It does sound like there might be a problem with the concrete portion of the room but we would also like to know as much detail as possible on how hvac is laid out. I hate chasing rabbits all over then be told one was hidden beneat a bucket from the start. It appears you arn't looking to jump right on this so I'll only get you started with seeing what can be done and how much effect it will have. The nuts and bolts of remediation can come later,depending how much your enthusiasm wanes after stareing down the barrels of realality. Use quick reading digital thermometer to read A outdoor ambent, B air temp in inclosed space beneath floor C soil temp beneath floor at 2" 6" and 12" below grade (test near center of room and midways between columns,not next to columns nor block walls.) Repeat soil test within 2 inches of 2 or 3 columns near center of room and 2 or 3 locations 2 inches from inside of block wall. Record and average readings for each test set. You can't use a pos device nor curupt readings with breath,body heat,lights and such. The differentials can be much smaller than you are acustomed to in daily life but are meaningful non the less. Try doing this after 3 or more days ( longer the better) of fairly consistant sun.wind,precipt and temps ( day night flux is inevetable as long as there isn't changing weather patterns.) A good thermometer isn't expensive and can be used for many different things for years to come. I try to keep secret the device I used for steak last yesterday was used last week on a boiler system. After you assemble and chart your data, you should have an aah haa moment.

True. There will be heat lost to the ground. Nonetheless, do not conclude there is no point at all in having a layer of insulation: any break is better than no break at all. Think of wearing one layer of light clothing as protection against the cold: better than bare skin. Any small R (e.g. R2, R3) still directs more heat upwards than not. To place radiant heat directly on the slab means more and more heat is lost as floor temperature rises, not just 50% but a far greater fraction. "thin" , "not very" , "useful" -- terms undefined. A separation layer is better than none. Not to overthink this, just to drum it in. "Something" is more than nothing, and this is the significant point, not whether the R factor appears low or high. Once your floor temperature rises, you want to prevent sending most of it into the ground, which is what happens when there is no insulation / isolation / separation at all. Note that the definition of R rating testing does not allow one to distinguish between heat transfer occurring by conduction or by radiation. In many circumstances this is not critical. If you want to get into this level of detail, it will take a few dense paragraphs. Suffice to say that R ratings will be misleading and inaccurate for this application so do not rely on the apparent differences between R numbers. They are all small and that is OK. They are almost meaningless in this situation of a slab-floor separator with a thickness of from 2 to 16 mm. It is obvious that with electric cables you have more room for a thicker insulation layer. HTH -david

I would not be too quick to sell that liquid bonding material short. Back in the early 80's I was working maintenance for a large municipal school department and we an area where we needed to use the tubes of liquid nail, but like you, we were a bit skepticl of how well that stuff works, so we decided to give it a test. I took a 4' piece of 2x4 and cut off about 6", then I used the liquid nail to sister the cutoff piece to the side of the remaining 2x4 on one end and I put it in a vise to clamp it. I left it in the vise overnight, and the next morning I took it out and clamped the short piece in the vise and tried breaking the joint by both lifting an twisting on the longer piece of 2x4. After two of us grabbed the 2x4 and pulled on it as hard as we could causing a twisting force on the joint, the joint finally failed, but when we examined where the two pieces of wood had been joined we discovered the glue had not failed. The wood on the shorter piece of 2x4 had split amd sheared away, which accounts for why the joint finally failed, but the actual wood to wood glue joint was still intact.

As you know, resistance electric heat is much more expensive than heat pump. Almost surely, natural gas-fueled heat is significantly less expensive than heat pump. My experience with high-wall mini split heat pumps (seven of them in a seven-room home) is that they are great ceiling heaters. Your experience might be better with slab. I have a raised home with virtually no insulation under the floors. (Coupling the home to the earth can work better since the temperature is more moderate than air.) In my house, the air temperature gets very polarized when the fan speed slows as the units approach set temperature. Ceiling fans improve the situation. With mini splits, you have no heat during defrost cycles. (I honestly don't miss it, but some people might. Note that sinceI have three outdoor units not all in defrost at the same time.) A bunch of mini splits are going to increase maintenance and probably repair costs compared to a central system. Just think about it, multiple filters to get up on a ladder and clean, multiple blower motors, control boards, vane motors and such that potentially go South and need to be replaced,.... Mine need a lot of laborious cleaning to get the crud out that accumulates on the blower cage, in the coils, in the drip tray,.... You have multiple drain lines that can clog and there is no overflow safety. Putting heat on your slab, as is, will heat your slab and the earth beneath very effectively. The earth is an infinite sink for bleeding off heat or cool. Heat transfer by conduction through the concrete is much more effective than by convection into the living space. You need to maximize heat transfer to the living space and insulate below it. Radiant heat, especially on a slab, is not responsive. It is the least responsive compared to radiators/convectors (middle) or forced air (fastest). You don't just turn up the t-stat and have warm floors a few minutes later. What type of heat do you have now and why are you abandoning it? In other words, how are your floors, and everything else being heated now? What is your climate like?