Why is solid still being offered as a viable option over radiant? While I understand that some installations of solid over radiant perform fine, many don't even when "done right".
There is nothing more frustrating than trying to explain to a customer why they shouldn't do it, only to have them pull out a NOFMA sheet that says "wood flooring and radiant heating can perform very well together".
Why can't they just say that it's a bad idea, unless evaluated to be suitable by a qualified professional?

Some of those NOFMA recommendations were written a long time ago and are not considered applicable for todays homes. Many manufacturers of radiant panel subfloor systems don't recommend solid over their systems subfloors as well. If people would take the time to research their floors as much as they do their kitchen sink faucets, they'd see that solid over radiant is a problem waiting to happen. And today we have very fine engineered floors that look so close to solid that only a real hardwood pro could tell the difference. Owen's PlankFloor is one. Also, if people would choose to use solid parquet flooring again, they could have it on radiant as well. There are many great looking patterns. But everyone wants what everyone else has, strip or plank. I ran into to this exact problem about 8 months ago during the planning stages for a very large custom home. The customer wanted solid N. American Black Walnut plank over a radiant panel subfloor. I told her I'd do it but to look at the info regarding solid installs over radiant. Once she had a chance to let all the facts soak in, she decided an engineered floor would be the better choice. Sometimes, you just have to hang in there and educate you customer with the latest info, not outdated recommendations from NOFMA.