Adding 3/4 plywood or cement board to subfloor with staple-up hydronic heating

RMChad

We have PEX tubes running under our 3/4" OSB subfloor. On top of the floor we have tile in places and carpet in other places. We want to put luxury vinyl plank in now everywhere and need to raise the level of the floor where the carpet is by 3/4"

I'm considering 3/4" plywood or 1/4" + 1/2" sheets of cement board (I can't find 3/4" cement board). The cement board has a much lower R value, but would the plywood be suitable?

Dave H_2

That would all depend upon the install from below (plates or no plates), the water temperature the floor is currently running at, is it the main heat, where are you in the country and the heat loss.

All this goes into the calcs in order to properly setup a radiant floor.

If you were to add the extra layer of plywood, of course it adds more r-value to the floor and the result would be to raise the water temperature to compensate for it.

If you are maxed out in temp and no plates are used, then you probably need to add plates. Plates can drop the supply water temp up to 30 degrees.

If its not the main heat for the space, then go for it; its job is to create warm floors

Dave H.

HomerJSmith

Cement board such as Hardy backer have a very low R value as you said. I would go for a smaller plywood thickness and use a 1/2" cement board to keep the supply water temp as low as possible. Using two board would be better than plywood, tho.

I can't tell you how vinyl flooring will fair over a radiant floor. Shrinkage ???

Paul Pollets

How did you snake 3/4" pex within the joist bays? Is there 1 loop per bay? I presume there are no transfer plates? I would be very careful adding any more R value without plates so water temperatures in the loops can keep the floor warm without creating excessive joist bay temps.

PC7060

Go with single layer of backer board.  I doubt 1/4” is worth the trouble of adding another layer.  Remember you will need to apply a layer of thinset under the cement board and fasten it to the subfloor use approved screws. 

Be careful the screws don’t hit one of the tubes or push the heat plates away from the subfloor. 

RMChad

@Paul Pollets, the OSB is 3/4" not the pex. That would be tough to work with! I plan to move forward with cement board to minimize the R value.

RMChad

@PC7060, thank you. That makes sense!

RMChad

Thanks for the comments! I plan to use 5/8" cement board, which I found at a building supply store. I'll cover this with floor leveler as needed to match the height of the existing tile. Then we'll put the vinyl plank on top.

RMChad

New thought -- my floor installer believes that the floor leveling compound may crack as the floor flexes, so we are back to a 1/2" + 1/4" cement board to provide low R and some thermal mass.

PC7060

Be sure to use thinset between all layers otherwise the inevitable gaps will kill the thermal transfer. 

And be careful of the tube!!!

hot_rod

Be mindful of the weight of cement board 

Zman

RMChad said:

New thought -- my floor installer believes that the floor leveling compound may crack as the floor flexes, so we are back to a 1/2" + 1/4" cement board to provide low R and some thermal mass.

I would use the 5/8" and adhere it to the OSB below with thinset. The thinset will take up the 1/8" and provide a squeak free floor.

RMChad

Hi Zman -- that makes the most sense to me. It will provide better thermal transfer and bring the height up to 3/4". Thanks for the idea.