I have a customer that plans to install Uponor Joist trak radiant heat at some point after we complete his oak flooring for him.The system is designed to be installed from under neath existing plywood subfloors. I let him know that rift and quartered is always recommended when dealing with radiant heat. He feels that the joist trak system wont achieve temperatures that will cause problems . What do you think from your experiences ?

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I dont have any experience with this system. But I would ask myself ...What does he base this statement on? Sounds like something he wants to believe. Protect yourself, dont let him rope you into doing something you'll regret , your the one that will be stuck holding the ball.

Yup. R&Q is the only way to go. Try for 2 1/4 inch as well, will give the minimum amount of winter shrinkage. It doesn't matter much what type of radiant heat system is installed, they all operate under the same principle of heating up the floor system, which in turn radiates into the air space.