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I live on the third floor of an apartment complex, meaning I have two neighbors beneath me who essentially heats my apartment.

I want to install floor heating through a water heat pump and I have read it's recommended to insulate at least 25cm (10"). This is not possible, as the space between the floors is only 20cm, and it's wise to leave 25% free. I also dont know what the 25cm is based on, maybe it's only for ground floors because 25cm insulation is a lot.

I am considering leaving as it is right now, without insulation, as I believe that will trap the neighbor's heat and I wont enjoy their "free heat". Or am I missing something?

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    I can't do the math on this, but as heat travels inexorably from hot to cold, you will probably be donating more heat to them than they are to you, with or without insulation.
    – Tetsujin
    Dec 21, 2021 at 19:00
  • The floor is a common structural element shared by you and your neighbor, so legally you can't remodel the floor without his/her consent, especially if you need to utilize the space below the floor. If you are going to place the heating element on top of the floor, then, you need adequate ceiling height after installation, also, you need to figure out whether the floor is adequate to support the additional weight, and probably need to inform the building management before commence the work.
    – r13
    Dec 21, 2021 at 20:05
  • Perhaps you could get away with using the same material you can place behind radiators, that deflects the heat back into your house, much more then that it insulates.
    – Luuklag
    Dec 21, 2021 at 21:49
  • who says it is wise to leave 25% empty and for what reason do they think it wise?
    – Fresh Codemonger
    Dec 23, 2021 at 1:14

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That insulation advice refers to situations where you're against unconditioned space, not where you're against someone else's conditioned (heated) space. You can do with much less insulation in that case and not expect to lose much energy since the rate of heat transfer is proportional to the temperature gradient, and you won't have a very large one between your heated floor and the rooms below. If you can insulate even half of what's recommended I'd expect satisfactory results.

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  • I would keep the status quo, it should be drastically reducing you heating bill. If you still want to insulate talk to the neighbors below you will be saving them on there heating bill while adding to yours.
    – Gil
    Dec 21, 2021 at 23:13
  • @Gil, did you mean to comment on the question? I'm not sure how this relates to my answer, nor is it clear what the "status quo" is or how it would reduce heating costs.
    – isherwood
    Dec 22, 2021 at 13:37
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You state your neighbors are acuntally heating your apartment. If you shut that off with insulation you will have to make it up with your in floor heat and pay the energy cost. Consider cold air is heavier than warm air hence it pushes warm air up. This is mainly because cold air is denser than warm air. I have two floors, a basement that is heated with hot water and a forced air on the main floor. There is no insulation between them but the walls are well insulated. The main floor heating runs considerable less then before the installation of the hot water heat and the lower gas bill reflects this. I get pseudo double use of the basement heat. I would suggest putting in the floor heat, it will cover you if the neighbors go on an extended vacation or there heating plant fails.

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You want to insulate as much as possible.

Typically you heat your floor to a temperature higher than the air temperature of your underfloor neighbors ceiling.

Your hydronic heating probably runs at 35C at least. Their ceiling air is probably going to be 22C (depends what they heat to and when relative to when you heat). So your floor will want to move 13C difference to them (at least). If you insulate you slow that transfer and speed the heating of your floor. If you don't insulate you'll be increasing their ceiling temperature instead of your floor.

Even if you insulate their heating is helping you. With your floor heat off there ceiling heat will warm your flooring it just takes longer before their heat reaches you. Even in the case where they turn their heat off chances are they wouldn't let their unit go to less than 15C so your ceiling assembly would be at 15C instead of 8C of the earth or whatever your exterior temperature is.

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