I am soon to pour the concrete slab in my new 42x56 ft shop. I will install pex tubing in the slab and insulate under and on the perimeter of the slab but my question is this: Does anyone have knowledge of data about leaving an uninsulated “hole” near the middle of the slab so as to be connected to the thermal mass of the earth at that place? The soil under the slab will be fairly dry, and I’m looking for data that will help me decide to fully insulate or leave a hole that is ~10% of the total area of the slab I’m in sunny northeast Wyoming. The shop will be totally off grid and fully insulated. PV array/batteries/inverter supply the AC power. Solar water heating collectors of the design published on the Builditsolar website will provide the heat for the floor.

I don’t have data to precisely quantify the heat loss, however, it will be relatively small. The majority of heat loss through a slab is around the perimeter edges and from the area a couple feet inward of the perimeter edges. This is why slab heat loss is usually calculated based on the total slab exterior length and not on the total slab area. Given that your lack of insulation will be in the middle of the slab and is only 10% of the area of the total slab, I would speculate that your heat loss in this uninsulated area will only be something less than 3% more than what it would be if it were insulated. Sounds like a nice project and Builditsolar is a great site. What is your motivation for not insulating this section of slab?

Sail away...
My motivation goes like this...
I am only trying to keep this shop above freezing, which should be pretty easy to do given all the insulation and insolation. So, since the slab will be at a relatively low temp(40-50F..?) I thought that there could be some stasis provided if the concrete had intimate contact with the ground which might have a temp that is about the same...in short, more thermal mass. I’m happy to see the numbers you sent.
Thanks for your reply!

R-value of soil

https://inspectapedia.com/insulation/R-Value_of_Soil.php

Based on my reading I would not leave any uninsulated places in the middle...….. Some time and diligent googling will get results The temp takes a few feet or more to stabilize as the ground will freeze solid like concrete up to 4 feet deep in places...…..I just read a decent article on it recently....the general concensus was no as that is considered old thinking and has been proven highly variable. Insulating the entire thing gives a known constant, rather than a wild card, unless you have temp and moisture readings on file for a uninsulated slab thru the cold parts of the year.