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My city said that for my 2-1/2 story home, the foundation needs a 12 inch wide and 3 feet deep foundation with 3500 psi concrete poured within the mud wall (i.e., no need of foam board). I am planning to make the trench 16 inches wide and on the sides of the trench attach a 3ft high insulation board 1.5 inch thick. This board will act as vapor barrier to the concrete stem wall from both sides and insulate as well. There is no crawl space.

My question is, are there any issues with pouring 3500 psi concrete between the board into the trench?

Generally, I have seen people make the foundation and then stick board to the stem wall. I am hesitating doing the latter, as it will need a wider trench, which is very difficult without excavators.

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  • 12" wide for the whole 3 feet? This seems odd, normally people build shallow foundation walls with footings. In my area on good soil the typically specification is 24" wide footing that is 8" tall and you build your 8" thick concrete wall on top of that. 3500 psi is pretty weak - we typically go 4000 which allows the pump truck to add some water and weaken it. doing a volume calc your setup results in 12.5% more concrete - not a huge difference certainly your formwork is more simple if you just trench form it. sounds like slab on grade why do you want to insulate the concrete foundation? Apr 2, 2022 at 0:57
  • thanks. I was told that footing is not required, if I make a narrow trench of 12 inch and dig to the depth of 3 ft. In that case pouring the concrete will make the work much easier (told by the town engineer). This is the reason I thought of adding ICF as recommended. I will use 4000 PSI (although they mentioned 3000 psi is enough). Any comments please let me know. It is critical in the entire project.
    – marc Katz
    Apr 2, 2022 at 12:21

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You can purchase "insulating concrete forms" made of foam that are designed and reinforced to withstand concrete pressure (if not damaged while being installed - there are some horror stories of "blowouts" while filling these in above-grade applications. Less of an issue in a trench, but don't damage them anyway.)

Insulating concrete form image from cement.org

I agree with FreeMan's comment regarding the odds of your trench wall being flat enough that you don't just crack loose foamboard to bits.

Obligatory note to use reinforcing steel (it's cheap compared to the whole cost of the concrete job, and makes the concrete so much more durable/strong) and to provide a connection to the steel for an "Ufer" ground (concrete encased grounding electrode) near where your electrical panel will be located. It's one of the best grounding electrodes you can have, and costs practically nothing if you remember to install it when pouring concrete. The ties that hold the ICFs together have convenient notches to hold reinforcing steel in place.

As for doing this "without excavators" - you can certainly cancel your gym membership until the job is over. They can be rented (get some training and be very, very careful) or hired (with operator) and will make the job very much faster and easier - and safer. Can be done with Picks & Shovels but it is a LOT of work. You might need to ease into it gradually or you may get a lot of blisters that will slow you down (gloves advisable at first for that reason.) Realize that a 3 foot deep narrow trench can collapse, depending on your soils, and that may trap you by burying your legs. A bit deeper and it can kill you by preventing you from breathing. You'll definitely need to keep children out of the trench, as it's plenty deep to kill them in a collapse.

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  • Excellent recommendation. I note that in scanning the linked article, there is no mention of the foam acting as a vapor barrier, so water proofing still needs to be applied to the outside of the foam blocks once the pour is complete.
    – FreeMan
    Apr 1, 2022 at 15:18
  • Usually the cost of a machine rental will pay for itself in the time savings, nicer trench, and your back will be happy. By hand at least a day or more, machine probably an hour or two or less.
    – crip659
    Apr 1, 2022 at 17:15
  • My plan was to make a narrow trench. Waterproofing will require me to make a bigger trench right? or may be I add waterproofing to the ICF before adding ICF in the trench. would that be fine?
    – marc Katz
    Apr 2, 2022 at 12:23
  • As recommended, I am planning to use ICF and pour concrete along with UFER ground. My town mentioned for this addition of 18 X 30 home, there is no need for Rebar. I understand rebar is always good. or can I just add vertical steel rebar at 48 inch spacing and hold it vertically using concrete block at the bottom and then pour concrete?
    – marc Katz
    Apr 2, 2022 at 12:25
  • Vertical rebar is doing very little, if anything, for the strength of your wall. Horizontal (or X-pattern to be fancy.) Bend a 20 foot bar at each corner (so it bends around the corner, and there isn't a joint right there) overlap 2 feet and tie together with the next bar. If minimal, lower part of the wall, but at least 3" above the bottom. You might also inquire if you can reduce the size of the concrete if it's properly reinforced, which could save some serious money as of the last price of concrete I saw.
    – Ecnerwal
    Apr 2, 2022 at 20:32

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