Hi everyone! New to the forums, I'm an Army vertical construction engineer, but for my purposes here, its more as the Homeowner/DIY.

So, recently I took the plunge and bought an "Endless Pool" 8'x15' swim current above ground pool. My wife and I have opted to build an addition onto our garage to house it (heated space for year round use).

I'd like to build a 12x20 shed (monothlic/floating slab on grade w/ thickened edge) with the most energy efficient construction that reasonably fits the budget. I won't easily shy away from a challenge if needed. At the moment, I am gathering and doing some Google research on my best options for the building envelope (I live in Utah).

Some questions I have:
- Under slab insulation: Will just buying 2" 4x8 XPS sheets at a big box store be sufficient on top of the vapor barrier? I tend to think I could go much thicker (especially seeing posted videos of others), but at such a low quantity for only a 12x20 building, I'm not really sure where I go locally to get an order for a project that small (the only place I know of, needs an order by the semi-truckload). Would it be easier perhaps to just stack (2) - 2" pieces together in an alternating seam pattern to achieve an "R-20"ish rating. Is that possible? Should I go even thicker than that?

- Having the slab somewhat warm is important since the pool water makes contact with the slab with little more than a thin pool liner between them.

- Double stud/Staggered Stud cellulose filled walls for some thermal bridge limiting, or just go with 2x6 frame stuff with rigid foam board, or something else perhaps? I can easily do blown-in for the attic.

- Just FYI, I intend to put fiber cement on the outside to roughly match the rest of the house. Whether the walls are cellulose or foam filled, I can easily add 1.5" or 2" sheets of XPS to the exterior, prior to finishing with the fiber cement siding.

- The pool is heated by a natural gas heater

Sorry if all that seems a bit garbled together, I'm still trying to wrap my mind around shifting gears from building things the Army way (for blast protection) to move instead towards saving energy and keeping the heating/cooling costs down.

Thanks for reading,
-Jeff

Roughly, you flatten the area, lay down the foam, add sand or gravel to the middle area (so that the concrete is thicker at the edges), and pour on top of this. The foam has to extend beyond the slab (wings) for this frost protected shallow foundation design. Is humidity (or water) an issue with this room? If so, I'd avoid wood and consider SCIPs or SIPs.

That makes sense. The pool cover keeps the humidity done to almost nothing, and a simple dehumidifier during use I am told by the manufacturer is almost always sufficient for indoor use (it isn't a hot tub, just 8-12 degrees warmer than room temp). I guess my next question would be, while I will have foam underneath, what would be the best way to limit thermal bridging from the edges of the slab and the exterior walls, I'm having trouble seeing you that would all match up.

Exterior foam needs to cover the slab edge. It can bump outwards, since logically, you want thicker exterior foam over the slab edge than the wall (that has other insulation in it).

Right, yeah I came across this site from the DOE

http://www.ornl.gov/sci/buildingsfoundations/handbook/section4-2.shtml

Figure 4-9 is what I would be doing, but my concern here is that by leaving the "footer" or thickened edge portion open on the bottom (no foam) as depicted here, am I still going to loss a lot of heat during the winter in my slab (thereby sucking the heat out of the above ground pool)?? This doesn't really allow for a thermal break this way. The FPSF process saves a substantial amount in excavation and labor, but I don't want to turn around and lose it all in the heating bill. I just wonder how well this actually performs, or if I am just worried about something insignificant.

You CAN put foam under the footings; it is being done on many Passive House buildings. you simply have to buy foam that is rated for the load. GO-Logic shows this on one of the videos on their website and Branch River plastics makes such a product. You can probably find references to this on the website for Building Science.com

Don't use the ORNL design. Keep looking and you will find some correct ones.

Soil often supports only 15-20 psi and it's easy to find foam that exceeds that (at which point you are fine).

Jeff,
Since you said you are up to a challenge! Perlite! & a wood foundation! Perlite is expanded rock, a good insulator at r3/" and quite economical at about $1.25/ cubic foot. Perlite in bags under the slab is "greener" and more economical than plastic foam and doesn't have creep and strength issues. The challenge will be finding a local to you source. Then there is the wood foundation for the building around the slab. Pressure treated wood (wood basement grade) can give you a well insulated foundation for the walls and roof with Roxul bat insulation.
One other comment, If you do double stud walls use aligned framing. The small reduction in heat loss, usually r 1 or less, is offset by a substantially greater moisture/condensation concern.

Jeff - you mentioned that this is an addition to an existing garage. What are the foundation and structure of the existing part?

I will be putting the 12x20 addition on the south facing side of a 25x25 garage (I'm doing some solar heating on the roof). I'm not entirely sure if the front is a floating footer/thickened edge or what, but I do know that the back (where the addition goes) is a poured concrete wall down to the 30" frost line (the rear of the house has a walk out basement) and the original garage sits on that same stem wall system. Because of this, the roof of the addition will just 13' mono trusses (lean-to roof) as even at 12' high, on the high side against the existing garage, it is still a about 4 feet below edge of the roof line.

I hope that made sense.

I'm good with an excavator, and really good a laying block, so I'm half tempted to stick with what I am used to and just go down to the frost line. I'd isolate the footer with one 2x4 wall to a then thermally disconnected floating slab on 4" of foam with the 2nd/double studded wall resting on it. R20 on the floor, R40ish wall, R50 ceiling....along with some descent thermal breaks. I even have half a mind to do radiant heating on the slab, with a solar system (which might actually be enough to space heat and heat the pool water at the same time).

I've certainly been thinking about all the options....it'll be interesting to see what I end up doing, its bound to change at least another time or too, haha.

Jeff - if you try to put a slab foundation next to a stem wall foundation and tie the walls and roof together you will have problems. The slab is designed to float under frost conditions and the existing building won't. The simplest is to do at least a ICF stem wall to match the existing concrete.Should work out close to cinder block but lead to a much warmer floor.

Just to be clear, a frost protected slab doesn't float under frost conditions, it completely avoids the ground freezing beneath it.