Installing a radiant floor under wood works fine, but here are some considerations:
The best flooring materials with a radiant floor:
Inert materials such as ceramic, stone or concrete are the best from the point of view health and effectiveness, they have the quickest heat transfer and they don’t contract and expand with changes in temperatures. Installing a wooden floor is not a problem, but as you suspected, you will need to be careful about product selection.
Radiant floor performance with wood:
There will be a small difference in heat delivery with a wood floor, wood is about R1 per inch, so it will slow the heat transfer slightly. You would notice this most if the floor is dead cold and you activate it, as it will take a little longer to get up to temperature than it would if it were masonry or concrete. But if you keep your house at a balanced temperature you wouldn't experience any discomfort.
And since the heat is inside the insulated building envelope, it will eventually find its way to the air. So don’t concern yourself worrying about a lack of efficiency as some installers suggest, the heat will get there eventually. If you’re not a big fan of really ‘hot’ floors, wood will actually add to your comfort as the heat transfer is not as noticeable.
For your above grade floors, some heat will be lost downwards, but this is not a problem as it will simply be adding heat to the room below. What is important, is to have sufficient insulation below the basement slab floor. The greater the temperature differential between the floor and the ground below, the greater the rate of heat loss will be. Therefore, heating a floor is not a reason not to insulate it, that is a reason to insulate it even more.
While Building Code will let you get away with less, we would not recommend heating a slab on grade in any Canadian climate with any less than four inches of foam insulation. Six inches and sometimes more, depending on your specific climate, would save you energy and money in the long run.
As for damage to the wood with radiant heat:
It is important to first check with the manufacturer of the flooring you are considering to find out if any warranty is still valid when installed over a radiant heating system. The floor must be able to expand and contract without cracking due to changes in relative humidity.
Some hardwood species perform reasonably well in this capacity, but we would recommend that you first consider engineered hardwood for added durability. Some bamboo or cork flooring materials are also suitable. Be conscious of the chemical make-up of the finish, heating wood with VOCs increases the rate of off-gassing. To sum all that up, our choice for wood on top of a radiant heat source would be an FSC-certified, pre-finished engineered wood with low or zero VOCs.
As I read this I am curious as to the heating load loss on your cement pads thermal mass by putting this product down
Hi Lee,
I think you are asking if the cork insulates it and reduces the heat transfer, right? It would in a minor way, but the insulation value of 3/16ths of an inch of cork is next to nothing compared to the 8 inches of insulation underneath (R32), so it really wasn't a concern for us.
How does the cork material hold up to UV rays if located in the south facing room areas?
We have it over the whole floor including in front of the south facade and after about 5 years It's doing fine. There isn't really any noticeable fading any more than you'd expect from wood floors etc.
Mike, are you still glad you put radiant heat in the floors, given that you have a cork layer to keep your feet warm? If I do cork over the slab, should I still use radiant heat as well as a mini split? It's very hard to build a house with few, and simple, systems! Also, I'd love to see a lessons-learned, or what-to-do-differently video on the Edelweisse house!
Hi Ann,
Yes, very glad. Even with 8 inches of insulation and a layer of cork, it wasn't super warm when we operated it with only the heat pump. Radiant heating is a very comfortable heat source compared to forced air. Since you are walking on it all the time, and particularly with polished concrete floors that have no additional covering, not having heat will always be noticeable since your body will (almost) always be warmer than your house, and so the floor will actually be sucking the heat from you feet anytime you're in contact with it. A lot of Passive House projects I've seen start with a well insulated slab on grade but omit floor heating based only on heat load calculations and not wanting to invest in the seemingly high cost of a heating system. Saving money on infrastruture is always great, but comfort in your home is great too! Here is an article that covers that, so have a read ahead of building just to be sure - How to design a home for thermal comfort
Also, our latest demo house has a solar air heated slab on grade floor, so it is very efficient but also comfortable.
Is your concrete slab part of the thermal mass of the building? If so, does the cork has a negative impact on the performance of the thermal mass or with only 1/8 it is negligeable? Do you recommend other material to cover the concrete when we want to use the slab as a thermal mass?
thank you!