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thirstyross t1_isse95p wrote

Reply to comment by Prudent_Falafel_7265 in Radiant Floor Question by movey_mcmoverson

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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