mini splits in a cold climate,Peterborough Ontario.All of my decisions are based on 2by 6 construction with roxul or blown cellulose and 4 inches of foam on the exterior .It is a slab on grade with 2 inches of foam under the slab .I will also go r50 in the attic ….this presumably will give me sufficient insulation to heat with a mini split on each floor as we do get a few days at -25c .I do have a propane back up source of heat ….here is my question .If we decide to build ICF ,is there enough insulation value to heat this way ….or how do I get enough insulation ? thanks in advance …Bob

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  1. Expert Member
    Dana Dorsett | | #1

    You can get to fairly high-R using assymetric ICFs, such as Quad-Lock's slip-in panelized system:

    From a price/performance point of view there are cheaper ways to get there using materials other than concrete & EPS, but there's no question that you can get the performance you need with an ICF approach.

    At R25+ (whole-wall, all thermal bridging accounted for) the window choices will be a bigger ultimate determinant of heat load, and the thermal mass of ICFs reduces and delays the peak heat loads.

    R50 in the attic is probably on the skimpy side if you're reducing heat load do mini-split type levels. It's usually cheaper on a $/ BTU/hr-reduction to take it up to R75 in the attic than adding an other inch to the wall foam.

  2. GBA Editor
    Martin Holladay | | #2

    Here are links to GBA product reviews of ICFs with above-average R-values:

    Logix XRV forms:

    Reward Wall System Boost-R panels:

  3. bobhol | | #3

    Thanks for the replies ....I take what you are saying is from a cost perspective go with the 2 by 6 and extra exterior insulation but to upgrade my attic insulation.Should I be going with 2 inches of ext. foam or 4 inches ? Is the difference in R value worth the cost ,especially if I spend my money in the attic and slab?...regards,Bob

  4. GBA Editor
    Martin Holladay | | #4

    Q. "Should I be going with 2 inches of ext. foam or 4 inches?"

    A. The answer depends on your climate zone. Here is a link to an article that explains what you need to know: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    I not sure whether your part of Ontario is more similar to Climate Zone 5 or Climate Zone 6. If it is similar to Climate Zone 6, you would need at least R-11.25 of foam on the exterior of a 2x6 wall.

  5. jinmtvt | | #5

    There are many different insulation values with diff ICF manufac.

    Some now have 3" on each side, which is the minimum i would recommend in your situation.
    Quad lock have some good options with thicker exterior foam and higher R values and is Canadian.

    As always with ICF, an experienced team of worker is more important than all other factors combined.

  6. Expert Member
    Dana Dorsett | | #6

    Peterborough Ontario climate is comparable to the warmer half of US climate zone 6, with mid-winter temperature averages comparable to Madison WI.

    With 2x6 studwalls full of Roxul or cellulose and wood or fiber cement siding you have a whole-wall R of about R14, and to hit mini-split type heat loads you'd likely need R30+, which would take 3" (R19) of exterior polyiso, and 4" (R25) would probably still be worth it. That is plenty of exterior R to be protective from a dew-point point of view, and no interior vapor barrier would be required (or desired.)

    With ~R40 above grade walls and R75 attic you'd want about 3" (R12) of EPS under the slab, and an R20 foundation wall (ICFs are fine) to be reasonably consistent.

  7. Expert Member
    Dana Dorsett | | #7

    Jin: The 3" + 3" ICF is a somewhat lower performer than a 2" + 4" , since the 3+3 isolates the thermal mass more severely from the interior.

    The 2.25" + 6.26" (labeled "R38 TMO in the Quad-Lock parlance) would be about right for something you could heat reasonably with mini-splits:

    While the steady state R-value @ 20C (with allowances for siding & gypsum) is really closer to R36 than the labeled R38, it's average performance would be more like R40 in Peterborough's climate, partly from the mass-effect, partly from the higher-R performance that EPS exhibits at lower temperatures.

  8. bobhol | | #8

    Thank you all for the great I will discuss these options with the builder and see what he says and.also my boss and see what she says.. lol....regards,Bob

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