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Community and Q&A

Exterior insulation with ICF

Dustin_7022224 | Posted in Energy Efficiency and Durability on

We’re trying to continue making progress on our pretty good ICF home in climate zone 5 and are at a crossroads on the exterior work.  The walls above ground are 4″ of concrete, 2.5″ EPS foam on each side of the concrete – so about R-23.

For reference, we will have an insulated, un-vented roof with conditioned attic.  Planning on R-23 over the roof deck and R-15 in the rafter cavities.

We have covered the roof deck with water & ice shield to help improve air tightness (belt and suspenders), which folds over the edges onto the ICF.  The rafters end inside a fascia board, flush with the exterior walls.  We will install applied or ‘false’ rafter tails/eaves and rakes in the near future. 

In the interim, we have had some water intrusion, I think with water coming off the roof, down the ICF, and inside at the window bucks and cold joint between the top of the basement ICF and the above grade ICF.  I’m hoping that getting the overhangs on the roof and the siding up with a rainscreen will take care of all that.

The garage is for all intents and purposes detached/adjacent and is 2×6 stick framed and sheathed with Zip.  The concrete floor has PEX over rigid foam for radiant heat.  We plan to keep the garage maybe 50F in the winter.  I think we have plenty of room in the walls for insulation, but is there any major (cost-effective) advantage to adding exterior foam over the Zip on the garage? 

Appreciate any suggestions, particularly with regard to if it would be really worthwhile to add a little more insulation on the exterior of the ICF on the house.

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  1. Expert Member
    Michael Maines | | #1

    Dustin, relying on roof overhangs to take care of water intrusion is not a safe strategy. You need to find and deal with the water problem. I don't have experience building with ICFs above grade but below grade I know they require a separate waterproofing layer (though often omitted by builders). Does the ICF manufacturer have requirements, or details for flashing windows and doors?

    A couple of relatively minor quibbles: 5" of EPS will be R-20 to R-21, not R-23. No EPS is R-4.6/in, and concrete has essentially no insulating value. The manufacturer may be showing "effective R-value" which takes the thermal mass into account, but that is situation dependent.

    Although the Pretty Good House approach is very flexible and open-source, it does not sound to me like you are building a PGH. A PGH minimizes embodied carbon emissions; building a house entirely of foam and concrete comes with a large carbon footprint. A PGH in cold climates should have an envelope approaching R-40 walls and R-60 roof; although that can be reduced, especially with the help of energy modeling, you are going slightly over code-minimum on the walls and exactly code-minimum in the roof--in fact most of us in the green building world consider R-49 code-minimum for roofs in zones 5 and 6, not the R-38 exception allowed when insulating fully over the top of the walls.

    You will likely have better than code-minimum air tightness, which is good. You just won't have a Pretty Good House.

    1. Dustin_7022224 | | #2

      Thanks Michael. Food for thought in time to make changes.

      Any feedback on the garage with regard to exterior insulation over the Zip?

    2. Expert Member
      Dana Dorsett | | #14

      >"5" of EPS will be R-20 to R-21, not R-23. No EPS is R-4.6/in, and concrete has essentially no insulating value."

      At an average temp of 40F through the foam (+10F outside, +70F indoors) Type-II EPS (commonly used in ICFs) is good for R4.5/inch. With air films, gypsum board, siding, etc it's slightly better than R23 clear-wall under wintertime peak conditions, maybe R22-ish at the wintertime average conditions in most zone 5 locations.

      Even with 4" concrete walls the thermal mass benefit is real enough to take a chunk off the peak and average heating/cooling loads, and it more than meets code minimums without fatter insulation on th exterior. Whether it's "worth it" to go higher R on the exterior of the walls depends a bit on the other insulation and window specs.

      >"Planning on R-23 over the roof deck and R-15 in the rafter cavities."

      Explain that a bit. Code min on an R-value basis is R49- you're at only R38 center cavity, though it probably just squeaks by code minimum performance on a U-factor basis with a tiny bit of marging if the R23 is continuous, not thermally bridged by framing. Is that 4" of fiber faced roofing polyiso above the roof deck with R15 fiberglass batts between the rafters, or something else? What is the rafter spacing & depth?

      In a zone 5 climate a continuous R23 on the exterior of the roof deck provides sufficient dew point control at the roof deck for up to about R32 in cavity fill. Going with R30 rock wool in 2x8 rafters or R30 fiberglass in 2x10 rafters would be a pretty cheap upgrade from the R15, and most likely more benefit than adding another inch of foam to the exterior of the ICF.

