ICF Alternative
I’m looking to build an energy efficient, rectangular building with a focus on longevity.
ICF has intrigued me but I’m not a fan of the foam, which brings me here. I have a bit of a silly question:
Could I apply Rockwool (via an adhesive) to a piece of plywood to use as concrete forms? Specifically, on the interior of the wall. My thinking is, creating an ICF-like sandwich where I’ll have Rockwool glued to the plywood form on the interior, pour the concrete, then fasten the plywood to the concrete once set, and remove bracing. On top of the plywood, I’d Sheetrock the home (so I have a continuous anchor for artwork, etc..
On the exterior, strip the forms and apply thick Rockwool insulation followed by a cladding. I’d waterproof the concrete before applying the insulation, obviously.
Is this a crazy concept? Seems fairly easy (though costly— again, longevity).
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Replies
Instead of inventing something new, you can look at wood composite ICF. These use wood fiber with cement binder as the forms so there is no foam.
HomeBulderGuy,
Aside from the huge labor and materials expenses involved, you would have thermal issues thanks to the forms ties that you would need to hold it all together. The ties do have a larger effect than expected, a fact that NREL wall tests demonstrated. You would likely encounter condensation issues on both faces depending on your climate zone. The plywood and Rockwool interior side would pose problems for placing wiring runs and both faces would require long and fussy fasteners to anchor into the concrete. I can't guess how you would set windows and doors. Reversing the sandwich and putting the foam in the middle still suffers from the ties degrading the overall performance.
The wood fiber/cement ICF-like forms are more resistant to fire and vermin, but the final walls do not perform much above a frame wall of similar thickness. Other wood fiber/cement technologies have somewhat better thermal characteristics, but they are not for high compressive loads and the structural supports required echo timber framing done in concrete. For milder climate zones it might be viable. Other attempts to make cementitious panels have had varying success. Try googling cement faced SIP panels and read up.
If you are able to find an affordable source, understand the thermal limitations and can find an engineer experienced with designing for the material, AAC blocks might be your grail. I know a person that used them and so far as I know he is quite happy with the result. I can't judge how long the house will stand. I will note that while the walls may be durable, windows and roofs are still subject to age failure.
Historical buildings of stone, mud brick, or similar may well be durable, but rarely energy efficient. People like to think that adobe is timeless, but like wattle and daub their secret is in endless upkeep. Pyramids seem to last pretty well though. Not sure about the ventilation or lighting on those ;).
Thank you to the both of you!
Akos: I will look into those products more. I read briefly about them but couldn’t find much info. What I do like about ICF is the continuous concrete.
Onslow: thank you for such a detailed response! I guess I’m just struggling to find information based on what I would like— continuous concrete walls with exterior insulation and a plywood backer on the interior underneath my 5/8 drywall (again, for art).
My question was asked because like an ICF wall, I figured I could use the Rockwool/plywood as a “leave in place” form. However, I do need to think about drying. Maybe this will be more complex than I had initially thought.
I will be using full-bed stone cladding. I would like Rockwool Insulation. I would like a continuous concrete structure.
With the ICF, I’m worried about the foam holding up over time, mainly from animals— termites, mice, etc. I understand there are additives that can be added to reduce this. But, spending $30/sf on Shou Sugi Ban and paying a fortune for full-bed stone, I want it to be done correctly.
It’s easy to get carried away with wall thickness (4” stone, 1” air gap, 6” insulation, 8” wall, 3.5” stud wall, 3/4” ply, 5/8” drywall) so I’m trying to find a balance here. I do plan on large, triple pane fixed and tilt/turn windows that are an R7 (minimum).
I’m just stuck here. Maybe I’m overthinking it and ICF would be sufficient.
EDIT: It’s late and I’m just thinking. What if I poured a 8” concrete wall (8” because wall height is upwards of 20’ for a single story. Then I take pressure treated 2x4 sleepers and fasten them to the 8” core, and nailed another 2x4 on top of the first, bringing it to 3” in depth, which I can place 3” comfortboard in between. Next, I take another 3” layer and stagger them, sandwiching the second layer between ZIP and the sleepers. I can then install the full-bed stone and have the sleepers to anchor to, which are secured to the concrete core. That way, the wood sleepers serve as a thermal break since no anchor goes to the core AND that second layer of comforboard covers those sleepers and seams are offset.
Are you in the northeast US? This product would suit your needs: https://www.comfortblock.com/. Not exactly what you're asking for but roughly R-30 and easy to install.