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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorAshley
    • CommentTimeNov 1st 2020
     
    Hi,

    I am looking to insulate the external wall of my bathroom. The wall is 1970s built with brick / 50mm cavity (with CWI) / block. I'd like to fit 50mm of celotex, 12mm marmo multiboard (or similar) where the shower is and then tile. For the non-wet areas of the bathroom I intend to fit moisture resistant plasterboard over the celotex and then tile.

    I have space for a maximum of 50mm insulation + 12.5mm platerboard + maybe a few mm more if needed.

    I have read loads on various sites (and on here) about the recommended approach to fixing the marmox boards over celotex, and that is to batten out the wall and then attach the marmox boards. With this approach, I have 2 choices:

    1. batten out the wall (300mm centers), cut and fit celotex inbetween and then board over. Whilst cutting celotext is a faff, the battens fixed to the wall worsens my overall insulation

    2. celotex the wall (would probably glue it on with insta stik), fix in place with battens over the insulation and then fix the marmox boards on to those battens. For this I would need 25mm celotex over the wall, 25mm battens and then cut and fit more 25mm celotex in the gaps around the battens

    However, what I'd like to do is fit 50mm celotex (glued and mechanically fixed to the wall), and then fit the marmox multiboard directly on top of this, again glued and mechanically fixed to the wall. To me this gives the best insulation, however, the advice from Marmox is that board wouldn't be stable enough for the tiles.

    I have recently used my approach for fixing celotext and plasterboard to bedroom wall (to be skim coated) and the end result is an extremely strong fixing. So I was wondering, has anyone fitted these marmox type board in the way I'd like to do it? Are the manufacturers being overly cautious?

    Thanks in advance for any advice / suggestions,

    Ashley
    • CommentAuthortony
    • CommentTimeNov 1st 2020
     
    I can’t see how it would not be stable, the sheet insulation is effectively incompressible

    I would add mechanical fixings. I would not use tiles but go panels for shower/bathrooms instead, possibly no need for the backing board then either
    • CommentAuthorEd Davies
    • CommentTimeNov 1st 2020
     
    Don't forget that you (or somebody else) might need to be able to fit hand holds in and around the shower at some point.
    • CommentAuthorowlman
    • CommentTimeNov 1st 2020
     
    Why are you complicating things by using 3 different materials, Celotex, Marmox, and plasterboard? Why don't you just use 50mm Marmox and then you've got the perfect insulated tiling surface, or have I missed something. BTW personally I don't like tiling onto PB, you're tiling onto cardboard.
  1.  
    As I have posted many times in the past my attic shower room, refurbished over 30 years ago, has XPS on adhesive dabs (*yes, with lots of cold air channels - we knew no better then - it was the manuf's recommended fixing system!!) and no mech fixings. Tiles directly onto the XPS - no primer, PVA, nowt. Absolutely rock-sold, still. I cannot see how your Marmox (XPS) with a GRP (I think) layer would perform any less satisfactorily. Like Owlman I think you are complicating it with Celotex and Marmox over.

    Claims may have changed but 'back in the day' IIRC, XPS had a claimed lambda of 0.030W/mK. Celotex has 0.022. ** Just looked at a Kingspan XPS board and they claim 0.035. This would change the 'all XPS' calc below to a U value of 0.28.**

    Assumed filled cavity U value 0.55 (per Part L1B) then R = 1.81. R of 50mm Celotex 2.27; R of 12mm XPS 0.4 Total R 4.48. 1/4.48 = U = 0.22W/m2K

    Repeat for 60mm XPS and no Celotex:

    Base case wall as is: R = 1.81

    60mm XPS R = 2.0

    Total R = 3.81

    1/3.81= U = 0.263W/m2K

    Calcs are approx but indicative.

    You could forget the trade name and just buy 60 (or 65 if you can get it) XPS.

    Hope that helps.

