Electric floor warming systems are for floor warming only. They do not heat the room. If your slab is uninsulated, then in your cold environment your floors won't even really get warm as the heat will be sucked out through that slab into the outdoors. It'll be a waste of money. I'm really surprised that you actually have slab construction in such a cold climate. Slab requires footings to be dug down below the frost level so it won't heave, and in your area, that's down low enough that it's not that much further to do a basement. And to do a slab without insulating it is also strange. I would not put hard surface flooring in such a situation where you cannot install the sleepers and additional insulation. Your best bet for comfortable flooring will be for carpet. The padding will give a small amount of insulation between you and that freezing cold slab.

Thanks, franztini, for taking time to share your thoughts. I agree with you- Warmboard is great for new construction, it would be quick solution. In your situation, you probably already have some insulation in the slab from your previous radiant heat. My slab has no insulation at all. It is desired that insulation over slab has R=19. This would require 4" thick insulation. I can't raise floor that much and would be ok with R=10 (maybe even lower). Roth panel seems as a great option for slab since it is made of insulation. Also, aluminum on top may heat floor faster. Though, for some reason, I have tough time to find contractors in NJ that have experience with Roth panels. Also, comments related to Roth panels I saw on web seem to be older. I was wondering if there were any issues with RP. The drawbacks - it may be time consuming to install (especially for hardwood flooring) and R value is only 4.5 and no additional insulation can be added underneath. Though, I would still consider if people have good experience with it. Also, there could be some additional tricks to improve insulation such as insulation on perimeter (don't know much about it). I looked at HeatPly. Because it is made of plywood, I would need additional insulation. Also, you may note it doesn't have the benefit of having aluminum on top to heat floor faster. Though it is great that it is provided with boiler and tubing. If you don't need insulation, it may work well for you. I found out that in order to insulate under Warmboard, I would need to use sleepers (rigid insulation will not work). While this will be the right way to provide insulation, this will require raising the floor level. I am taking time to make the right decision but have really hard time with it and the cold weather is around the corner.

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.