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Engineered Hardwood Over Radiant Heat: Species, Plank Width, Glue vs Float, the Surface Temperature Question, Humidity, Start-Up, and How We Do It Over Our Heated Slabs
Flooring & Tile
Engineered Hardwood Over Radiant Heat: It Works, It Has Conditions, and Here Is Every One of Them
Most of the houses I build have engineered hardwood glued straight to a heated concrete slab, and most of those floors look the same at ten years as they did at ten days. The ones that did not were never the fault of the heat. They were a plank too wide for the room’s humidity, a floor floated on a foam pad over a slab that was still wet, or a boiler somebody turned up to 55 C in a cold snap. Wood over radiant is a set of conditions, and if you meet all of them it is the most comfortable floor there is. Miss one and you get to learn about wood movement at your own expense.
Two numbers, then the reasons
On our slabs the water goes into the tubing at about 40 C and the floor surface runs 30 to 37 C, and that is how I have run glued engineered over heated concrete for thirty years without a floor coming back on me. Now the other number. Every engineered hardwood sheet I have ever read caps the floor surface at 27 to 29 C, and the warranty hangs on that line. Both are true. The first is my experience; the second is what the manufacturer will stand behind in writing. Run warmer than the sheet and you are relying on the builder and the product, not the warranty, and you should know that before you do it. A floor sensor or a mixing valve on the zone is how you know the surface number instead of guessing it. Some sheets also cap the rate of change: no more than a couple of degrees a day. The radiant compatibility article explains why the makers set the number where they do and what happens well past it.
The mechanism is simple. Wood holds a moisture content that follows the air around it. Warm the bottom of a plank and its moisture content drops; the bottom shrinks and the plank cups upward at the edges, then dries from the top and checks along the grain. Engineered flooring fights this with a plywood core whose plies run crosswise, so the plank as a whole stays put while the face does its small amount of moving. It is a better product over heat than solid wood for exactly that reason, and the engineered vs solid page is the longer argument.
Species and width
| Choice | Over radiant | Why |
|---|---|---|
| White oak, red oak | Good | Moderate movement, forgiving, every maker rates them. The default for a reason. |
| Walnut, American cherry | Good | Stable species; cherry darkens in sun, which has nothing to do with heat. |
| Hickory, hard maple | Careful | Dense, moves more with moisture, shows gaps sooner. Narrow planks only, and a maker who says yes in writing. |
| Exotics (jatoba, ipe and the like) | Depends | Some are very stable, some are not, and the sheets vary. Read it. |
| Plank width up to 5 in. | Good | Each plank moves a little; small planks, small gaps. |
| Plank width 7 in. and up | Careful | Only with a thick multi-ply core, a sawn (not sliced) face, and the house held within a tight humidity band. Beautiful, and unforgiving. |
| Core: multi-ply birch or poplar plywood | Best | Cross-laminated, stiff, bonds well to adhesive. |
| Core: HDF | Avoid | Fibreboard swells if the slab ever gives up moisture. Floating product mostly, and this is not a floating application. |
The wear layer is the other spec to read; over radiant a 3 mm or 4 mm sawn face on a plywood core is the sweet spot.
Glue it down. Do not float it over a slab
A floating floor sits on a foam pad with an air film under it. Over a heated slab that pad and that air are insulation between the heat and the room, about R 1 on top of the wood itself, and the floor has a hollow sound under foot that nobody likes once they have heard a glued floor. Glue-down puts the plank in full contact with the concrete. The heat moves through, the plank is held flat against movement, and there is nothing to squeak.
The glue is a urethane or modified-silane adhesive rated for radiant and for the moisture reading of your slab; some include a moisture barrier in the same trowel pass, which is worth paying for over any slab under five years old. Nail-down does not apply here, because there is nothing to nail into. Nailing over a radiant joist system is a plywood-subfloor case, and a different page.
