Regarding to my old problem: Tiles above concrete, with electrical conduits

I am thinking about using XPS to raise my concrete attic subfloor, so that I can bury existing electrical conduits.


  • I am thinking about raising the subfloor by ~40mm with XPS boards. I intend to use "styrodur", a harder variant of XPS.
  • The boards would be attached by flexible tile glue to the concrete.
  • I have spare tile glue, suited for XPS boards according to my tilelayer.
  • I'd leave "channels" in the raised floor to bury the conduits.
  • Those "channels" would then be filled with mortar. Yes, embedding those plastic conduits in mortar or concrete is up to code here.
  • Above the xps boards I'd lay the cheapest laminate boards I can find.
  • The laminate would be that tongue and groove variant, that could be "clicked" into one another.
  • The "room" is the low part of my attic. It would be used as a storage room. It isn't high enough for an adult to stand, one can only move crouching or crawling.
  • Every cable with 230V power (I am in Europe) is secured with AFCI, RCD, circuit braker and surge protector. The cables are up to code and correctly dimensioned. There are cables with 24V DC for the led lighting, those are secured with fast acting fuses. The network and TV cables are obviously without overcurrent protection.
  • I laid spare empty conduits for further upgrades.

Theoretical benefits

  • There would be an additional insulation for the rooms under my attic.
  • I would have a nice smooth surface in the storage while minimizing height.

Possible issues

  • I am unsure about the need for an vapour barrier under the XPS boards.
  • XPS is flammable. While I see no immediate danger of setting fire to the XPS by the electrical cables, I want to be on the safe side in case of an lightning strike (some cables come from an external TV antenna). Therefor I want to embedd the conduits in mortar.

Just to make it clear: Before converting the attic I had absolute security, because near the cables everything was non-flammable, just concrete, bricks and mortar. I am now planning to bring in flammable material and lots of additional cabling, so I want to have security back to (ideally) absolute for my peace of mind. Therefor "overkill" ideas are welcome.

Would that be an viable plan? Do I invite trouble with that?

  • Flammability would be my concern as well (I'm afraid I don't have an answer on that one), but I'm mainly curious about what goes on top of the XPS. It compresses very easily, so if the laminate boards are not or poorly interconnected, and directly on top of the xps and you put your heel down on a seam, or place a table leg, it might push the adjacent boards down and damage the xps underneath. You'll need to make sure said load is spread.
    – MiG
    Feb 16, 2022 at 10:29
  • @MiG great point. I've updated the question Feb 16, 2022 at 11:00
  • :) note that tongue and groove still relies on a (relatively) solid underfloor. After all, you can also dismantle it by rotating the adjacent board up. Easiest way to deal with this is if you have two layers of that cheap flooring, one perpendicular || on the other =. This would eliminate this problem.
    – MiG
    Feb 16, 2022 at 11:08
  • 1
    XPS panels can be had in different compression strengths, imparting different firmness to the floor. Some if it (the Styrofoam we're familiar with in the US as cheap coolers) will crumble in your hands. Other kinds are used as insulation under concrete floors at the bottom of a building. It will probably cost more to get the higher strength stuff, but it would make for a more stable base for the flooring.
    – FreeMan
    Feb 16, 2022 at 12:37
  • The burning in the event of a fire is a HUGE concern. Some chemicals have code restrictions because one inhale and you are knocked out.
    – Evil Elf
    Feb 16, 2022 at 12:46

1 Answer 1


I am unsure about the need for an vapour barrier under the XPS boards.

For insulation purposes, usually XPS has a sufficiently low moisture permeability (see "perm rating") that a separate vapour barrier is not required.

Compare the rating of your product (and its thickness), to what is recommended or required in your climate zone, where exactly in Europe. Oldenburg and Almeria will be very different.

The guidelines will also depend on your application: attic or above/below ground walls, or basement floor.

I want to be on the safe side in case of an lightning strike

If lightning is a factor, all wiring entering your house must be clamped and earthed before the point of entry by a separate earthing conductor which is run outside the house.

It is wise to keep the cable fire-insulated until all electrical charge can be discharged to earth. Cement is far better than XPS. Embedding the cabling in metal tubing, e.g. "EMT" electrical conduit in North America, is an excellent and practical alternative.

XPS is flammable and provides an increased fire risk over other fill methods (cement, plywood with gutters for cabling...). In North America it is approved for insulation, including contact with cabling.

However, the front facing side of electrical boxes must not allow sparks or flames from the wiring to reach the insulation or any other in-wall flammables like wall-board paper backing, or wooden framing.

For high-voltage wire runs, a further consideration is the stipulation of allowable cable zones (Europe) and clearances (North America). Concerns arise when it is reasonably possible that a fastener (screw, nail) or drill might penetrate from the floor / sub-floor from above, or from the ceiling from underneath, which could damage or short the cabling.

Approved metallic rigid (EMT) or flexible (BX/Armoured Cable) would provide an adequate mechanical shield and fire barrier.

For TV antenna see also: Grounding tv antenna in attic?

For amateur radio antenna, see also Should I ground outdoor speaker wire? If so, how? for some references on grounding the wire itself.

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