Hello,

We are first time buyers and need some advice ASAP. We are in the process of buying a mid victorian terrace and have just got the survey back. Pretty much everything was up to scratch apart from the 1960's groundfloor bathroom extension (off kitchen) which has walls only 120mm thick making it prone to condensation and damp and heat loss. It has a flat roof. The surveyor has highly recommended that we get internal dry linings ("water resistant insulation board") so that it meets building regulations. However if we do this he has said it is highly unlikely we will be able to fit a bath in. It is not showing any signs of damp yet.

To be fair the bathroom was our only real project - it is outdated with some wierd semi partition between the bath and sink and the toilet. We were going to strip it when we had saved up enough and redo it (new suit and hopefully knock down the wall). But the insulation thing is a bit of a blow as is the possibility of no bath. Does anybody have any idea how much this type of insulation costs and should we ask for some money off the price we are planning to pay? We are very eager to be as environmentally friendly as possible in any of our renovations.

Any advice greatly appreciated.

What length have you got brickwork to brickwork for the bath? You can get baths in all sizes, not just standard 1700mm.

KT, sounds as if the surveyor is talking about tilebacker board.
The better ones are based on extruded polystyrene ("waterproof") insulation with a very thin reinforced render onto which you can tile directly. The insulation is available in different thicknesses.
Tilebacker is extremely popular in Germany and that is where the major manufacturers are i.e. Wedi, Stadur, Lux.
However Wedi have a UK business and sell through some of the tiling merchants. Have a look at www.wedi.co.uk

If you have insulated the walls and filled the joists with glass fibre insulation then water molecules will be floating around that bathroom searching for the cold area to condense and will be attracted to the roof. The water molecules will penetrate the ceiling boards through the glass fibre and condense on the cold flat roof decking. This will in turn form globules of water which will drip on the glass fibre insulation and saturate it. This will reduce the effectiveness of the insulation and the moisture will rot the wooden joist out. The build up you suggested is an unventilated cold roof. Even ventilated cold roofs have been banned in Scotland because of the major problems they cause.

My advice would be to put a warm roof using rigid insulation on the existing flat roofing membrane (which will serve as a vapour barrier). Then cover with a weatherproof membrane. This is a “warm roof” flat roof system which is the only workable solution if you seal the building up. The insulation needs to be 0.2 u -value for building regulations. Depending on the thickness the following rigid insulation meets building regulations when I checked with suppliers last week:

110mm Celotex TA3

110mm Celotex XR3000

150mm Knauf polyfoam Roof board.

The best people to talk to are the technical departments of insulation suppliers. You may want to use expanded polystyrene because it has less embodied energy. However, aesthetically you will have a thicker roof.

As we thermally insulate and make buildings less permeable there are going to be a lot of rotting flat roof decks in the future.

Cheers Steve