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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

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    Some photos would help by the way!
    – MiG
    May 12, 2022 at 14:53
  • When I lived in Israel, everything was built out of concrete. It would slowly heat in the sunlight during the day, giving off heat inside the building over night when it would get rather chilly. Note that this was a slow process and took 12 hours or so of sunshine to heat the concrete enough for heating at night. You can do this, but it won't be a quick process and it'll be subject to the concerns listed in the answers provided so far. TBH, I'm not sure if the building walls were insulated at all or just simply poured concrete.
    – FreeMan
    May 12, 2022 at 15:01

2 Answers 2

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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.

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    Note that the question says "rooms that are functionally a fully-enclosed 2nd-floor balcony" so it's no longer (if it ever was) a typical balcony that you'd rarely use. It's rooms that are extended over an unheated/exposed area below, as I read it (and I've lived in places built that way - first-floor porch under otherwise normal room on second floor of house. It does make the heating/insulating tricky.)
    – Ecnerwal
    May 12, 2022 at 12:37
  • Indeed, the area was once a wooden exterior balcony, but at some point the previous owners converted it to interior rooms by replacing it with beam+block floor/ceiling, and building a 2-layer (brick, air space, brick) wall around.
    – swiftcoder
    May 12, 2022 at 20:00
  • OK! Then my comments about thermal mass miss the point. You could inject insulation in the air space between the bricks. How is the floor insulated? Do you live in a cold climate?
    – bobflux
    May 12, 2022 at 20:38
  • The concrete beams are fairly effective thermal bridges at the moment, I'm unclear whether insulation in the breeze block would help all that much? Fairly moderate climate, typically the winter temperatures don't go more than a couple of degrees below freezing at night (and almost always above freezing during the day).
    – swiftcoder
    May 13, 2022 at 9:24
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    Oh, they're called breeze block? Anyway, yeah, this type of floor is notorious for leaking a lot of heat due to the beams. If you want to isolate it, and you can't put the insulation on top, it'll have to be outside. But there will be thermal bridges through the walls then, so you'll end up wrapping the whole room in insulation.
    – bobflux
    May 13, 2022 at 9:40
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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.

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  • Not well insulated at all. The concrete beams in the floor are bridging low temperatures from the cold wind travelling through the space underneath the balcony. The floors were very cold this past winter, hence my desire to insulate underneath. The walls are significantly better than the floor, as they at least have an air space between the two layers of brick. And I've upgraded the windows to double-glazing.
    – swiftcoder
    May 13, 2022 at 9:19
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    And adding insulation (including walls and ceiling) is a good idea period, but I'm a bit concerned that a fair amount of heat seems to travel all the way to the concrete column. What was the outdoor temperature when these thermal images were taken? You could consider packing those in wood, which is both impact resistant and provides some insulation.
    – MiG
    May 13, 2022 at 9:53
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    The concrete beams are shaped like an inverted 'T', and the base is 12cm wide x 5cm tall. I don't have any sufficiently exposed end profile to tell how wide the top of the beam is. The exterior temperature wasn't recorded when those photos were taken, but given the time of year I'd hazard a guess it's around 10 C
    – swiftcoder
    May 19, 2022 at 10:22
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    You might be able to inject insulation into the cavities, but the thermal bridges will remain. I would definitely consider applying insulation underneath, with some sort of weather and impact proofing layer underneath. And wrapping the columns in a similar fashion.
    – MiG
    May 23, 2022 at 8:47
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    That will probably either require a great deal of energy to have any effect in your balcony-room, or not work at all. Radiant heating is normally applied on top, embedded in cement so that it can impart most of its energy in the user space, the floor below it provides some dampening. Thermal energy travels very slowly through cement and stone, and because it has to heat up the increased thickness, the temperature goes down over distance. Furthermore, the cavities also provide a little bit of insulation (can do some convection and radiation, no conduction), worsening that situation even more.
    – MiG
    May 24, 2022 at 19:56

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