Foundation Design Difficulty

I’m struggling to use @medeek foundation to model the design I want:

Layout File:
Foundation Cross Section.layout (526.6 KB)

I’m wanting a Frost Protected Shallow Foundation (FPSF).

  • Basically a Stem Wall foundation that extends above grade to roughly 30", with continuous insulation including the footing on the exterior.
  • INSIDE that will be a slab without footing (piggybacking on the stem wall footing, actually), insulated below with appropriate fill below the insulation.
  • Since the crawl space will have both the water supply coming up from a buried (below frost level) lateral - and a holding tank for greywater (once I figure out a way to pump it out!), I plan on conditioning the crawl space to keep it above 40 F.

NOT shown on the picture or layout file is what’s happening above this! But just FYI:

  • Sill Plate
  • Floor and Rim Joists - 2 x ??? (probably 8 to span 12’ cabin width) - with a SMALL amount of insulation. I don’t mind if the main cabin heat “leaks” into the crawl space, but I still intend to have the crawl space as it’s own “zone” at 40 F!
  • Subfloor
  • Etc
  • 2x4 stick built walls w/ sheathing/insulation on outside to match with insulation on exterior of foundation

My questions are simple:

  • How do I model this? I prefer to use @Medeek Foundation, but aren’t wedded to it.
  • What do you think of this approach?

I do plan to somehow make this an airtight design (overall), Heated/Cooled via Mini-Split (main area), with Energy Recovery Ventilator (ERV) for the cabin and a couple of through floor vents to keep the air in the crawlspace from going stale! Perhaps the floor vents will be enough to heat the crawl space - at least to the low extent I want it heated!

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The problem I see with this stemwall/footing combo is that the stemwall is not centered over the footing. In all my experience I usually have seen the stemwall centered or mostly centered over any footing it is resting on. The reason for this is that you don’t want there to be an overturning moment on your footing.

Hi Nathaniel (aka @medeek)!

Ha! That may be the LEAST of the problems that comes up!

I’ve got no problems with centering the stem wall on the footing - perhaps making the footing 12" Wide by 6" (or so) deep. I’d ADD 6" to the stemwall height to get at least 1’ of the exterior insulation below grade - and in the process picking up another 6" of height in the crawl space.

From what I’ve Googled, while the frost depth where I’ll be building is “officially” 24" (county wide number), it’s actually less where I’ll be building, more like around 12". And given that FPSF designs (also per Google research) don’t have the foundation reach even TO the frost line, I think 12" of insulation below grade is a bit of over-engineering - but I don’t mind a bit of over-engineering for a building that will have a (roughly) 60’ perimeter - it’s just not that much extra material. I don’t (yet) have a drainage plan, but the site is nearly level - so I’ll cross that bridge and plan for French Drains (or not!) later!

My worries are more about:

  • How to model it - preferably with your Foundation extension
  • Whether the approach shown should work (in principle) for a minimally conditioned (the crawl space) area below the main (and only) fully conditioned storey. If yes, I can start deciding exactly how think the various insulation layers will be - and how to make the crawl space (including, possibly, the slab) can be made within the air tight envelope!

See why I’m searching for a CA Certified engineer for collaborative design?

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Yes, that struck me. The footing is usually twice as wide as the wall thickness and as at least as thick as the wall thickness. Depending on you’re Seismic Design Category, you may need certain rebar. Shallow insulated footing not only have insulation going down, there’s an outward leg too. In the IRC it looks like it’s covered in R403.3 Frost-protected shallow foundations.

Only where Air Freezing Degree Days (AFDD) are over 2,000. Otherwise the “outward leg” isn’t required. See this. Look at the California listings - the site isn’t in Nevada or Sierra Counties, so the AFDD is 1,500 or less.

But I’m just an interested and motivated amateur! I really do need to find a California Engineer willing to work with me on a design, not only for the foundation, but for other systems in the house - Air Quality and HVAC with a bit of structural thrown in.

I should probably also mention that the site is in one of the few areas of California in Seismic Hazard Zone 0! Even with that, I don’t mind adding rebar and actually anticipate a few pieces - but didn’t bother to show that in my sketch.

@medeek As I think about this a bit more, the slab portion of my proposed design won’t actually be bearing any weight! It’s there because I’ve done a LOT of crawl space work when I did communications wiring professionally in the 80’s and 90’s - and I want a finished surface so I can put a car creeper in it and maneuver around it easily.

Thus I’m thinking that trying to do the entire foundation using Medeek Foundation is overkill. Yes, use your extension for the stem wall, but just do a quick and dirty 3" thick box on the inside of the stemwall, resting on the stemwall footing at the edges - and just forego the bells and whistles on the model (text labels and detailed assignment to tags) for the crawl space floor (which probably shouldn’t even be called a slab!)

Your thoughts?