How thick should my concrete walls be?

Question

I've been drafting a house for a couple years now and through many iterations I discovered one of the biggest flaws in how I was drafting was in not knowing what I would be building the house out of.

After looking up the differences between Concrete and Lumber construction I settled on the idea of drafting a concrete house. I found some nice ICF designs and have since been designing the house with ICFs in mind. However, recently I've been doubting the information I've been given. See I keep getting conflicting information on how thick ICFs should be.

How thick should an ICF Exterior and Interior Wall be in my drafting document?

Bonus: What parts take up what amount of space? How thick is the Core Concrete, insulation, etc.? And what tolerances should I have for drywall to go over the walls?

Answer 1

ICF block comes in core sizes of 4, 6 and 8 inches, and the insulation is 2.5 inches per side. Most of the ICF houses I've worked on were exterior and load bearing only. In Ontario, other jurisdictions may be different; any time you're dealing with load bearing walls you need to have an engineer certify it and submit it to your local building authority. I've never heard of any engineer putting their stamp on someone else's drawing (I'm sure it does occasionally happen) because they are absorbing the liability for someone else's work.

From my experience, foundation walls are always 8" + 5" for the insulation and I've seen 6" on upper levels. The 4" blocks, I would imagine, are for partition walls. They are very comfortable houses to live in. The rated insulation is pretty much what you get, as opposed to a wooden structure. With lots of extra framing and pack points, along with poor insulators and difficult spots to insulate, the actual R-value of your typical wooden structure is far less than the R-value listed on the bags of insulation or what the spray foam guy is telling you. Wood is a poor insulator.

ICF is also very tight so a HRV or ERV in a warmer climate is a necessity. Also, consider concrete floors over corrugated metal with radiant floor heating, if you are in the north. The drywall can go on a stud wall directly on the face of the block. Most seem to use steel studs. Hope this answers at least some of your questions.

Answer 2

ICF walls are sized based on loading on the walls. This is not standard.

Thickness, steel reinforcement, grade of concrete, etc. is based on: 1) roof loads, 2) seismic loads, 3) wind loads, 4) soil capacity

1) Long span roof trusses, open beam ceilings, etc. all contribute to wall loads and must be designed into the ICF walls.

2) If you live in a seismically active area, additional reinforcement and thickness of walls are required. (BTW, heavy buildings, like concrete, act poorly in seismic areas, while light buildings, like wood framed homes, act great in seismic areas.)

3) If you live in a “high wind” area, additional reinforcement may be necessary.

4) Regardless of where you live, you’ll provide larger footings because a concrete house weighs more than a wood frame house.

In addition, ICF construction has some inherent issues: additional cost of construction, inability to move walls in the future, additional costs for installation of electrical wiring, etc.

This type of construction is not for the inexperienced. It takes awhile to figure out how to do it...and mistakes are costly to correct.

Here’s a site that helps explain ICF problems: https://homeguides.sfgate.com/problems-insulated-concrete-form-walls-89771.html

So, to answer your question, I’d gather all that info about seismic zone, soil bearing capacity, etc. and take it to a structural engineer (or the LOCAL ICF franchise dealer) to determine wall thickness, size and spacing of rebar, etc.