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I have a couple of rooms that are functionally a fully-enclosed 2nd-floor balcony, with a concrete beam + breeze block system as the base, and floor tile cemented directly over that (no insulation that I'm aware of). I want to add radiant heating (water-based), but I don't have space to add it above the concrete - raising the tile would cause the floor to be above the level in the rest of the house, and the interior doorways have limited height as well (solid granite lintels, not feasible to open upwards).

My plan is to add a couple of inches of insulation to the exposed underside of the concrete before next winter, and I'm wondering how effective it would be to attach PEX-style tubing to the underside of the concrete before the insulation.

I can't find anyone who has attempted this before, but I'm wondering if it is a reasonable idea, and what the potential pitfalls might be. I assume it's going to take a fair bit of time, but that it will eventually heat all the way through the concrete - or am I going to lose all my heat downward and out despite the insulation?

Front of balcony Underneath of balcony

These thermal images are from the energy certification 5 years ago, before the windows were replaced with double-glazing (apologies for the google translate): enter image description here

swiftcoder
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If the floor is built with concrete blocks like so:

enter image description here

Then... there's a lot of air in there. If you put the hot water pipes below it, this won't conduct heat well enough for it to act as underfloor heating.

Yeah, you could put the pipes in the holes and then fill them, but that would cause a lot of destruction, and it wouldn't be effective for another reason:

If it's a balcony, presumably you won't be there all day. So you need a system that heats quickly when you want to go there, which means low thermal inertia. In addition, if you have high thermal inertia, when you leave the balcony, all the heat stored in the mass will be wasted (and you paid for that heat). So, a thick heavy concrete slab is not ideal for these reasons.

If possible, I would suggest heating the ceiling of your balcony instead. If you have a drywall ceiling, you could replace it with variotherm (or other similar solutions). There are many ways to do this. Then stuff the plenum with insulation. This would give you a radiant heating ceiling, and low thermal mass, so you can switch it on or off when you're actually there.

bobflux
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How well insulated is the full enclosure? The balcony construction sounds like it's a giant thermal bridge, which would mean with a slow system like that (or any heating system in general), a fair amount of your heat would escape into the environment.

Also, as Bobflux mentioned, there might be air gaps that would severely reduce radiant heating function. And if it's a solid concrete block (which is less likely), the thickness might dampen out any heating efforts at the bottom.

Assuming the balustrade is closed and properly insulated, you could consider vertical radiant heating on the inside of the closed surfaces (including the building side minus door opening), and perhaps even the ceiling (provided it's backed by insulation, or you're giving the neighbours upstairs free heating on their balcony). You'll still lose a bit of space, but at least the door won't be an issue.

The energy efficiency issue remains though. This is probably out of season, but if you can rent or have access to a thermal camera, heat up your balcony space for a good 4-6 hours on a cold day or night (for example with a mobile heater), then go outside and check from multiple angles where it starts glowing. Those are the spots you'll need to insulate more.

MiG
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