What it costs over a heated slab: mid-grade engineered at $5.25 a square foot, glue-down labour $6.50, adhesive $1.00, so $12.75 installed. A 900 sq ft main floor is $11,475 plus about 10% waste on the material, $470, call it $11,950 before HST. The floating version is $9.25 and I would not put it over a slab. These are the engineered hardwood cost page rates.
Humidity is the other half of the job
The sheet says 35 to 55% relative humidity in the house, year round, and over radiant I would aim for 40 to 50 and hold it there. The heated slab is not what dries the wood out; the Ontario winter is, when the outside air at minus 20 holds almost no moisture and every air change drops the house toward 20% RH. A tight ICF house helps because there are fewer air changes to dry it, and the houses we build hold humidity through January with a small humidifier or none at all. A leaky house needs a humidifier on the furnace, and it needs to be running before the floor goes in.
Check the wood with a pin meter before installation: it should be within 2 points of what the installer expects it to settle at in the house, which for a heated Ontario house is 6 to 8%. The acclimation page covers the five to seven days and why installers skip them. Over radiant, the acclimation happens with the heat running.
The start-up sequence
- Slab dry first. Under 75% RH by in-situ probe, or under 3 lb MVER by calcium chloride, at the adhesive maker’s number. A 4 in. slab wants 60 to 90 days, and running the heat shortens that.
- Heat on for two weeks before the flooring arrives. Three on our jobs. This drives the last moisture out of the slab and gets the house to living conditions. Acclimate the wood in that house, not in the garage.
- Turn the water down to warm, not off, the day before installation so the adhesive cures at a sane temperature. Most adhesives want the slab between 15 and 25 C.
- Ramp up slowly afterwards. No more than 2 to 3 C a day on the water temperature, back to the design setting over a week. Never crank it.
- Set the outdoor reset and leave it. A modern boiler or heat pump control lowers the water temperature as the weather warms. The floor sees the smallest swings that way and the fuel bill is the smallest too.
- Humidity 40 to 50%, and a hygrometer on the wall so you know, instead of guessing from the gaps.
Questions people ask
Can you put engineered hardwood over radiant heat?
Yes, and it is the standard wood floor over hydronic slabs. Glue it down, put a floor sensor or mixing valve on the zone so you know the surface temperature, hold the house at 35 to 55% humidity, and pick a plywood-core product with a maker’s radiant approval in writing. Solid hardwood is possible with more restrictions; engineered is the safer choice.
What temperature is too hot for a hardwood floor over radiant?
The sheet says 27 C, a few say 29, and the warranty stops there. Our slabs run water at about 40 C and a surface of 30 to 37 C under engineered floors that have been fine for thirty years; past the sheet you are trusting that experience and the product. If the system needs water over 45 C to heat the house, the problem is the house, not the floor.
Is it better to glue or float engineered hardwood over a heated slab?
Glue. Full contact with the slab means better heat transfer, a solid feel and no pad acting as insulation. Floating over a slab is quicker and cheaper and the floor sounds like it.
Why is my hardwood floor gapping over radiant heat?
Low humidity in the house, almost always, not the heat itself. Check the RH; if it is under 35% in winter, add a humidifier and the gaps will close by spring. Seasonal gaps that close in summer are normal movement, not a claim.
How wide can the planks be over radiant heat?
Up to 5 in. without much thought. 7 in. and wider only with a thick multi-ply core, a sawn face, a maker who says yes in writing and a house you can hold within a 10-point humidity band all year.
Check the floor you want against the slab you have
The radiant flooring compatibility checker takes your floor and your heating system and tells you yes, no or the conditions. If you are building new, the system itself is priced on the hydronic radiant cost page, and the heated slabs under our own houses are on ICFhome.ca.
Sources and further reading
- National Wood Flooring Association: the installation guidelines every hardwood warranty points to: subfloor flatness, moisture limits, acclimation, fastening schedules and the radiant-heat chapter.
- Mirage Hardwood Floors: Quebec-made solid and engineered hardwood; their installation guides give the moisture, acclimation and radiant limits in plain numbers.
- Uponor Canada: hydronic radiant tubing and controls; their design manual is where the surface temperature and floor R-value tables come from.
