I don't know why your duro rep said not to use a floating floor over radiant heat. Their own specifications (available here: Duro Design Specifications) say you can !?.
Anyway, I've installed cork floors over radiant heat both as floating and tile several times with no problems.
It is a common installation- you just need to follow certain steps to assure a good install. In brief: proper acclimation of the material, having the heat on "medium" (max surface temp. of 85F) 5 days before and during installation, a vapour barrier under the floor, and adequate expansion space at the perimeter, doorways, etc..
Definitely get an installer who has experience laying over radiant heat floors!
As for your question about the insulation value of cork blocking the heat from a radiant heating system. I get this question often, and the answer briefly is that cork is entirely compatible with a radiant heat floor. Here is the answer and rationale in detail, explained in point form:
1. A radiant heating system is not a quick-response system- i.e. it relies on thermal mass and heat radiation. That's part of its efficiency and why it is comfortable- the floor constantly radiates warmth upwards.
2. The insulative value of a material refers to how well it slows down the transmission of heat. The heat doesn't dissappear or get absorbed by the material- it just takes longer the flow through. So in practice, floor coverings do not affect the overall operation of a radiant heat floor because the heat will eventually get through. Remember- a radiant heat floor is not a quick response system- you don't just walk into a room and crank up the thermostat. (Of course don't do something silly like install a radiant floor over a drafty, cold basement, because you'd lose all the heat out the basement first)
3. Yes the insulation value per inch of cork is higher than that of wood but a cork tile is only 3/16" thick. I did the math a long time ago and the total R value of 3/16" of cork is almost exactly the same as 3/4" of a typical wood. So putting down cork tile is no worse than putting down 3/4" of wood.
4. That being said, a floating cork floor will have a bigger R value than a cork tile floor, because it is on a 5/16" mdf core, and thus will slow the heat transmission a bit more. I don't tend to use a floating floor, unless specified or the price differential is less than the added bit of surface preparation to bring a floor up to cork tile standards.
5. Finally, a site-finished cork tile floor will not gap visibly due to expansion/contraction of the radiant heat floor. Cork is a truly resillient material with excellent rebounding properties (95% recovery after 50% compression!) After all, cork itself is commonly used for control joints and gaskets.
Lengthy answer, but I'm hoping if the issue comes up again, we can just link to this post.
I hope I covered everything- If I've missed something, I'd be glad to follow up.
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Sawtooth design, big windows
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