Rob is right;

But, alarm bells should go off when an online retailer is quoted, in a misleading statement, on technical matters, to which they themselves, have a tenuous grip, at best- present company excepted.

We collect thousands of dollars every week, correcting common mistakes, in radiant floor heating systems, for folks who "did their research". I would much rather they called before they started or
used an experienced designer, as the industry would be better off with happy customers who only had to pay once.

Here is a quote more appropriate to the subject. "The man that tells the truth should have one foot in the stirrup"

OP, Your insulation values are close to mine of: R65/35

Elemonate thermal bridges & minimize infiltration.
Get a good HRV unit AND use really good windows.

All of my windows are at least tripple pane with argon & Low-E, most also have a outer storm window.

I have the much maligned suspended PEX under subfloors, 1 loop (2 pipes) in each 16" TJI space.
My tile floors always feel cold to bare feet.
With house kept at 68* my floors only warm up to about 74* when its -20* outside.

BBP