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ICFs and a walk-out basement

user-1012653 | Posted in Green Building Techniques on

As I am firming up my design, we will have a walk-out basement. The plan is to use an ICF basement, with 12″ double stud walls for the upper part of the house.

My original thought was to do the ICFs on the 3 “below grade” sides, and then change to the double stud wall on the walk-out side to save cost and make for easier construction of windows, doors, etc. (plus if a RO gets framed wrong or something changes, we are fixing wood and not concrete…).
Besides that, the other reason is it is a long (60′) exposed wall on the north face. I was thinking the r-40+ double stud would be a much better option then the r-25ish ICF wall.

Are there any thoughts on this? Would it still make a better/stronger basement ‘system’ if all the walls were ICF? I realize the length is quite long, which is why I am curious if it would even help make any difference or not.

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  1. user-946029 | | #1

    Don't discount the mass of the ICF wall. This often doesn't show up in R-value discussions, but it quantifiable and does make a difference in reducing air infiltration.

    PCA sells an HVAC sizing CD that enables you to do system calcs. It might be worth the $60 to determine which route to go.

  2. dickrussell | | #2

    Jesse, where is the house? I am guessing zone 5 or 6, given your use of an R40 wall. Bear in mind that the "thermal mass" effect of concrete is most useful in climates where there is a lot of outside temperature swing above and below the inside temperature. In prolonged cold weather, the mass really doesn't help with temperature profile. True, it can provide a very airtight wall, but that can be achieved easily with a framed wall also.

    Without knowing more, I would guess that reducing the heat loss through that wall by 37% with the R40 framed wall would more than offset any gain there might be from thermal mass effect of the concrete in an ICF wall.

  3. user-1012653 | | #3

    That is exactly correct and the reason for my question. I am in zone 6, and this is the reason I put r-25 for my ICF and not higher, as I am not counting the mass.

  4. user-988403 | | #4

    I agree with Dick. The advantages of the high R-value are much higher than the potential thermal mass. Thermal mass is only a benefit if you want to store "free heat" (mostly provided by the sun during the day) and release it when it is needed (during the cold nights). In a super insulated passive solar design thermal mass could be important even without large outside temperature swings, because you might invite more "free heat" in, during a cold but sunny day than you need at this point. With thermal mass you can store the extra heat and release it during the night. Also it prevents the structure from overheating during the summer as long as you have cooler nights. However, I am not so sure about the benefits of the thermal mass in ICF walls because of the interior insulation. I don`t think you can effectively store solar energy in a ICF wall and release it when needed. Any thoughts on this?

  5. user-757117 | | #5

    Jesse, am I understanding correctly that the walk-out side is 60' long and faces north?
    Just a thought, but if the site allows, you may want to consider putting the walk-out on the south side...
    I would stick-frame the walk-out side for all the reasons you mentioned.

    If the walk-out portion of the basement is ICF construction...
    How will the exterior cladding transition from the wood framed wall above to the ICF wall below?
    Is it possible to keep the cladding all on the same plane or would it have to "bump out" at the transition point?

  6. user-1012653 | | #6

    I agree completely, which is also why (against the recommendation of every ICF person I know) am only using it for my foundation and nothing above grade. Any report given by ORNL or Build Science shows ICF in heating dominate climates have a marginal benefit over code built stick frame, and they usually claim the added tightness of ICF is to blame.

    Lucas- no, unfortunately the site slopes to the north with the road on the south side. However my details I am doing align the exterior face of the ZIP sheathing with the exterior face of the foam on the ICF form. This gives me a continuous smooth plane. I plan on running my peal and stick water proofer up within this area, and then taping off the seam between the ZIP and the foam to give me an air tight seal at my box sill plate. My plan is to put 1.5" of XPS foam behind the ZIP over the rim board to create a thermal break at that point as well. Stone veneer will go from grade and up the wall to below the window sill.

  7. user-946029 | | #7

    What kind of insulation are you using in your double stud walls?

  8. user-1012653 | | #8

    blown fiberglass most likely

  9. user-946029 | | #9


    Fiberglass? Ugh.

