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I installed a water heating floor system with 17 mm pipes and studded panels.

This is how a studded panel with a pipe looks like:

studded panels

It has 52 mm height (the polystyrene has 30 mm).

Now I want to add a screed over the heating floor system and I'm looking for the lightest solution that doesn't load the slab too much. The screed should uniform the pipe temperature and assure a solid base for the finishes.

The screed should be:

  • floating
  • compatible with the heating floor system
    • enough elasticity
    • a good heating conductor
  • thin
  • light
  • resistant
  • ready to add wood flooring or the ceramic tiles (depending on the room) on it

Until now I analyzed some classical solutions like:

  • normal cement-sand screed => 132 kg/m2

    • at least 45 mm over the pipes (=> ~60 mm in fact)
    • density 2200 kg/m3
  • anhydrite screed => 100 kg/m2

    • at least 35 mm over the pipes (=> ~50 mm in fact)
    • density 2000 kg/m3
  • perlite screed => 66 kg/m2

    • at least 45 mm over the pipes (=> ~60 mm in fact)
    • density 1100 kg/m3
    • bad heat conductor

Do you have any better suggestion?

It would be nice if I could mitigate the thickness of the screed over the pipes to at most 25 mm.

layers

  • Hello, and welcome to Home Improvement. Good question; hope the answer is useful. And, you should probably take our tour so you'll know how best to participate here. – Daniel Griscom Jul 20 at 11:46
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We have underfloor heating, pipes like you describe with a 50mm screed of light concrete, well cement, surrounding and covering the pipes and heavy tiles (12mm thick) as the final floor surface.

The heat transfer is fine but the response is slow - but we designed for that as the floor stores heat to be gently released as we also have the sun heating it during the day in winter.

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A wood floor is going to slow down the response a lot. Wood over cement will usually be on furring strips on the concrete. (How will you miss your pipes? Glue the strips to the floor? Put the blocks of wood between the pipes to attach the wood to later?)

I've actually never seen someone put down an extra layer of concrete so that they can put wood on top. Usually this is done, and either followed up with tile, or with an embossed concrete finish. (I've seen patios done that look like well laid flagstone.)

You may want to reconsider the design:

  • Why is a screed above the pipes needed at all? Cut strips of 3/4" plywood, lay between the pipes, and attach your hardwood floor to that. The strips can be attached to the sub slab with a hilti. This may be interesting at the loops, and you may have to skip a nail or two where the board is just in the wrong place. I would consider glueing the edges at those spots.
  • Are you set on wood? Could a conventional screed just to cover the pipes, then tile with thinset mortar be a viable alternative. I've seen tile with very convincing wood grain patterns on it.
  • Wood is a dynamic material. In addition to expanding and contracting with temperature changes, it will also do so with humidity changes. A warm floor will dry and shrink. When you turn the heat off in the summer, it will absorb humidity from the air and expand. This normally happens with a wood floors in a house, but most of the time the house is fairly closely temperature regulated. You don't have 1 face of the planks heated 20 degrees or so warmer during parts of the year.

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