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Boarding and tiling over Celotex insulation

Discussion in 'Tiling' started by jamie765, 29 Jun 2020.

  1. jamie765

    jamie765

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    I have a studded bathroom with 90mm insulation between studs and 15mm insulation over the studs, which I intend to board and fully tile.

    Want to fit 12mm Hardiebacker board in the shower area and 12mm moisture plasterboard on the rest of the walls, then tile.

    Just called Hardiebacker and they said their boards should only be fitted direct to the studs, NOT have any insulation in between (even if only 15mm)? As they can't guarantee it'll hold the weight?

    How can this be dealt with? Would the same issue not arise when plasterboarding over the 15mm insulation and tiling on top?

    I would have thought long screws were the answer but now am a bit confused on which direction to go, any guidance?
     
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  3. Mr Chibs

    Mr Chibs

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    Lots of screws should be fine, you are using studs of 90mm, so there's plenty of depth to go through and hold, I have celotex 50mm over wood, with boards and haven't had an issue.

    As a thought, what about the STS tiling board that the likes of wickes sell?
    Around 10mm thick and a lot less weight... if you are worried?
     
  4. jamie765

    jamie765

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    Thanks, when you say you have 50mm Celotex over wood, is that boarded and tiled?

    I wasn't concerned about it until I called Hardiebacker this morning just to double check and they told me their boards need to be fitted direct to the stud, not what I wanted to hear...

    However I'm sure this is a common occurrence in bathroom external wall insulations where tiles are laid on plasterboard with insulation behind, no? I'd rather not use STS tiling board/Marmox etc on ALL external walls if I don't have to, particularly since I've already fitted the 15mm insulation over studs and purchased the Hardiebacker and moisture resistant boards, ready to fit!
     
  5. Mr Chibs

    Mr Chibs

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    Yes, boarded and tiled, floor to ceiling.
    Most of mine are on plasterboard, but they been doubled 2 x 12.5mm up on some walls.

    Nothing has fallen off so far

    Don’t think I would worry, what centres are your studs?
     
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  7. jamie765

    jamie765

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    Studs are at 450mm centres

    I'm thinking to put 20mm Marmox board in the shower area instead, and continue with the 15mm PIR behind 12.5mm moisture resistant plasterboard on the rest of the external bathroom walls (and just moisture resistant plasterboard direct to studs on the two internal bathroom walls).

    Although it's strange you can't tile over Hardeibacker on top of 15mm insulation but you can tile over 60mm thick Marmox board, which has 58mm of insulation behind it anyway!
     
  8. Mr Chibs

    Mr Chibs

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    Why can you not tile over HB on top of PIR? Weight?

    Marmox I think is like the STS board previously mentioned. Two thin cement based layers with PIR (similar) centre all bonded together.
    If a 60mm marmox is allowed, then this must be just a gluing issue.

    Not wanting to doubt, but I've got a 50mm celotex over a batten, with 25mm of plasterboard, 3mm skim tile adhesive and tiles, over a year and half later, nothing has moved, and I don't expect it to.

    I know its going to work out dearer, but if you are concerned, could you not ditch the 15mm celotex and replace with thin marmox/sts, you'll still benefit from covering the studs, but will be able to HB over it... apply a cement slurry as glue and screw to studs.
     
  9. jamie765

    jamie765

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    I guess it's the weight yeah/issue of having PIR in between the Hardiebacker/plasterboard and the studs. I didn't think it would be an issue especially as its only 15mm PIR, however Hardiebacker told me their products should be fixed direct to the studs or battens.

    I'd rather be safe than sorry, however if its normally practice to just use long screws when sandwiching PIR in between studs and plasterboards/cement boards then I'd much rather go down that route, especially as I've already fit all the 15mm PIR!

    There's two external walls which I've put the 15mm PIR over, plan was to Hardiebacker just over the external wall which has the shower in it, and moisture resistant plasterboard the rest of the walls (ie. Moisture plasterboard on the two internal walls and rest of the external walls too).

    I guess my original question was, is it OK to use long screws with plasterboard over PIR insulation, then fully tile.

    FYI I calculated the 12mm Hardiebacker to be 14.37kg per m2, and Knauf 12m.5mm moisture resistant plasterboard to be 9.5kg per m2. I'll also likely skip the skimming before tiling, to reduce weight load (and walls would/should be even anyway as they're all newly built)
     
  10. Mr Chibs

    Mr Chibs

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    Personally it wouldn’t bother me.
    I used 90mm screws. Job done.

    Generally when screwing plasterboard on top of pir, it’s the plasterboard that allows the screw to pull through if you overtighten, this won’t be a problem for Hardiebacker, as it’s tough as old boots.
     
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