ICF + 2×4 superinsulated walls
Has anyone built an ICF home with 2×4 walls on the interior of the ICF forms for added insulation? This seems like a relatively easy way to get a super-insulated wall given the ICF form will basically get you R25 and then the interior 2×4 will get you R15 (assuming you used mineral wool). Are there any moisture/condensation issues with this? I’d think the interior framing/insulation wouldn’t be subject to moisture issues given it would always be kept at a temperature close to the inside of the home.
I’m currently building a home with 2” exterior mineral wool insulation and 2×6 framing. While the final product will be great, its been very expensive, difficult, and time consuming. I think the ICF approach I mention above would be cheaper, easier, get me a higher R Value and have better air sealing.
I’ve also thought about double stud walls, but I don’t like the potential for moisture issues.
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Hey there RodRob15 -
Building Science Corp has a good write-up on ICF under their Enclosures That Work section: https://buildingscience.com/documents/enclosures-that-work/high-r-value-wall-assemblies/high-r-wall-icf-wall-construction. Note that their analysis for a typical ICF wall is R20; I don't see how an ICF can be R25.
I have always thought of above-grade ICF has a relatively expensive wall, but of course it's always with the question: compared to what? I don't see any moisture concerns with this approach; you would probably spec elements of this wall to dry to the interior.
The fact that the ICF is structural means that you can configure the interior 2 by 4 wall to be non-structural: 24-inch on center, single top plate, 2 by 3s even.
Thanks Pete,
I rounded up the R Value. I used Fox Blocks for the foundation on my current build, which are R23. I think this probably differs between manufacturers and the quality of EPS that's used. I'd love to see some graphite enhanced EPS forms in the future, which get R5 per inch.
I'm doing all of the work myself, so I just look at things in terms of materials cost. I've also been using mostly premium materials, not just code level commodity garbage (i.e. 2x6 framing, 1/2'' plywood for sheathing, 2'' mineral wool rigid external insulation, 2x4 rainscreen, Hardie fiber cement siding). In particular, the 2'' mineral wool external insulation was very expensive. With that being said, I imagine the materials cost for an ICF above-grade home wouldn't be that different, but I think it would be so much easier and faster to build. If a contractor was hired, I could just see an ICF home being much cheaper given the labor savings. That would at least be true where I'm building (West Virginia) where the 'perfect wall' type construction with external insulation & rainscreen is not common.
Next house I build will probably be ICF, so I'll get to see if my beliefs on pricing are true. Building my foundation with fox blocks was so easy. I know ICF isn't considered the greenest product, but I think that's debatable.
forgot to add that I'm using a European house wrap and tape for air sealing, which was also quite expensive
rodrob15,
Who was your ICF contractor?
I bought the ICFs through a local concrete supply company and then did all the work myself. It's about as DIY as it gets for building a foundation. My father and I basically had the forms ready for a pour in about 2 days. I did the pour as well with a pump truck.
Hi rodrob15.
I'm wondering how your DIY ICF house came out ? Pro's for ICF is the storm/tornado resistance here in Iowa, but I don't like the fact you end up with only R23 with fox blocks. And don't buy their energy stick system pitch since there are 1/8" gaps every 8" with the additional 2" of insulation.
I imagine you might be finished building your ICF house by now and in hind site, would you have built the house any differently?
Thanks!
If you model ICF wall alone and the ICF with the extra wall using BEopt, my guess is you will not recover cost if the extra wall given today’s fuel and interest costs.
Walta
Other brands make bigger blocks with more foam. Some even have it on the correct side, meaning the outside. You can get an R40 block. Don't know how cost effective. Also thought about gluing extra foam sheets on the outside, but again, don't know the details.
The fact that you can bury your electrical in the ICF foam and attach finish walls directly is a major selling point. Fox Blocks sells foam strips that you insert into the blocks to increase the R value about 8 I think. If you want a six inch concrete wall you use eight inch blocks. I see no reason you can't use ten inch blocks with strips on both sides. The result is an R16 improvement in the same space as the proposed wall. You could even cut your own strips and put them in any block brand.
Id have to respectfully disagree with the additional R16. The plastic ties separate the 8" x2" foam strips inserted inside about 1/8" I'm guessing. In other words, every 8" you get a 1/8" gap and you cant stagger the seams. The image that comes to mind is Joe Lstiburek personal barn where he covered the roof with 8" thick foam instead of staggering 2" or 4" thick foam sheets.
https://www.google.com/search?client=firefox-b-1-d&biw=1280&bih=674&tbm=isch&sa=1&ei=rVxqXZfMNMXwtAXPj4boDQ&q=Joe+Lstiburek+barn+roof+gaps+staggered&oq=Joe+Lstiburek+barn+roof+gaps+staggered&gs_l=img.12...0.0..111365...0.0..0.0.0.......0......gws-wiz-img.4PR218pFV2U&ved=0ahUKEwjX1q-lg63kAhVFOK0KHc-HAd0Q4dUDCAY
Installing two layers of 2" foam with staggered seams on the outside would be more effective.
I don't think that is a problem in this case. You've got the monolithic concrete air barrier. And you have the outside foam from the block which overlaps. And if this is a problem it would exist on all ICF as the blocks just butt together anyway without overlap. Hardly a guaranteed airtight seal.
But 2" of additional foam on the outside might still be a better idea.