    IMO, you're headed down the wrong road. So I e-mailed a link to this discussion thread to an old colleague of mine. He's a licensed PE in 20 states and one Canadian province. Here is his response:

    "He would be better off doing the whole house out of ICF. Even with the double stud configuration, unless he is using polystyrene foam in the wall, the old saying comes to mind “fiberglass is a filter”. Air doesn’t blow through ICF walls.

    If the focus is on R-value, he can get an ICF system (quad lock?) that has 4” of foam on the outside and 2” on the inside. He is forgetting that mass helps.

    What hasn't been mentioned is that the windows and doors are where all the heat loss is. Windows are R-1. They are the “hole in the bottom of the pail of water” … all of the heat leaks out through the windows."

    Finally, I'll ask if you're anywhere near a tornado or flood-prone area or a source of loud noise (school, highway, airport, hospital, fire station). If so, the durability and soundproofing will be hard to beat.

    *Neither myself nor the engineer work for an ICF company.

  10. user-755799 | | #10


    Air doesn't blow through fiberglass when all sides are air sealed. While I'm not a big fan of fiberglass for a few reasons (I would have chose cellulose), it is effective when air-sealed. I'm probably not alone on this site in that I've with built houses with a variety of systems: ICF, SIPS, double studs, They all have their pros and cons and the system you chose depends on your goals. High r-value is not an attribute of ICF's.

  11. dickrussell | | #11

    Mike, I find it annoying when someone like your old colleague says something like:

    "What hasn't been mentioned is that the windows and doors are where all the heat loss is. Windows are R-1. They are the “hole in the bottom of the pail of water” … all of the heat leaks out through the windows."

    Statements like that detract from whatever other useful thoughts he may have offered. No one in his right mind would put a single-pane window or shoddy door in a new house that is designed as superinsulated. All one has to do is look at the heat loss distribution of a good energy model for a house to see that wall losses, however small, do contribute substantially to the total. As my father (a banker) used to say, "It all adds up."

  12. user-1012653 | | #12

    Fiberglass, while not as "green" (depending on your definition) as cellulose, it also has some advantages that make it the correct choice for me. When blown at pressures of 1.8-2+ psf, it has similar air convection current reductions as dense packed cellulose, and at a slightly higher r per inch. Talking with a highly skilled local insulator passionate about energy savings (as well as getting other opinions all over the US) it seems to be much prefered install over cellulose for various install reasons. While it is not made from completely recycled products (new fiberglass products are using more and more recycled glass) my "green" argument will lie with I can reduce building materials while achieving similar r value of a cellulose wall (only need 10" walls vs 12" walls=less foundation, roofing, etc).

    I must say, I respectfully disagree with a lot of what your colleague claims. First off, with proper air sealing, single stud, double stud...any kind of stud wall can perform just as tight, sometimes tighter, then ICF walls. Like wood install, ICF is only as good as the install. If there are voids in the concrete due to poor vibration, those spots leak air as well. Gaps around window bucks and other openings also have to be air sealed and detailed. Some of the most efficient houses in the world are built hardly ever using ICF, but typically double stud or thick SIPs. In heat dominated climates, the thermal mass gains is minimal at best according to independent studies listed above. Going with thicker foam forms greatly increases price over an already expensive product. I have priced out ICF all the way up to the trusses, and my materials cost nearly tripled for a wall with half the r value. Hitting similar r values will make the price close to 5x the cost of a double stud shell.
    However I will agree with you on tornadoes, and we are in tornado alley. I have actually designed several FEMA 361 saferooms using ICF forms (12" thick CONCRETE) as well as precast planks. However since we will have a basement, there will be a "true" saferoom with concrete cap in there. ICF walls are only safe if your roof doesn't come tumbling down on you...(in which case the roof diaphragm is now eliminated thus the chance of the ICF walls falling on you greatly increases...)

    Finally on windows....I am not sure what glass he is using to have an r-1. Most typical dual pane windows from Pella, Andersen, Marvin, etc will have r values around 3+, with triple pane options from r5-10+. Also with properly tuned glass and designed house, the windows can become a heat gain net positive and not a reduction.

