I need some help on my DIY project for underfloor heating system in an apartment.

The question is on the screed requirements provided that I have the following (non traditional) setup. There is polystyrene insulation that is glued to the concrete base. Then there are whole OSB plates freely positioned on the insulation. I plan to fix the heating tubes to the OSB and to implement a floating screed over it (sand and cement). Finally I plan to have a tiles finish.

First of all do you see any potential problems with this setup? Please mind that any changes with it will be expensive to make so I need to make this work.

  • Are you sure you want to put mortar or cement directly on OSB? The OSB may just want to swell up and deform from the moisture. Or the mortar may not adhere well to the OSB. You may want to consider cement backer board like Hardibacker or Durock, or another conventional tile/mortar substrate. I believe I have seen it as thin as 1/4 inch. And I assume you own and not rent the apartment, right? I wouldn't want to try to remove this later... – Grunthos Jun 2 '14 at 14:48
  • If pouring a cement/sand mix over the tubing, there's no need for tile backer (or anything other than XPS insulation) UNDER it - and it's an expense that's not needed as well. – Ecnerwal Jun 2 '14 at 15:12
  • Thanks for commenting! Yes I'm concerned about OSB swelling or deforming and I was thinking about putting polyethilene between the tubes and the OSB. However, the sand/cement mixture will not adhere to the polyethilene for sure and this is worrying me as well. Yes, I own the place and yes a removal will be a nightmare th . – vkamenen Jun 2 '14 at 20:04

Skip the OSB - attach the tubing to the insulation. XPS insulation plays well in concrete/cement applications. OSB Does NOT. I can't imagine what lead you to this particular "non-traditional" idea, but run away from it before you find your whole floor disintegrating under your feet.

Responding to comment - the insulation glued to the slab should not have any appreciable gaps - if it does, fill them with more insulation. If it is, in fact, XPS (extruded - as it should be - closed cell foam, smooth-surfaced, usually pink, blue or sometimes green) rather than EPS (expanded - crumbly white stuff) it stands up to foot traffic just fine. Tubing clips are typically screwed right into it. If you have glued EPS to the slab, then you'd be well served to buy the thinnest sheets of XPS you can find (1" or 25mm is typically the thinnest) and place that over the EPS, without leaving any gaps this time. It usually has a tongue and groove edge treatment to help with that.

These are folks to whom my relationship is "just a happy customer" with plenty of detailed information about installing radiant heat in a manner that will last. I installed mine over 2" XPS, walked all over it running tubing and reinforcing steel, and didn't make so much as a dent in the insulation. That page has links to both a written manual and a video.

  • Thank you very much for the help! One of the downsides of the XPS is that it will get deformed from the tubes and the clips while doing the work for pouring the sand/cement over it. Also pieces from the mixture will get between the plates and worsen the insulation. Any advice on how to deal with these? I would vote for your answer but the system is preventing it as is my first post here – vkamenen Jun 2 '14 at 20:30
  • Thanks for the help and for the link! What I have is XPS for sure and it gets deformed a bit when I step on the iron mesh over it. This is one of the problems with this setup, the other being that I dont't know how to get the tubes reside in the middle of the screed and not on its bottom. – vkamenen Jun 3 '14 at 23:14

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