I wouldn’t trust the neoprene backing on all the LVP I’ve installed to stand up to a hot wire.

Honestly, I wouldn’t trust LVP around a hot wire (but that’s me personally).

You need to call the number on the box and speak to someone with technical familiarity with the product. They’ll tell you what installation methods they’ll warranty, which is definitely what you want.

Radiant heat should have NO product above it that acts as a vapour barrier as it severely limits heat radiation. LVP will act as a vapour barrier and is not a candidate for radiant heat.

Aren’t most shower waterproofing systems— whether they’re elastomeric coverings like red guard or hydroban, or troweled-on barriers like kerdi all class 1 vapor barriers?

Edit: red guard and kerdi are .36 and .9 perms respectively, making them class 2 vapor retarders. Still rather vapor closed, and probably more vapor closed than LVP when looked at as an assembly rather than the perm rating of the center of on plank of flooring.

I put slate tile over mine. I wouldn’t recommend lvp.

You might get some heat to the touch of the foot for some minor comfort improvement but for any appreciable room heat, which I think OP will need for his application in a garage conversion, the radiant will not provide enough heat for overall room heat it that’s what he’s after. In Ontario building code for new construction for instance, as far as radiant heat as the primary source of building heat, any vapour barrier must be BENEATH the heating source tube or cable to mitigate vapour to be less than 45 ng/Pa•s•m2 . Usually poly is best placed below any R10 insulation foam as well which the radiant heat would be tied on to or a puck panel used. Again, OP isn’t strictly following code I guess (because I bet he’s not even allowed in most municipalities to convert a garage to livable space) but in terms of just getting good bang for his buck, that cable or mat isn’t going to heat much without a thermal mass (being embedded in concrete) or with LVP above it.

Here’s a YouTube video of a product available: https://youtu.be/3H-oCZ1sW7o

From everything I’ve read and heard from floor installers, LVP or any kind of floating floor and radiant heating don’t play well together.

I just did a bathroom, floor wire, self-leveler to just above the wires, then epoxy, looks terrific was easy and fairly inexpensive.

Honestly, I would avoid radiant heating. I have it in my 2k sq ft ranch style home here in the Bay Area (prior owner put it in) and I find it more finicky than the normal forced air type of heating. Maybe just me....

I’ve always been hesitant about the slab and earth beneath just acting like a bottomless heat sink for all the energy being radiated into it.

Sub slab insulation should be above code minimum by a bit if you want to not just dump money down the drain, I’m thinking.

The bottom line is you want to ask the manufacture of the product you want to buy if it is acceptable for over radiant flooring systems. Anything other than that is just guess work from everyone else. The manufacture will give you a definite yes or no.

That said, I wouldn't really recommend it. I've done a laminate floor over heat that was cleared by the manufacture, but there isn't much thermal mass there. The floor got warm but it didn't really feel warm.

The benefit with a tile floor is that while it takes a while to heat up, it holds the heat. When you walk on it, the floor feels warm. If you put cold wet shoes on a vinyl floor,they will suck out all of the heat in the area and the floor won't turn on unless the sensor is in that area.

That's just my personal opinion. I'm not saying it's wrong but it depends on what you expect to get out of it. You might be just as well off wiring an electric heater on the wall and calling it a day.

You can absolutely buy a variety of flooring that is radiant heated floor safe.

Coretec LVP can go over radiant heat (and is a floating floor), and I actually just bought some engineered hardwood flooring that is designed to be installed over radiant heat as a floating floor (gonna delivered today).

A vapour barrier stops vapour, not heat. You can absolutely install some brands of LVP (f.ex Coretec) over a radiant heated slab without issue.

It’s not a question of stopping heat. What he wants to stop is hydrostatic pressure and vapour from his slab mixing with warm air. Without a vapour barrier BELOW his heat source - and preferably some insulation as well OP could be creating ideal conditions for condensation below his flooring in the cold/hot mixing of vapour, especially if a low perm flooring is sandwiching the vapour within and slowing any potential drying upwards. His heating cable will be working full time to dry vapour rather than heat his room space and running full time adding virtually no warmth to the room. OP is better off with a space heater. If this was an above- grade bathroom he’d have no problems. His specific installation over a slab likely leaking vapour is a recipe for ineffective heat that will run full time. $$$

That's not what the guy said though. He said you can't put LVP above radiant heat because it will be a vapour barrier AND severely limit heat radiation. That is categorically false, heat will radiate through a vapour barrier without issue, and that's why we need insulation.

I absolutely agree that a concrete slab which is heated with radiant, should have insulation + vapour barrier below and around it to limit heat loss in directions you don't want, but that isn't what the guy I was replying to was talking about.

I’m still not following this model of vapor drive and how it interacts with a temperature difference.

A warmer floor will drive vapor away more effectively than a cold floor.

A colder floor will more readily condense atmospheric vapor than a warmer one.

What am I missing here?