Unless you put some insulation between the radiation that responds to the first floor and the basement space, every time the first floor zone is on it'll be heating the basement. In most installations MORE heat would end up in the basement due to the insulating effects of finish flooring, carpets, furniture, etc.

In a fully conditioned basement a radiant-barrier ~1" below the floor and R11-R13 batting should suffice. (NRT.Rob will probably recommend more. :-) ) R-19 batts/no-radiant barrier would perform similarly. R10 of iso board or XPS would be fine too. But it DOES need something or the basement will likely become essentially an uncontrolled somewhat overheated zone.

BTW: Builders SHOULD use insulation on the forced hot air ducts, even within conditioned space, at least on the outbound side. R6 is a mandatory minimum in some locations. But they should also seal all the joints with mastic and pressure test 'em for leakage too, but only in CA is it mandated by law.

hello all,
Well, very informative as usual. I did not remember to check the State to state regulations yet.

Thanks for the reminder that they can be different building codes and regs in each place you wish to build or renovate.

I hope to have a home with radiant floor heating soon.  It has been a very cold winter for much of North America and it would sure feel good.


When you need answers and need them fast.....flooring and carpet

We also have un-insulated radiant tubing under the main floor with the unfinished basement below it, but used as a guest bedroom / kids playroom with a family room / kids playroom. I haven’t been heating the cement floor, but since the radiant from the floor above is not insulated it is warmer down there. A question I have is if we insulate under the tubing will more heat actually go up? Or does it really just limit the heating of the space below (because we want it to do that). I do understand the control side of this, if we did heat the basement floor warm as well I am sure it would get warmer down there than the main floor.

Yes and no. The amount of heat that moves from one region (the hot water tube) to another region (the room above or below) depends on the temperature differential and thermal resistance in the path. If you add insulation under the tube and keep the tube temperature the same as it is now, less heat will move into the space below and the same amount will move into the space above. What you will find is you input less heat into the water at the heat source thus cutting down the total heat load. If you continue to inject the same amount of heat into the water as you do now, the insulation below the tube will cause the tube temp to go up. That will drive more heat into the space above. Your control system regulates the temperature of the water in the tube, right? Does this help?

I am circulating the water from the pool through the tubes, it is at 89F so that stays the same no matter how much I am pulling out. I do notice if I run the pump in low speed the water leaving the tubes will be 85F with the pump in high speed the water leaving will be closer to 87F. So I am guessing in my case I would just get a higher return temp on the tubes if I insulated under them and a cooler basement. I do plan to insulate it before the ceiling goes in.