I'm looking at having underfloor heating installed in my living room, and I thought as I'll be taking up the floorboards, it would make sense to do subfloor insulation in the living room before installing the new floor. I've seen a guide here - https://www.eco-home-essentials.co.uk/underfloor-insulation.html and read comments from this forum and it looks like using an airtightness membrane is strongly recommended. However as I'll be doing this one room at a time (I probably won't change the floors in the other downstairs rooms for 4-5 years), I was wondering if this could make it difficult to form a single airtight layer when the other rooms are eventually done?

I'm also finding it difficult to find an installer with suitable knowledge in the Reading (Berkshire) area. Many insulation companies offer PIR insulation, or spray on insulation, with no mention of airtightness membranes and hygroscopic materials. I have considered doing it myself, which would help a lot financially but at the end of the day I want it to be done once and done right, and I haven't done any of this kind of stuff before. I was wondering if anyone can recommend any nationwide installers that would do a good job on this?

Posted By: tonyHow deep is your void?

52cm

You could consider fully filling the void with eps beads or Leca, need not to be in a Radon or landfill gas area,

Certainly you should insulate well which for means U=0.1 or better

Posted By: tonyYou could consider fully filling the void with eps beads or Leca, need not to be in a Radon or landfill gas area,

Certainly you should insulate well which for means U=0.1 or better

Unfortunately I am in a high Radon area

To make the room 'fully air tight' (or as good as possible) you would need to remove the skirting boards, and stick the air tight membrane to the walls beneath. Then replace the skirtingboard.

Between rooms, I guess you just overlap the membrane at the threshold and join with double sided tape.