Vinyl Plank Over Radiant Heat: Product Limits, the Manufacturer Temperature Cap, Expansion, Glue-Down vs Floating, Electric Mats, and the Dark Plank in the South Window

The two numbers, and what the sheet has to say

Same as for wood: on our slabs the water goes in at about 40 C and the floor surface runs 30 to 37 C, and glued vinyl plank has lived on those slabs in basements and rental suites for years without trouble. The sheet says something else: most vinyl makers cap the surface at 27 C, a few at 29, and the warranty is tied to that number. Both are true; mine is experience, theirs is what they will stand behind. Run warmer than the sheet and the warranty is not what is protecting your floor; the builder’s judgement and the product are. The sheet also has to say three more things or the product is not for this job: that it is approved over hydronic radiant at all, the maximum run length before a transition is needed, and the expansion gap at the walls. The radiant compatibility article has the rule for every floor; this page is the vinyl fine print.

What goes wrong well past the sheet’s number is not dramatic. The planks soften a little, the locking joints relax, and a floating floor that was tight in October has visible end gaps by February. Dark planks also give off a plastic smell for a while when they run warm, which nobody mentions in the showroom.

SPC, WPC and the flexible stuff

Prices from the vinyl plank cost page: mid-grade material $3.25, click labour $2.75, glue-down labour $3.75. A 500 sq ft basement in glue-down LVP is $3,500 plus 10% waste on material, $165, so about $3,665 before HST.

Glue-down over a heated slab, and why

A floating vinyl floor has an attached pad or a loose one, and an air film under it. Both are insulation, R 0.2 to R 0.5 between the slab and your foot, which is not much, but it is not nothing, and it is the layer that lets the planks move. Glue-down LVP is bonded to the concrete across its whole back. The heat goes straight through 2 mm of vinyl, the planks cannot walk, and there are no end gaps because there is nowhere for them to open.

The conditions are the slab’s: flat to 3/16 in. in 10 ft, dry to under 75% RH or 3 lb MVER, and clean. A heated slab that is still giving off moisture will lift glue-down vinyl in bubbles, and the heat makes it happen faster. Run the heat for two weeks before the floor goes in, test the slab, and use an adhesive the maker rates for radiant and for your moisture reading. The vinyl over concrete page covers the moisture testing and the mould-under-waterproof-planks problem, which a heated slab does not cause but will not cure either.

If the floor floats anyway, because the client wants the attached pad or the product only comes that way: 3/8 in. gap at every wall and fixed object, a T-moulding every 30 to 40 ft of run or whatever shorter number the sheet gives, and no heavy island or cabinet pinning the floor down in the middle. Pinned floating vinyl over radiant buckles at the pin.

Electric mats are a different question

A hydronic slab is four inches of concrete spreading warm water evenly across the whole floor; the surface varies by a degree from one spot to the next. An electric mat is a cable every 3 in. with no mass over it. Directly under a floating plank, the cable line runs well past the sheet’s 27 C while the thermostat reads the average, and the plank softens along the lines. Most vinyl makers say no to electric mats under a floating floor for that reason, and the ones that allow it want a layer of self-levelling compound over the mat first so there is something to spread the heat. The electric heated floor page is where electric belongs: under tile.

The dark plank in the south window

This one has nothing to do with the heating system and it catches people every year. A dark grey SPC plank in front of a south-facing patio door, on a sunny day in March with snow on the ground reflecting into the room, reaches 40 C and more at the surface. The radiant slab under it is innocent; the sun did it. The planks grow, the floor has nowhere to go, and it peaks at the nearest joint or lifts at the door. I have seen it in houses with no floor heat at all.

Over a heated slab the sun adds to a floor that is already running in the thirties, so the margin is smaller. Lighter colours, glue-down in the sunny rooms, blinds that actually get used, and an expansion gap the sheet calls for. If the client insists on a near-black plank at a south window, I ask them to sign for it. Peaking and buckling of a floating floor are an installer problem when the gap was short and a homeowner problem when the blinds stayed open.

Rules of thumb

1. Find the word “radiant” on the sheet and the surface temperature beside it. If either is missing, it is a different product. Then put a floor sensor or a mixing valve on the zone, so the surface number is something you know, not something you hope.
2. Glue it down over a heated slab. Better heat, no gaps, no run-length limit, no pinning problem.
3. If it floats, respect the run length and the gap. The gap is for expansion that will happen every heating season.
4. Heat on two weeks before, slab tested, then install at a warm setting, not a hot one. Ramp back up over a few days.
5. No electric mat directly under floating vinyl. Tile over the mat, or a hydronic slab under the vinyl.
6. Think about the sun before the colour. The slab is not the hottest thing a dark floor meets.

Questions people ask

Sources and further reading

• Torlys: Canadian maker of engineered, vinyl and laminate floors; the installation and radiant-heat instructions are a good example of what the sheet in the box says.
• Uponor Canada: hydronic radiant tubing and controls; their design manual is where the surface temperature and floor R-value tables come from.
• Nuheat: electric floor heating mats and thermostats; the installation manual covers embedding, sensors and the floors it can go under.