boxcar, The foam is waterproof, so if you lay it under crushed stone with no place to drain; it won't. However, if you have the patience to fiddle with the crushed stone, grading and packing it firmly to the center or one end and can dig out a drywell, you could lay the foam over it, leaving the top of the drywell open from the foam, then more stone on top, and the water would drain to the drywell. But it would result in a much nicer and easier to keep clean floor, not to mention tracking the stuff, to just overlay the foam with concrete patio blocks or bricks. As long as the crushed stone is well and truly packed, the foam really will support the bricks or blocks, that will lay easily and could be butted tightly to avoid the need to fill between them. You could even broom mortar into any cracks, wet it and when it dried, would have a really solid floor. BTW, just in case you aren't familiar with drywells- They are just holes dug down to below frost line, lined in the old days with mortared brick or stones and now, plastic liners that can be bought for the purpose. Or, you can use a length of fairly wide plastic culvert pipe. You could also use the same heavy plastic channel drain as I did, except that instead of the section with the drain drop leading to a sump pit, it would go directly into a drywell. There are a number of options, but for any drywell, always- the deeper the better, then fill it with large stones to the top of the liner and either set a made for the purpose cap or channel drain drop section over it, or top it off with with 1/2 inch hardward cloth and lay a piece of window screening over that, to prevent anything other than water from seeping into it. I drain my GH with a channel drain that connects to a sump pit, fitted with a sump pump, that empties the pit to a 150 ft. long line, but I fitted out my GH out to serve as a tropical fish hatchery, so often need to dump hundreds of gallons of water at a time, and have a large utility sink out there too, with H & C water, so it's a different GH use and set-up than most. I also laid OSB over the foam, then topped that with 3/4 inch concrete board, but that was because I wanted a glazed quarry tile floor and it requires far more elaborate under pinnings than do patio blocks or bricks. The tile also lines the kneewall, but there too, my needs were different. All you really need for a GH in which you only grow plants, is a means to drain off excess water from the plants and avoid too much dampness that might cause fungus on them as well as the interior walls of the GH, although the ability to hose down a concrete block or brick floor with cold water is a nice extra, for keeping the GH clean and help cool it on hot days. It really all depends on HOW you want to use your GH, and WHEN in relation to your climate, and how much time, effort and money you are willing/able to spend to fit it out for your own maximum use and enjoyment. After all, a garden GH is really only for pleasure, because unless you are feeding an army of kids, for what it costs to build and maintain one, it's cheaper to shop for veggies at Whole Foods and buy started plants at any nursery. So by all means, put a lot of thought and effort into your foundation and if, after you have the GH you need to back off on the fittings until your wallet recoups; do so and in the end, you will have what you really want. We spent an entire season preparing the foundation for my GH and by the time we actually got to erecting it, all we had time for before winter set in was to install the heater, but as anxious as I was to use it, patience paid off. (Of course, we are also a pair of arthritic old poops, so everything takes us longer now.)

You didn't mention the recent day and night time air temperatures so its not possible to know the effect on the concrete but if the day time temperature during the pour was above freezing there should be no problem because the concrete was warm from the mixer, the snow and the forms will help to insulate the concrete, the chemical accelerators will greatly shorten the time needed to reach adequate strength, and the heat of hydration from the on-going chemical reaction between cement and water will continue to generate heat. I'm surprised anyone would be using board forms but it will provide a great old time wall finish if there is an unfinished basement.

Concrete can certainly be poured in cold weather! If it couldn't, most concrete construction would come to a screeching halt in winter and that just does not happen. In fact, it is more desirable to pour in colder as opposed to hotter weather as the curing process slows in cooler weather and the concrete develops better strength. The concrete industry has done extensive studies on cold weather curing and there are various methods and materials that should be utilized to that end. Most concrete contractors in cold winter climates are very familiar with these additives and processes. Just make sure your guys do. btw, cement is a just an ingredient in concrete and is a very fine powder......your slab will be a concrete slab, not a cement one :-)) Cold weather concreting.

It may be helpful to differentiate the components of a cast-in-place concrete foundation: A concrete footing and a cast-in-place concrete foundation wall. Typically, the foundation wall is constructed on a previously constructed concrete footing. It is important that the footing not be placed on frozen ground or risk shifting and settling afterwards. I suspect the builder is referring to chemical additives which can be added to the concrete during formulation to lower the freezing point. That affects the concrete, but it doesn't do anything with respect to frozen ground.