Wood is about R1 per inch, so yes, a thicker plank will have more resistance to heat movement than a thinner plank. But when you're talking about 3/4" for solid strip hardwood versus 5/8" or 1/2" for an engineered plank, that's a differential of R-0.25 or R-0.125, it's not terribly significant.

It's not the thickness, but the stability, that's the main advantage that an engineered plank will give over a solid strip hardwood. The ply structure is generally more stable and really does a nice job of minimizing movement.

Quarter-sawn, along with choosing a stable species, will be your best bet when going the solid wood route. But in general, an engineered plank will still be more stable than quartersawn.

You can get around that a bit by using less wide strips. Example, if movement occurred, an 8" wide plank floor would have wider gaps between planks than a 2" wide strip floor. Going with narrower strips can hide movement.

Glue-down is preferable to me over floating. Bostik's is my preferred adhesive.

Aluminum Oxide from the factory is more durable than site-finished.

There are engineered strips that have a thicker wear layer. Some of the engineered floorings 15 or 20 years ago only had a 1/16th" veneer of "show wood". It took a careful screening to refinish them. So look for a thicker wear layer, it'll resist punctures from dropped items and you can get several screening refinishings or even full-fledged sanding refinishes out of them.

Going with solid wood strips, mesquite, teak, wenge, padauk, purpleheart, those are "exotics" that are all fairly stable. Maple isn't. Brazilian cherry can have a bit of movement, though I have installed 5" wide quartersawn BC planks with no ill effects.

Much of the success or failure with solid wood over RFH is species choice, flat versus rift or quartersawn, and the overall design of the RFH system. I shoot for a minimum of 3 times the "R" factor underneath the RFH loops that there is above the RFH loops. That keeps the heat going where it's designed and intended to go, and allows for low water temps in the loops.

We used Plank Floor by Colonial Craft, maple, 2nd & better. This is a 3/16" site-finished over 7-ply engineered product. The flooring was glued and nailed to sleepers set in the GypCrete. We've been extremely pleased so far.

We aren't too hard on floors, though - no kids, one golden retriever.

HTH,

Jay

Here is a link that might be useful: Plank Floor (species list above right)