      1. Dustin_7022224 | | #15

        Sorry Dana, I was mistaken when I originally explained my plan for the roof insulation.

        "I was reading some notes of mine earlier when I referenced the plans for R-38 in the roof. If we go with Roxul ComfortBatts at R-32, we should have a total assembly of R-55 and still maintain the code required minimum of 41% of the insulation above the sheathing."

        Appreciate your feedback. Your suggestion is my current plan and reinforces my decision.

  2. Expert Member
    RICHARD EVANS | | #3

    I think the Nudura ICFs have 2 5/8" of EPS on each side. This would get you to R21-22. Nudura claims that it is the "equivalent of R-50 (!!) because of thermal mass". I think we can put that into the same category as R-15 Cedar Shingles and R-10 reflective paint. :-) There are 'GPS' ICFs too with a a higher R value. I think these are more common in Europe.

    Anyway, regarding water intrusion, some ICF manufacturers offer a peel and stick membrane designed specifically to adhere to their product. I would cover your walls with it and and lap the Grace on your roof OVER the self-adhered membrane on the wall. This systems works well even below grade where ICFs are commonly used.

    I would double check your flashing at windows and doors. You shouldn't be getting leaks.

    Regarding insulation- forget the walls and focus on your roof. I can't imagine that you are using 2x4 rafters (R-15) so there is some opportunity to add more fluffy insulation to get this closer to R-50.

  3. krom | | #4

    I don't recall carbon being involved in the pgh that started on here.. (the nicest thing I can say about carbon argument against concrete is that its ridiculous) all the carbon for all the concrete used in residential construction world wide doesn't add up to a grain of sand on the beach.

    10,20,40,60 for R values, low infiltration, reasonably simple roof lines, good overhangs, good windows etc.

    The garage is a pretty easy thing to do a manual J on, or use an online heating calculator to determine the payback period for additional insulation. Hopefully you've got at least R10, with R20 preferred under the a heated slab.

    1. Expert Member
      Michael Maines | | #13

      I'm pretty familiar with the PGH standard. Originally it called for low embodied energy, and when it was revisited a couple of years ago, embodied carbon was added to the list.

      1. Dustin_7022224 | | #16

        For those of us who prefer to pick and choose 'standards' or are otherwise trying to strive for PGH, but limited by budget or other restraints... I guess I'm building a house that is a pretty good compromise. You can call it a PGC-IMHO if you want.

  4. Expert Member
    Akos | | #5

    PGH R values only make sense if you are insulating with low cost materials such as batts, blown in insulation or dense pack cellulose. The math doesn't work if your insulation is expensive.

    There is very little financial justification for much above R25 assembly(roof or walls) in Zone5. The cost of the extra bit of insulation to get to PGH levels will never pay for itself in energy savings unless your energy costs are very high.

    Not a fan of ICF, but I would say the OP's setup is actually pretty decent for the climate.

  5. Dustin_7022224 | | #6

    The house is already under construction and the ICF is completed and most framing (roof, interior walls, garage) is done. So, Nudura is out, although I don't know they offer much advantage over BuildBlock. I appreciate even more so now the disadvantages to ICF construction and am not sure I would choose it if I had to do things over again, but again, the concrete is poured so I'm trying to make the best decisions that I can with what I have left to do.

    Here is what the manufacturer says, "There is much misunderstanding regarding vapor barrier and water resistive barrier requirements on above-grade ICF applications. Some, but not all, EIFS systems have code approval to install on ICF without a vapor barrier. " Funny, huh?

    I appreciate the comments Rick and Akos with regard to focusing on the methods I've chosen thus far and putting more insulation into the roof as a financially conscious strategy. I was reading some notes of mine earlier when I referenced the plans for R-38 in the roof. If we go with Roxul ComfortBatts at R-32, we should have a total assembly of R-55 and still maintain the code required minimum of 41% of the insulation above the sheathing.

    The house is oriented with most windows facing south with a 15' wall on that high side, under the shed roof. We will have a light shelf between the lower and upper windows to reflect some summer sun (in addition to shades/blinds) and allow the lower winter direct sunlight in through the upper windows, and an exterior trellis with angled boards to help do the same for the lower windows. The floor below, in those rooms which are the kitchen and living room, is polished concrete, so we are hoping it will provide some thermal mass benefit.

    I'm leaning more towards skipping the additional exterior insulation, installing Tyvek all the way around, shingled under the ice & water shield which is already on as described in my original post. I think this will make it easier to detail the window and door flashings and hopefully deal with our water problem.

