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I have an underfloor heating element laid on a 6mm thick insulation board. When I walk on it, I can feel that its slightly springy.

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I want to pour a self-levelling compound on top as a finish but I did this before in my other room and the concrete ended up springing slightly and resulted in cracks along some places - mainly in the gaps between the heating elements because these also provide some reinforcement as opposed to just the bare foam board. So I stuck a strip off a recycled cardboard roll into the gaps, and it's less springy already.

But I worry that its not enough and wish I could do more.

Maybe some thin plastic stiff sheet which would have small holes in them (as opposed to a plain sheet) so as to not block any of the heat and yet provide some reinforcement for the concrete. But where do I find any suitable plastic tiles like that?

The concrete will be as deep as 1.5cm or more at some places, and vanishingly thin at others because the difference in height is 2.7cm all of which I can't level without creating a step into the room and shortening the doors.

Alternatively, I thought of just putting a carpet straight on top of the heating element. But then I worry that furniture like a chair with thin legs could directly press onto the heating element causing damage.

Laminate flooring would be ideal but that would require too many specialized tools and for my crazy uneven floor it would result in an ugly job like it was before.. so a new more-levelled floor would look better I think.

The reason for the big difference in height is because it must have cracked from some pressure underneath. Some places in the room are just higher up without a crack so I'm not just levelling to cover a crack.

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I drilled holes into the subfloor for levelling screws and a bit of air escaped and even now I'm starting to feel a few bubbles forming underneath the insulation board. This is one of my other worries that pressure could also crack the concrete. But maybe I just drill a few tiny holes into the resulting levelled compound to allow air to escape?

Maybe I mix my own cement and sand (results in a very strong cement) and use floor tiles with a levelling ruler, and tile straight on top of the heating elements? But not sure how cosy that is for a living room.

I just want a second opinion.

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    System we used had channels in the insulation for the pipe, then concrete on top - a fine mix. No springing and no issues after 15 years. Could your heating element have less springiness if it was the other way up?
    – Solar Mike
    Commented Jul 2, 2022 at 20:01
  • It's mainly the 6mm insulation board that is springy. But if there were something stiff spreading a pressure point to a larger area, it would help. The heating element is electric, its just a plastic roll with electrodes at its sides, so flipping it upside down make no difference. Commented Jul 2, 2022 at 20:19
  • I have installed both electric and liquid in floor heat, both were the same as solar mike had and mine worked great my foam was not even close to the same thickness but I had 6” concrete over that and I would park my international 3 ton work van on it with no problems that slab was intact 15 years later 18x 32’ for that bay.
    – Ed Beal
    Commented Jul 14, 2022 at 19:00
  • 15cm thickness of cement and thinner insulation, makes sense. I'm talking about 1-2cm cement because it's for indoors living room, ground floor. Commented Jul 15, 2022 at 8:49

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So I've read that flowing screed may be stronger than semi-dry screed, so this is just to exclude that option;

But first major aspect is the depth of the screed. If screed goes over isolation of 5mm, it needs a certain minimum depth of at least 10mm (can't remember where I've seen this number) to provide enough strength (because its "floating").

This I didn't account for the last time. The minimum depth probably depends on the exact screed used and also the thickness of isolation.

Second major aspect is the drying time. What I was recommended was, after the cement hardens and dries the first time, pour a bucket of water over the whole area to allow the cement to suck up all the water and keep it moist for at least 21 days. It was suggested to use some sheet or blanket to prevent evaporation or to keep water in one place and also water it with a hose (or a bucket) when it gets dry. I've read that cement that is allowed to settle in this way can become twice as strong compared to cement that is not treated in this way and only left to settle for say 4 days.


UPDATE:

Springiness And I can certainly see the difference, the first 3 weeks the floor was still slightly springy (I ended up with 15mm thickness) but then after water started evaporating from within the cement which took another 2 to 3 weeks, it no longer feels bouncy, so I have a much more confidence in the project.

Lesson Learned Turning on the heating element was not a good idea if the cement is still internally wet because it becomes electrically conductive and I could feel the AC in my hand when I touched it. But this disappeared when the cement got drier.

Cracks I have left the room in peace without walking on it excessively for many weeks but a few cracks appeared anyway. I think this is very normal. I'll just need to make sure that I only continue laying the final layer until I can fully turn on the heating, and see or hear no more cracks forming.

Requirement adjusted I ended up not trying to use the screed for levelling purpose, but only to create the top layer over the heating element.

The requirement of the floor being level is wrong in my case - I should've levelled it before laying up the heating element. If I did try to level the floor at this later stage, some areas would've ended up being heated up at a much slower rate than other areas.

And originally I planned on 21mm on average which would've resulted in the costs as shown below but because I didn't need to level it after all, I also could reduce thickness down to 15mm to save costs and still provide required thickness.

So given 1 bag (£16.99) covers up to 12mm thickness for 1m squared; a 15m2 room at 21mm I'd need about 28 bags, that is about 477£.

After reducing to 15mm (everywhere regardless of local levelness), I still needed 23 bags, though calculation said 19 bags.


Approach I didn't go for: And as an alternative, I looked at polycarbonate plastics (i.e "lexan") which provide the greatest strength (to spread a pressure point to a larger area); But they are also very thermally insulative (not good!) but also electrically insulative (which is good for electric heating elements) so a perforated sheet might be ideal, but haven't found practically any shop where to buy exactly this and not sure what exact pattern would be ideal and how much area per squared meter should be open holes vs filled with the material.

And to address the bubbles forming under the flooring I added many punctures in the insulation board to allow the air to pass through the insulation (under which the air gets accumulated) and before pouring a levelling screed, I attached small plastic thin hollow tubes (like straws) at each point vertically.

And I fixed the bottom of the small tubes with a hot glue gun, so they don't tip over when pouring the compound. That way I won't need to worry about the air building up underneath the finished concrete.

The reason for being so careful is because the concrete floor had cracked in the past and I can see the 2cm difference in height. So I don't want to repeat that history.

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