Internal wall insulation in a flat

ComicGeek

FreeBear wrote: »

So whilst the thermal wallpaper will do something, it is no where as effective as 50mm of foam. To meet current building regs, 75mm would be the minimum. The downside to using too little insulation is that the dew point could end up inside the wall - This becomes a problem with (interstitial) condensation and damp gets trapped in the wall. The small room that I'm refurbishing had been insulated on the walls with 5mm of polystyrene. On stripping the walls, I found the plaster to be soaking wet and falling off the wall in places. Really not good...

The external wall improvement U-value of 0.3 hasn't been specifically set to deal with condensation - we don't want people to think that they must reach 75mm insulation or not bother.

Assuming that there aren't any underlying issues with damp in the wall, or routes for warm, moist internal air to bypass the polystyrene and come into direct contact with the wall, you really shouldn't get a continuous build-up of interstitial condensation as the condensation should evaporate during summer months. I suppose if the wall is north facing, like the OP's, then that might delay the evaporation process in summer. The damp doesn't get trapped in the wall, it moves from a high vapour pressure to a low vapour pressure.

What that particular brand of thermal wallpaper is trying to do is raise the internal surface temperature of the wall by increasing its reflective properties, and increase the amount of radiant heat experienced by people in the room. It wouldn't be my first solution, but if I was desperate and couldn't do anything else I would try it. We shouldn't discount the psychological effect of small changes in heating - my wife can always guess when I've turned the heating down by 1 degree!

FreeBear

ComicGeek wrote: »

The external wall improvement U-value of 0.3 hasn't been specifically set to deal with condensation - we don't want people to think that they must reach 75mm insulation or not bother.

No argument from me on that count. For a number of reasons, I'm only able to use 50mm of insulation boards on the room I'm renovating. It falls short of the target U-value of 0.3 W/m²K at an estimated 0.344 W/m²K. To mitigate this, a new window is being fitted and the loft insulation increased - Building regs allows for cases where the target U-value can not be achieved for technical or functional reasons. In which case, the best possible level of insulation is permitted with a payback of 15 years or less.