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 12.5mm 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?

Thanks...have already fitted the 15mm insulation and already purchased the Hardiebacker and moisture resistant plasterboards ready to fit, so ideally want to make use of them all.

If I did buy Jackoboard/Marmox instead to use in the shower area, what about the rest of the bathrooms external walls, where I was planning to put moisture resistant plasterboard over the 15mm insulation, and tile over? Or would I have to put Jackoboard on ALL external walls instead?

Perhaps ditch the tiles. Horrible things with porous grout. Go for a sheet material, wet wall or whatever.

Ref insulation over the studs. I built an entire 4 bed house with 50mm over the timber studs (Rockwool Flexi between) and a huge brick barn conversion (265sqm) nearly twenty years ago too. I was using Fermacell in the all areas even vaguely damp and everywhere with anything weighty hanging off the walls. No problems to report.

Moisture resistant plasterboard is a pretty cheap way of getting a noticeably better performance, so I would use it rather than ordinary plasterboard. But I expect the ordinary stuff would be OK.

I agree with Fred about the surface treatment. My wife hates cleaning grout so we have no tiles and went with sheet surfaces everywhere. They're a lot better than they used to be.

If youre insulating behind tiles on internal and external walls, the walls are likely to stay warmer and water will dry quicker. If you wipe down after showering with a window cleaners wiper youll get most of the water off and tiles will be dry in no time. This should keep any mould in check........ having insulated our internal walls and wiping down, we're hoping so!!

LoftDIY87 - I have used Hardibacker board for the walls of a shower enclosure, admittedly not with insulation behind as the walls were both internal. However I really cannot see why, with sufficiently long screws, you cannot use the Hardi board in your situation.

OK the stuff is heavy but if the bottom sheet is resting on the floor then the screws are only keeping the board in place rather than taking any weight. Celotex is not easily crushed, so there is no fear of the Hardi board cracking as you tighten the screws – maybe don’t over-tighten though, just tighten by hand for the last few turns?

Re tiles vs sheet material. I have always gone for tiles but I think if I ever did a shower enclosure again I would opt for the latter. We use one of those Karcher window cleaning suction gadgets and it is brilliant at keeping the grout clean but it is quite a faff to use – probably takes longer to dry the tiles than to have a shower!

If you are using Hardie backer on wet areas you should still fully tank the Hardie backer (either with a taking paste or tanking membrane) as it's water resistant only, not water proof. If you use a XPS board such as jackoboard or wediboard etc then these are waterproof and only need tanking at the joints (with a tanking tape). As you've got some Hardie already, grab yourself some jackoboard washers and just go overboard with the fixings through the insulation you have and in to the studs behind.. Hardie won't guarantee the work but you should be okay with a common sense approach

With regards to the rest of the walls that you plan to tile, personally I would not waste your money on MR boards and use standard boards instead. I only use MR boards for ceilings in bathrooms and for any walls that aren't to be tiled. And then taping and jointing instead of plastering as they're not designed to be plastered

Not sure i'd use it over screws but whats this stuff? Looks like an adhesive for fixing Hardie Backer board to insulation (among other things) prior to tiling...


https://www.ctdtrade.co.uk/preparation/tile-backer-boards/hardie-backer-fixing-foam

Quote..

Hardie Backer Fixing Foam
D0008368
Type: Board Adhesive
Brand: James Hardie
Size: 750ml

Key Features: Ready to tile and plaster in 1 hour

It can be applied on all common surfaces such as...... snip....insulation board..snip...polystyrene...

Thanks for comments all.

After all your input I think I will stick with the 15mm PIR behind Hardiebacker in the shower area and 15mm PIR behind moisture plasterboard in the other external wall areas. Seeing as I have already purchased these materials (and fit the 15mm PIR), will be a waste to redo it all if the current setup will still securely hold with long screws.

Speaking of, would 65-70mm screws be OK? This would be 12mm through the Hardiebacker/plasterboard, 15mm through the PIR insulation and 38mm/43mm into the stud for 65/70mm respectively.

These Turbo Golds for the Hardiebacker: https://www.screwfix.com/c/screws-nails-fixings/woodscrews/cat840066?brand=turbogold#category=cat840066&brand=turbogold&productlength=70_mm

And drywall screws for the plasterboard: https://www.toolstation.com/drywall-black-phosphate-phillips-screw/p42417

Re: Hardeibacker resting on the floor, they advise leaving a 2-3mm gap between boards and the floor, thus they wouldn't really be resting on each other and would only be fixed to the wall?

Hardiebacker has its own design of screw. I suspect they will be like a fermacell screw with a steeper countersink angle more suited to harder boards.

Regarding expansion gap and not having Hardiebackerboards resting on the floor. The official Hardie video:

https://www.jameshardie.co.uk/new-installation-video-installing-12mm-hardiebacker-onto-a-stud-wall/

shows that it is ok to leave expansion gaps and still have a secure fixing to the studs, relying on screws alone, so I don't think you have any worries as long as you use long enough screws as you have already mentioned. 3" (75mm) should be ample!

Posted By: ealingbadgerI want to be using stainless steel screws for the tiny additional cost that they represent overall.

I agree in principle but I would check what the Hardie warranty says about the use of their own screws.