The crawl space under the slab is surrounded on all 4 sides with 4" block, except for 2 vent holes and 1 2x3 opening. Also there is only dirt on the floor. This I don't understand. "You might get 1/2 way back to the crawlspace temperature with insulation." The top of the slab is already at least 25 degrees above the crawl space temp. "I'd glue 2 or 3 inches of XPS but it will not get you as warm as the crawl space area." When the outside temp is 40 for instance the crawl space under the slab is about the same temp. So if I add the XPS or insulation to the bottom side I should be able to keep the slab itself a few degrees warmer than the crawl space under it, do you think the underside of the slab is colder than the crawl space? The slab is pretty smooth on the bottom, I have no idea how they built it, but the bottem side looks as smooth as the top of any slab. I can buy DIY spray for about $700 that would give me 2 inches of R7.4 per inch. I haven't called a local company yet. The piers would still be ground temp I guess. As far as the air idea I was conceding making a couple inches air tight under the slab and hooking the heat pump into one end with a 3" duct and exiting the other end back to the return air. Its all under the house.

i'd like to add a few points. A slab is a thermal mass bigger than an elephant. Concrete is a heat sink. You do get all the heat back, if you own both sides of the slab. In that case a concrete slab is a heat source as well. A governor. Here you don't have that. A slab on grade is connected to mother earth. The more it acquires heat (from any source above), the more it loses heat to the ground underneath. The more you try to heat it, the more you spend on heating the earth underneath. That is a lot of mass. This explains why seeds germinate next to rocks. The rocks acquire heat from the sun and re-radiate that heat into the ground, triggering germination. Your slab on grade might be on earth that is generally dry and warm. Or not. The more moisture in the earth, the more slab heat is lost to the earth. More quickly. It may be easy to have a warm floor on a warm slab on grade. Do nothing more than install wood on an underlayment. It's difficult to turn a cold wet slab on grade into a warm floor. A layer of really good insulation is required. Now, you are walking on a raised floor. Since you asked about the new and thin electric heating cables, I'll guess that you can't raise the floor by much.

What you're proposing is certainly feasible using your existing boiler and some sort of 3 or 4 way mixing valve or assembly. Your floor would have to be isolated from the existing slab and you would not benefit from a thermal mass. However, you may have faster reaction times. Keep in mind that pipe spacing is related to water temperature so make sure of your design before starting and ordering materials. Do consult with a radiant in-floor specialist that understands hydronic boilers as well - you don't want surprises! SR

The short story is we chose radiant because it worked the best with the floorplan / material choices we made. The long story is how we got there. We had the choice of a basement, crawl space, or slab. Basements are common in my area because of the required 3' footers - it doesn't cost much to go deeper and have a basement. Financially they make sense, but I don't care for them. They are cold, dark, and low ceilings feel oppressive. Crawl spaces are nice to allow access, but the floors were crazy cold in winter and the amount of humidity could also turn into a mold problem. We chose a slab foundation before we had land, and it worked out since the land we built on would not have supported a basement. Our slab has xps foam board under and along the edges for insulation. Our floor plan is completely custom that I did in autocad myself. All the common areas are on the south side and open. The bedrooms and bathrooms are on the north side. With an open floor plan (some of it 17' tall rooms) and concrete floors it makes forced air ducting more difficult. My experience with forced air is that it blows dust and curtains, feels like a cold draft across your neck when it first kicks on and forces the cold air out before the heated air, and is noisy enough to have to raise the TV or conversation volume when its on. Choosing radiant floor heat with separate zones for the common and private areas seemed like the perfect choice for heating. But we do use our air conditioner for 6-8 weeks and the floorplan layout was critical in being able to use a wall unit to comfortably cool the whole house. I was planning on finishing the concrete with coloring and waxing of some kind, but my husband became friends with a person who polishes commercial concrete floors and we had him polish our floors. It looks wonderful, and is easy to clean. The radiant heat is quiet and even heating with about a 2 hour lag time. I also thought it would be warm on my toes, but I learned that it's not - even in the middle of winter, and want to make sure others are aware of that so they are not disappointed. It's not cold, but its not warm either. Our upstairs is a quality pad and carpeted which feels nicer to my bare feet than the cool concrete. Each house and owner is different so I hope you find the best solution for your situation.