    Nick

    Edit: **See comment between asterisks above**

    Edit 2: Ed Davies has a point re grab-handles, but in extremis those could be through-screwed to the masonry wall. Gun silicone into the hole after the plug and screw through the uncured silicone in an attempt to 'make good' the VCL. (For anyone not familiar with it, XPS is fully vapour-closed and thus is its own VCL, but not on the joints! Consider 'rubbing in' adjoining boards with a thin smear of silicone.
    • CommentAuthorJeff B
    • CommentTimeNov 1st 2020
     
    Posted By: tonyI can’t see how it would not be stable, the sheet insulation is effectively incompressible

    I would add mechanical fixings. I would not use tiles but go panels for shower/bathrooms instead, possibly no need for the backing board then either


    +1. Agree with Tony - if I had my time over I would have used wall panels not tiles in our main shower. I used plastic panels in our upstairs shower room - no future hassle with discoloured grout, mould etc, - brilliant!

    I am not familiar with Marmox but I have used Hardibacker board screwed to battens with Celotex under and between the battens in a similar situation to you.
    • CommentAuthorphiledge
    • CommentTimeNov 1st 2020
     
    Jackoboard straight onto the block and tiles straight onto the jackoboard after taping the joints and sealing the fixings. All waterproof and insulating😊
    • CommentAuthorAshley
    • CommentTimeNov 1st 2020
     
    Thanks for all the comments, and for the detailed r-value info Nick.

    As to why I am mixing Celotex + Marmox, and also Celotex + plasterboard, it is to do with the insulation levels for the thickness and the cost. These are the figures I am working to:

    r-value: from my calcs, 50mm celotex has an r value of 2.25, 50mm of marmox is 1.7 - so celotex is nearly 50% more.

    cost: celotex cost £17 / sheet + £32 for the marmox = £49, to do the same area in marmox would cost around £70. For the non- wet areas the cost difference is even greater.

    Given the area I'm working with, neither are deal breakers, but if I could have better insulation for cheaper cost then that is always going to be appealing

    thanks again...food for thought...
  2.  
    Another vote here for shower panel- you could glue the foam insulation to the wall, then glue the shower panel onto the insulation, no need for tile backer. It's very strong (basically plywood) and you can screw into it and easy to DIY.

    But - how flat and square are your walls? Mine turned out not to be (old house) and fitting flat sheet materials was a faff. Using battens had the advantage that they could be packed out to make a flat surface, though this is also a faff. Also the thickness of the batten never seems exactly to match the thickness of the insulation (so more packing). Hammer-in frame fixings worked well for us to fasten battens through insulation, once the batten was compressed tightly against the insulation it didn't move.
    •  
      CommentAuthordjh
    • CommentTimeNov 2nd 2020
     
    Shower panels come in (at least ) two types. There's the type that use plywood or similar as a backing where I worry about rot in time, and there's the type that use XPS as a backing, where I don't. We used the XPS type over a tile backer board, and used the proprietary adhesive to get a watertight finish. We have fixed a handrail into the XPS+tilebacker and it's rock solid. (Shower trays can be very slippery when new!). The tilebacker is mounted onto resilient bars BTW.
    • CommentAuthorJeff B
    • CommentTimeNov 2nd 2020
     
    Posted By: WillInAberdeenAnother vote here for shower panel- you could glue the foam insulation to the wall, then glue the shower panel onto the insulation, no need for tile backer. It's very strong (basically plywood) and you can screw into it and easy to DIY.

    But - how flat and square are your walls? Mine turned out not to be (old house) and fitting flat sheet materials was a faff. Using battens had the advantage that they could be packed out to make a flat surface, though this is also a faff. Also the thickness of the batten never seems exactly to match the thickness of the insulation (so more packing). Hammer-in frame fixings worked well for us to fasten battens through insulation, once the batten was compressed tightly against the insulation it didn't move.


    Agree that not having flat/square walls is a pain but having been there and done it (at my son's house) it is worth all the faff as it only has to be done once, hopefully! Once all the battens are in place the rest of the job is easy. I had never heard of Jackoboard until the name appeared earlier in this thread but I would definitely consider this if ever I have to do a shower wall job again.
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