    We already excavated the partial fill along the front basement wall, added a second drain tile below the footer, and ran it into its own sump pit. Having watched for water during heavy rainfall, it all seems to be coming in through a few joints in the forms, at the top of a couple of window bucks, and at the cold joint between the basement pour and the above grade pour. This is the north wall, where the shed roof pitches down and dumps all this water, which washes down the wall. Thus, I think adding the roof overhangs, building wrap, and a rainscreen under the siding will fix the remaining problem. I will have time, once the windows and these items are installed, to monitor for leaks again before we get close to any interior finishes and I still have time to identify and deal with problems.

    1. Expert Member
      RICHARD EVANS | | #7

      I still can't picture this working with Tyvek... You'll have to make sure that the cap nails/ staples hit the plastic ribs inside the ICF. And even if they do, they will have less holding power given that that these ribs are slightly recessed into the foam some. Longer, 1 1/2" RS cap nails would almost certainly work 'if' you can hit the ribs. Because the foam has a little give, I worry it might tear the Tyvek too. I would borrow some Tyvek and test tacking it to the ICF before going all in on this approach. It might work if you are careful.

      I'll reiterate that I think a Self Adhered membrane is the way to go with ICF. BuildBlock offers an option here that is probably pricey but I imagine will work very well. I can speak from experience that the membranes do not like sticking to EPS. BuildBlock offers an adhesive/primer to make this work better. Looks like a really bomber system to me. At a minimum, maybe you go this route on your North Wall only?

      1. Expert Member
        RICHARD EVANS | | #8


        I wasn't recommending Nudura by the way, I was just pointing out that its possible that your ICF wall does, in fact, have a R-value approaching R-23 as you described depending upon the manufacturer of your ICF blocks. You are correct, no advantage to Nudura.

      2. Dustin_7022224 | | #9


        If i can avoid the true waterproofing, I'd like to. Other than the material expense as you mention and labor to do it right, we're going to be in real trouble if the above grade walls are submerged in standing water as we're not in a flood plain. I just need to shed the water, thus I was thinking about the building wrap.

        I'm using furring strips to create a rainscreen. Maybe if we put those up as the Tyvek goes on, I'm thinking it will hold it in place. Would almost be better to do it that way anyhow, so we can keep track of all the ribs/attachment points. Thoughts?

        How about advice on spacing the furring strips for siding (no heavier than hardie plank). I know I need to look at manufacturer specs, but perhaps folks with experience space them closer.

  6. Expert Member
    Akos | | #10

    Whichever surface you consider as your WRB, if water is leaking in than your WRB is not detailed properly. In most cases with good overhangs and attention to details around the windows, this most likely won't be an issue. If you do have issues, it will be very hard to fix, I would rather fix it now when it is easy.

    If you want to use the ICF foam surface as your WRB (which is what it sounds like) than all the seams of the foam blocks need to be taped. That is a LOT of taping. Installing house wrap would be much simpler.

    You don't have to go nuts on fastening the house wrap since you'll be installing a rain screen. The rain screen will hold the house wrap in place. You can strategically tape it in place until the straps go on. Make sure you lap the sill flashing over the house wrap, hopefully you can peel enough of the peel and stick to get the house wrap under it.

  7. AP1210 | | #11

    I too have an ICF home that we built 20 years ago. I am replacing some windows due to leaks that have damaged windows. As I remove old windows, I see a large (1/2"+/-) gap between the 2x12 wood buck and the ICF/concrete. What I am doing is using some liquid flashing (Prosoco) to fill the gap then seal around the perimeter. What I have also found is that our brand of ICF has small vertical grooves in the surface at 1" on-center, and during heavy storms, water follows those grooves and can get in. Either an adhered membrane or cheap house wrap has alleviated the problem.
    I have also considered more insulation over the ICF, but with 6" of concrete and 3" of foam on either face, I figured that was good! And its a lot of work!!

    Good luck with your project.

    1. Dustin_7022224 | | #12

      Thanks to all for feedback. We have house wrap up on the north wall and the window bucks are all covered. It's tucked under the water & ice shield as it folds down over the end of the rafters and onto the face of the ICF walls. This has all but alleviated any water intrusion. The only problem we seem to have now is where the roof meets a dormer on both sides. Plan is to pull/cut back the water & ice shield here, block and foam any gaps, and seal with Prosco joint & gap filler, then re-cover with water & ice.

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