  13. user-946029 | | #13


    So that this doesn't devolve into an argument, I'll just go on record as having a different opinion and leave it at that.

    I did have to question one comment in your last post. You stated ".. the chance of the ICF walls falling on you greatly increases..)"

    Given that common ICF construction practice is to have reinforcement run continuously up from the foundation into any and all above-grade walls, and horizontally around the entire shell, I have no idea how the walls would ever fall on you.

  14. user-1012653 | | #14

    I would hardly call it an argument but rather a discussion. I do my best to leave my opinions at the door and try to base most things on factual findings. I encourage you to voice your thoughts, I am always interested to hearing other points of views. Its how people learn!

    With dealing with FEMA 361 design requirements, there are multiple structural issues that need to be addressed for a tornado proof shelter. Wall reinforcement, especially found in typical residential 6" ICF walls fail to come even close to meeting the structural requirements to not only withstand 200 mph wind loading, the 100 mph 2x4 puncture test, but also the internal design pressure resistance. One we did used 12" solid concrete with double vertical #6 12" oc.
    With the roof diaphragm gone, the (assumed) 9' tall wall, 6" ICF wall with minimal reinforcement becomes a sail in the wind, putting a tremendous amount of horizontal shear at the 1st floor wall connecting to the basement wall, which is typically a weak, cold joint already.
    Again, I will not disagree that an ICF house has a lot better chance then a double stud wall, but my goal is energy efficiency as number 1, but still getting safety in there as well. An all ICF structure, in my climate, simply can not compete anywhere near the double stud wall talking dollars invested.

  15. user-723121 | | #15

    Jesse, don't be swayed from using fiberglass wall insulation, properly installed it performs very well in cold climates. I have achieved sub 1 Btu/sf/hdd performance with double walls, fiberglass insulation and good solar orientation. This was with dual pane Low E windows and no sub-slab insulation in a 9,000 plus hdd climate.

  16. user-946029 | | #16

    "With dealing with FEMA 361 design requirements, there are multiple structural issues that need to be addressed for a tornado proof shelter. Wall reinforcement, especially found in typical residential 6" ICF walls fail to come even close to meeting the structural requirements to not only withstand 200 mph wind loading, the 100 mph 2x4 puncture test, but also the internal design pressure resistance."

    Jesse - You may have had a bad personal experience with an ICF wall at some point, but I can point to multiple examples where ICFs performed remarkably well in real-life incidents involving:

    Tornadoes -

    Hurricanes - (I can vouch for this, as I have been inside this house post-Katrina.)

    Blast testing - (At one point, I had the video of this event.)

    Automobile accidents -

  17. user-1012653 | | #17

    I will not disagree they are certainly better for storm damage prevention then any stick house. I was mainly referring to saferoom designs I have been a part of. The typical ICF home will not meet FEMA saferoom requirements, regardless if it stands up to F2-3 tornado. My point is ICF homes can give a false sense of security. You still have the risk of flying glass and debris inside the building, as well as possible roof collapse, etc. There is in fact an ICF home near me that stood up to a tornado that passed through last year. However the F5 tornado that passed through our area 3 years ago took down everything in its path, including 2 ICF homes. But with that being said, I would still rather be in an ICF home then a stick home during a twister, however when I have the option of a basement with a concrete stoop cap, that still trumps all, IMO of course. (not to mention a fraction of the price).

  18. user-946029 | | #18


    I agree, though it's because ICF homes aren't designed to FEMA saferoom requirements. If the windows aren't shatter-resistant, the doors and roof aren't secured, etc., then you've got a great shell and some cosmetic parts.

    I'm a little surprised the ICF homes lost the battle versus the EF5. Did anyone do an analysis to determine why?

    (I forgot to mention it in my previous post, but I do know video exists of a tornado going right over/through an ICF home with no visible damage depicted. I don't know the strength of the tornado, or where that footage now resides, but I did view the video 2 or 3 times.)

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