Hardwood over radiant heat in Ontario

Radiant does not forbid hardwood. It specifies it.

Hydronic radiant under hardwood is a normal, well-understood assembly. It is also the one where the wrong specification produces the most expensive failure, because the heat source is under the floor and runs for months.

The rule of thumb: engineered board, glued down, with a surface temperature limit and a commissioning schedule. Almost every radiant hardwood failure is a departure from one of those three.

Why engineered, and why not solid

Radiant drives a moisture gradient through the board — dry at the bottom where the heat is, wetter at the top. Solid hardwood responds to that gradient by cupping or, over time, by checking along the grain.

An engineered board is a plywood or HDF core with a hardwood wear layer, and the cross-laminated core is doing exactly the job the gradient demands: resisting dimensional change across the width. Quartersawn material is more stable again than plainsawn, which is why a quartersawn wear layer is worth asking about on a wide plank over radiant.

Narrower boards also move less in absolute terms. A 5-inch engineered plank over radiant is a materially safer specification than a 9-inch one, and the difference is visible in most mills' own warranty language.

The temperature limit is the whole ballgame

Most manufacturers cap surface temperature at about 27°C — roughly 80°F — measured at the floor, not at the thermostat. Above that the wood dries faster than it can equalise and the gradient wins.

This is a system design constraint, not a homeowner setting. A radiant loop designed to heat a poorly insulated room through a floor will want to run hotter than the floor allows. That is a problem to solve at design, with loop spacing and water temperature, not by asking the client to keep the thermostat down.

Commissioning: the step that gets skipped

A new slab with radiant in it has to be run before the floor goes on. The usual sequence is to bring the system up gradually over several days, hold it at operating temperature for a period, then bring it back down and let the slab reach the installation condition — and only then test slab moisture with in-situ probes.

Skipping commissioning means installing over a slab that has never been dried by its own heating system. The floor then does the drying, from above, in year one.

Ask any contractor quoting hardwood over radiant what their commissioning sequence is. If there is not one, they have not done this before.

Floating over radiant

Floating an engineered floor over radiant is the specific decision that voids most mill warranties, because the assembly puts an air gap and an underlayment between the heat and the wood. The heat has to conduct across something designed to insulate.

Glue-down transfers heat directly and holds the board against the gradient. It costs more and it is the right answer.