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Prudent_Falafel_7265 t1_isotwy4 wrote

Reply to comment by zedsmith in Radiant Floor Question by movey_mcmoverson

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.

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zedsmith t1_isov4ni wrote

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.

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Prudent_Falafel_7265 t1_isp4hf2 wrote

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.

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zedsmith t1_isqhftj wrote

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.

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thirstyross t1_isse95p wrote

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.

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Prudent_Falafel_7265 t1_issnshk wrote

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. $$$

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thirstyross t1_ist2w8a wrote

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.

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zedsmith t1_istmmfi wrote

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?

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