Is hydronic slab heat appropriate in a super insulated home?
I'm designing our new home in Anchorage, Alaska (building zone 7) to be insulated to R-40 or better ICF walls and an appropriate level of insulation in the roof and slab. I'm taking inspiration from Passive House but not pursuing the official certification. We've already decided on finished concrete floors throughout the home for durability and thermal mass. We are planning to utilize a Sanden air-to-water heat pump for domestic hot water and hydronic heat. I'm inclined to have the hydronic heat installed in the floor but I've heard that it's not necessary and will "overheat the house". One local supplier recommends the Minotair HRV with integrated hydronic heat loop, essentially an improved forced air heating system. My concern is that the slab will be uncomfortable to bare feet (all Alaskans take off their shoes in the home, even at parties) and I would regret not heating the floors. How did you execute in floor heat on the Kenogami house and keep it from overheating?
PS- I don't expect toasty warm floors, just not chilly floors. Thank you for your time!
I would personally not consider building a house on a polished slab on grade in a cold climate without heated floors. Heat moves from warm to cold, your body is 37C, and concrete is highly conductive. all those variables mean your floor will suck the heat right from your feet. The sensation is less pronounced with less conductive flooring materials such as wood, but concrete will always feel colder. It is well explained here –
How to design a home for thermal comfort
And I tell you the above from a personal experience with our last demo house which was the first LEED V4 in Canada and it also met Passive House performance requirements even though it we didn’t seek certification. We used the house as a teaching venue and one day when I had a class and the floor heat was not activated, but the air temperature was warm. People were asking if they could put their shoes on since their feet were cold, despite 8 inches of insulation below and even a 3/8” surface covering of cork.
I know slab on grade Passive Houses often don’t include floor heat, but you wouldn’t catch me building one. At the very least you could consider making it ‘radiant ready’ and running some pex tubes in case for the future. The problem with hydronic heating and the reason it overshoots the temperature is because there is so much latent heat in water that when it shuts off it will still continue to heat the concrete for a while.
As an alternative I would suggest you look at a Legalett slab on grade with air heated floors. There is not as much heat in air so it doesn’t over shoot as much, and the entire system is embedded in the floor so there would be no ‘boiler room effect’ of overheating at the manifold in your mechanical room.
Concrete is highly conductive, and even in a home kept at a typical room temperature it will still suck the heat right from your feet.