Can't find any sources to comment on the benefits, if any, of closed-cell poly (bio based) insulation sprayed directly on stapled-up PEX after it has been set in aluminum tracks between floor joists.  However, I have read about the necessity of keeping an air space between the PEX and more traditional insulation methods.  Question is, is it or is it not advisable to use spray foam insulation in such an application.  Thanks in advance...Cant imagine why it wouldn't be the cat's meow but you never know.

it's good, but I would advise a "shield" of cardboard, just to make sure the spray foam doesn't get between your aluminum plate and the subfloor. Foam can work itself into small cracks and that would reduce the plates' effectiveness.

Thanks Rob...appreciate this and any advice you might provide in the future.