Do the interior walls need to be plastered first before screeding the underfloor heating?

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I'm renovating an old house (built around 1900) and would like to install underfloor heating on the first floor. The old floor has already been removed, a new slab has been poured, the electrics and pipes have already been laid, and the next step will be underfloor heating.

Right now we can't start plastering right away because there are various reasons for delays. That's why my craftsmen are debating whether or not we can lay the screed directly on top of the underfloor heating, because the walls need to be plastered first. Now I have to decide if the interior walls should be plastered first, but I'm not sure. The screed will be adjacent to an old brick wall, which is also not perfectly level. The house is on a basement, so everything feels dry from below. The floor will be tiled afterwards. I understand there will be a mess later when we clean it up. Then I would try to cover the floor. But, in your opinion, is there anything seriously wrong with laying the screed directly to the "exposed" wall and removing it later?

  • I don't think you can cover it well.... Especially if the pipes are already laid. And underfloor heating is relatively sensitive.... I wouldn't let anyone run over it! 
  • For the screed layer, you could just put edge insulation tape directly on the wall without any additional plaster. Thus, the screed should hold up without damage. 

Answers

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Typical design: first the wall is plastered, then strips of insulation are glued to the outside, and finally the screed. The wall and screed should not touch each other for soundproofing reasons. If you plaster after that, you can't plaster up to the screed, but leave a gap. You will get the least satisfaction if you have to remove the plaster residue from the screed again.

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Yes, this is already a problem if the wall plaster is laid after the screed.

Just do it this way:

Typically, heating screeds require a 10mm thick edge joint before plastering the wall. When laying the screed, let it rest all around the brick wall, ideally on 20 mm thick screed strips (double) (10 mm each, which should be installed to prevent slipping). If wooden strips are used, remove them after the screed mortar has hardened and replace them with 10 mm thick adjustment strips before plastering. Two-layer adjustment strips also remain in the joint chamber in the future, wall plaster should be applied only up to the top edge of the screed.

For this purpose, 5 mm thick screed setting strips or MDF strips can also be laid at the floor/wall transition (i.e. lying on the screed surface for protection, the setting strips are close to the brick wall) while the wall plaster is being applied.

It's a little more work, but it will all work out!

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Depending on the construction of the exterior wall, the interior plaster is the airtight layer of the wall.

So plaster up to the slab first to make it airtight.

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