I agree with what everyone else said. Skip the plywood. Cork even transfers heat pretty well...for an insulator that is...over radiant floor heat. If you are floating the floor still use the 1/8 foam between the flooring and the cork.
Is it possible to install a cork floor over a radiant heating system?
Cork is a natural insulator and will affect slightly the transmission of heat. However, many installations of this type have been performed and without encountering any particular problem. Your floor may take longer to warm up but the heat will be retained more efficiently.
Only Natural Cork Floating Floor should be installed over radiant heating systems. Floating floors move as a unit since they are not attached to the sub-floor. That means visible signs of expansion/contraction take place at the perimeter only. Furthermore, adhesive used for installation of the tile could lose strength when exposed to high temperatures.
Certain precautions should be taken to minimize the effects of contraction and expansion:
Prior to installation over radiant heat, the slab needs to be heated and the temperature maintained at 75 degrees during installation.
As with any cork or wood floor installation, material must be acclimated in the room for 72 hours prior to installation.
All good info. The cork I was referring to is the underlayment cork. It is not flooring quality in that in is more pourous than the other. It allows for pretty good heat transfer.
Actually, I think tile does much better than wood and usually encourage folks not to do wood, of any kind over radiant heat. If the customer absolutely has to, then only vertical grain (quartersawn) or engineered. Some engineered flooring is not rated for radiant heat. The less layers is better cause every layer is an insulator and actually cause the heating system to overwork.
I dislike floating a real hardwood floor unless the planks are at least 5 inches wide. too much movement in my opinion.
When I do float I use Hushstep which is a rubber underlayment rather then foam or cork. It's cheaper then cork and thicker then the foam. It doesn't sound so hollow either. The Hushstep is only like 40 dollars per 100 ft roll. It's a 1/4" thick and also acts as a moisture barrier.
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