Do you need radiant heat in a slab on grade if it is well insulated?
If one builds a highly insulated house on an insulated slab are heated floors necessary?
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Whether or not you need radiant heat in a slab on grade foundation floor that is super insulated depends on your climate, the amount of insulation, any floor coverings you put on, and finally how you personally define comfort. See this page that talks in detail about human comfort in homes and how to achieve it –
How to design a home for thermal comfort and quality of life
The things in play are first of all conduction of heat, and how much heat will be conducted either way between your floor and your feet. Concrete is a conductor, so if the floor is warm or cold, you will be very aware of it. Wood does not conduct heat as much, so the effects would be less noticeable. So, if it is polished concrete and not heated, it will always feel a bit cool compared to a wood floor or carpet for example. Many builders of Passive House certified homes don’t include infloor heating as it is ‘not needed’, but occupants do sometimes find that to be uncomfortable, and after it’s too late to do anything about it.
Personally, I would not want a polished concrete floor in a cold climate without some injection of heat at least, but I also don’t *love* hot feet. So - the best solution as far as I’m concerned would be a very well-insulated slab and house so that you don’t need to super-heat the floor to warm the house, but I would also want to include a heat pump that would do most of the air heating in the home.
Heat pumps are the most efficient heating system, so with that combination you would have an efficient source of air heating, your feet would not be too hot or too cold, and for the spring and fall when you may want a bit of heat in the house and not heat the entire floor you have that in the heat pump. Having a backup heat source in case of failure is a nice redundancy as well. At the very least I would suggest you make it ‘radiant ready’ but installing tubes in the floor in case you or a future occupant would at least have the option of installing radiant heat.
Thank you. I know you are a fan of Legalette raft slabs with air heated flrs. Would you be able to give an indication of cost differences with this system over an infloor hot water system with a boiler?
Cost wise, installation of the air heated floors from Legalett (see here) are about the same as hydronic. The main benefits are - its installed by a construction crew rather than needing subcontractors, the heat is balanced a bit better, less latent heat in air than water means when the system shuts down it doesn't overshoot the temperature mark like hydrnonic can, less risk of system damage during install since its only air and not water (an accidental hole in an air tube means nothing but in hydronic its problem), the system is contained in the floor rather than having a manifold on a wall so it saves space and keeps all the heat in the floor. as for boilers, anothe benefit is that you can heat it with a boiler and water coil, or with an electric coil which saves space as the entire system is in the floor accessed by a hatch door. As for efficiency, there is no difference as it takes the same energy to generate heat no matter how you distribute it, be that forced air, baseboard radiators, hydronic or air heated floors.