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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    Please could anyone advise? We are replacing our bathroom and would like to improve insulation in the process. We live in a solid wall 1930’s property. The bathroom has two external walls with sloped skeilings. We currently get a lot of condensation build up in the colder months after a showering, especially on the skeilings.

    We are thinking to ask the tiler/plasterer to dry-line the external walls with insulation board, as well as the skeilings (the flat part of the ceiling has a thick layer of loft insulation on it). Does this sound a good idea? Should we also dry-line the flat part of the ceiling and the internal walls as well?!

    Been reading others comments and now I’m worried about cold spots and interstitial condensation –any advice welcome, we have no expertise in building matters and are pretty baffled!! Thanks.
    First thing - I always bang on about this! - you need Building Control approval, as you would be 'renovating' (adding or replacing a layer on) more than 50% of the external walls in a room. That means you will either need to improve the U value of the walls from the current 2.1W/m2K (or 1.7W/m2K depending on which version of SAP you look at) to 0.3W/m2K (about 65mm of Polyurethane - Pu), or you need to invoke one of the 'get-out clauses' (including the one about 'traditionally-built' buildings and breathability).

    I am much less comfortable with vapour-control methods of insulation than I am with breathable (water-vapour-permeable) ones, but you would want, for example, to think twice before using wood-fibre in a bathroom.

    If (as you should do) you are trying to get a really, really 'tight' vapour control layer (VCL), I would not recommend composite insulation/plasterboards, as you cannot guarantee the VCL on the joints. Use separate insulation boards and plasterboard instead.

    Enough for now, but plenty more help if you need it.
    • CommentAuthorslow
    • CommentTimeMay 14th 2018
    Several years ago I lined the bathroom upstairs in a solid brick Victorian terrace with insulating plasterboard (room too small for any other solution), using a thin adhesive over the entire sheet to stick it to the bricks. Unless any void is actually properly ventilated, the batten-off method allows a nasty black mould to develop in the void, because internal insulation often moves the dew point right in to the internal face of the external wall.

    I did the whole ceiling with insulated plasterboard too (flat area & sloped area). Since then I’ve repeated this on several properties, as the performance is great. As Nick Parsons points out above, this needs to be run past BC. I have never had any issues with tthe approach above, as they take the view that this is the most pragmatic solution in what is the small rear upstairs room, that in our Bristol terraces has two external walls and a shallow pitch roof void. I’ve solved mouldy, cold bathrooms & kids bedrooms in this same room this way, and BC have always been happy. I do provide them with dew point calcs, but they’re often less aware of options than any client that has done a bit of research.

    My preferred method, which I’ve done three times now to resolve cold rooms, is to add a hemp plaster at 25-30mm to the inner face of the external walls, insulated plasterboard to the ceiling, and paint them both with Earthborn claypaint. These rooms feel warmer (perception) than the plain insulated rooms, because the claypaint buffers the moisture levels suprisingly effectively, especially in bathrooms. I do also always put in a ventilation fan with a long run-on time, though.

    Have fun :)
    • CommentAuthortony
    • CommentTimeMay 14th 2018
    For me 80 or 90 mm insulated plasterboard foamed onto outside walls, insulate ceiling to a better standard then refit bathroom.
    Thank you very much to all three of you for your helpful advice. Really appreciate it.
    One other question: we have been recommended Marmox multiboard by a chartered surveyor, as the insulating board for the internal face of our two external walls. Apparently it is particularly suitable for bathrooms and wetrooms as its waterproof. It is a product familiar to tilers but less so to other trades. Does anyone have experience of insulating walls with this product please? Thanks very much.
    • CommentTimeMay 21st 2018
    I haven't used that specific product, but I have used XPS-based panels (Bushboard Nuance) in my bathrooms for the same reasons of waterproofness. I believe the Marmox product is good quality and considered it myself, but I went for an all-in-one product with a finished surface since I didn't care about the insulation value in my case.

    It looks like the joints between Multiboard panels are taped together to make a waterproof whole. With the Nuance, you glue the T&G joints together.
    Back to my mantra that tiling directly onto XPS worked very well for a shower cubicle for many years. In fact (memory worsens as I get old!) I think perhaps I did the same when I replaced the shower cubicle and re-hashed the room, so it probably still is working well.
    • CommentAuthorgyrogear
    • CommentTimeMay 21st 2018 edited
    Posted By: Nick Parsonstiling directly onto XPS worked very well

    +1 I have just finished mine and the tiles have not fallen off yet !

    I used 40 mm XPS, (well scratched with a sharp piece of Dexion), white adhesive mortar (for rigid-insulation) to wall, allowed it to dry for 48 hrs, then scratched the XPS again and tiled to it with the same mortar, also back-buttering the tiles.

    Thanks very much for your feedback - good to learn from your experiences.
    Taking on board all your comments and also those of the trades who have quoted for us, we are now thinking to use Marmox mulitboard under the tiles on the floor and two external walls (which includes the shower area) but not on the ceiling as we'd initially planned.

    The idea being that we might slightly improve the insulation of the room however not to the point where we have very warm areas and then colds spots. We will improve the layer of wool loft insulation on the flat part of the ceiling. Of course the skeilings will still be the coldest area however we get the point about maintaining good loft ventilation are concerned about over insulating and creating other problems.

    Does this sound a decent plan? Thank you!
    • CommentAuthorgyrogear
    • CommentTimeMay 25th 2018
    On my skeiling I added 80mm EPS then waterproof plasterboard, then PVC strip-panelling all sealed airtight.
    (I have a 35-yr-old warm roof).

    • CommentTimeMay 25th 2018
    Posted By: ordinarybodOf course the skeilings will still be the coldest area

    You said originally that the skeilings were the area most prone to condensation. If you do nothing to improve them, then they will become even more liable to condensation. I'd be tempted to add some hygroscopic insulation and surface treatment to that area, as long as nothing outboard is going to rot. If you're concerned, then I'd add some XPS and internally some lime and/or hemp plaster and clay paint.
    • CommentAuthorgoodevans
    • CommentTimeMay 27th 2018
    Is scratched xps not going to be equivalent to eps? I thought the nature of XPS is that it is only the 'skin' formed from the extrusion that made it 'waterproof'?

    That said my gut tell me that regular eps would be good for backing - as unlike many building materials it does not wick.
    • CommentTimeMay 27th 2018
    Posted By: goodevansIs scratched xps not going to be equivalent to eps? I thought the nature of XPS is that it is only the 'skin' formed from the extrusion that made it 'waterproof'?

    I don't know but I didn't think so. My impression was that XPS is uniform throughout.
    • CommentAuthorjfb
    • CommentTimeMay 27th 2018
    What ventilation do you have in the bathroom?
    I did not scratch my XPS before tiling on it. Nothing ever 'let go'.
    Thanks very much for your feedback everyone - we clearly need to weigh up the different ways suggested. We will also be upgrading our fan! Thanks again.
    • CommentTimeJun 9th 2018

    In case this is not too late:

    1) We use single-room MHRV in the bathroom (and kitchen) to reduce condensation issues:


    2) We did add more loft insulation in general, though we still have a problem spot in the bathroom.

    In general in the rest of the house (the bathroom has not yet been tackled) we used aerogel to dry-line, eg:


    with quite a lot of thought given to vapour control.


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