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Is it necessary to go beyond the standard icf r23?

wiscoguy | Posted in General Questions on

Wasn’t really sure where to post this I had my wall assembly all figured out amd was ready to go only to find out lumber prices were way way higher than even a few short months ago. So I have come full circle and I’m leaning towards an icf house that I would do everything myself as well.

Im just wondering if it’s worth going to the r35 blocks vs the r23 it’s double the cost. Almost seems like I’d be better off adding foam myself. The wall will perform pretty well it doesn’t leak air so my feeling is this wall will perform well in zone 6 where I live. Guess I’m just looking for opinions Positives and negatives to an icf build.


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  1. MaineLaxRef | | #1

    I am in the same situation. I am designing an ICF home in zone 6.

    I am planning 2 stories of ICF from the footer to the eave with 148 feet of perimeter wall. By my calculation, assuming a heat pump system with 300% efficiency, at R23 the heating/cooling annual costs would be about $240 for the walls. Changing to R35 would drop that to about $160, saving $80 per year.

    I was just quoted $16,000 for the R25 ICF forms. So, if the R35 would be $32,000, it would take 200 years to recoup the $16,000 delta. That is long term planning!

    1. wiscoguy | | #2

      Lol thanks that’s how I’m looking at it too

  2. jackofalltrades777 | | #3

    I am assuming you are taking about the Nudura ICF X35 Series?

    If so, was it really double the cost in the forms? I thought it was only a 20% price increase? I would double check on that pricing and get an official quote because I don't believe it is "double" the cost of the standard forms.

    If it was a Zone 4 or lower, then I would say it doesn't make financial sense to upgrade to the R-35 forms but Zone 5 and higher, it might make sense to upgrade to the higher R-Value.

    1. wiscoguy | | #4

      I was quoted almost double as well. For x35 there’s only one dealer In my area.

    2. MaineLaxRef | | #5

      I was told about 18.5% increase in what I had been quoted for R23 forms. I extrapolated (double) for R35, based on a rapid (and incorrect) assumption that there would be twice as much EPS.

      Upon reflection, an R35 form probably has 1.5 times as much EPS as an R23 form, so R35 would extrapolate to $24,000 for 148 feet of wall 17.5 - 18.0 feet tall. A delta of $8000 would only take $100 to payback (excluding cost of money, inflation, etc.).

      As I look at the mathematics for heat loss, my conclusion is that uncontrolled air infiltration is much more important than R-values, once you have achieved code. For my house, 2/3 of the estimated heating cost of $1500 per year comes from air at 1.0 ACH50. In other words, achieving R200 for floor, walls, windows, doors and ceiling would save me $500 a year, but improving to 0.5 ACH would save me $500 a year.

      Infinite R-value is impossible at any price, whereas reduced ACH is achievable with attention to detail. An investment with AeroBarrier ( could have a short payback.

  3. Trevor_Lambert | | #6

    If cost is a major consideration, isn't ICF one of the most expensive wall options? On top of it being the least green by a country mile.

    1. wiscoguy | | #7

      With current lumber prices it’s less for me. Also I would argue that concrete is farm more green then most matierials. Consider everything enclosed harvesting wood all the green houses gases used creating foam and it’s other issues of blower agents losing r value. I would say if anything it’s about even with wood framing when you consider all the products glues etc that go into wood framing.

      If I’m wrong please educate me but there a a lot of byproducts to stick framing besides just wood.

      1. tim_dilletante | | #9

        Late to the party, but I should chime in for posterity. One of the biggest issues with concrete is sand. Here's one explanation of the problems:

      2. tim_dilletante | | #10

        One of the biggest issues with concrete is sand. For a overview:

  4. sfortier | | #8

    I'm late to this thread but Polycrete Systems do sell an ICF block using Neopor that is R-29 effective (including interior and exterior finishes):

    Used those for my lake house and they were more cost effective than wood this summer where the price of a regular 2x4x8 reached $9.95 CDN and 3/4" plywood over $100...

  5. user-2890856 | | #11

    Several years ago ( 6-7 ) I designed a heating system for a gentleman in Ann Arbor Michigan , ICF R 23 . 4000 sq ft , single story , Forget the ceiling values but can look them up . Regardless , it was air sealed very well as you'd expect . R value does much less for loads than air sealing , this is a fact . That home which I tracked and have yearly spread sheets generated by the homeowner for the first 5 years . In those first five years his average gas usage was 40.00 monthly . This is for heating , clothes washer , cooking and laundry gas .

    My point being , R 35 would have done not a whole lot , the tightness is what mattered

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