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.

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 :)

Thank you very much to all three of you for your helpful advice. Really appreciate it.

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.

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.

gg

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!

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.

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.

I did not scratch my XPS before tiling on it. Nothing ever 'let go'.

Hi,

In case this is not too late:

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

http://www.earth.org.uk/MHRV-mechanical-heat-recovery-ventilation.html

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:

http://www.earth.org.uk/note-on-Spacetherm-aerogel-thermal-insulation.html#Spacetherm-F

with quite a lot of thought given to vapour control.

Rgds

Damon