DIY Doctor

Another external wall insulation question

Postby colourblind » Sat Feb 20, 2010 9:37 pm

Hi all
I was a bit confused by StoneyBoy's advice to HilaryN (?) not to externally insulate her/his wall as it would stop the walls from breathing and traps moisture inside...
My question is: what are all those external insulation systems good for?

I am considering doing an external insulation, or maybe internally using Warm-a-wall, or Rubbercrete (google ""
because our outside facing wall (non cavity) gets very cold in the winter, resulting in condensation and mould growth, please note that this only happens when the wall temperature is below a certain level, so it's not a damp problem.

What is the best solution for my problem??

Many thanks in advance
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Simply Build It

Postby stoneyboy » Fri Feb 26, 2010 11:09 pm

A solid wall - especially an exposed one - will always be damp. The wall will be totally wet on the outside if rain is being driven against it and the inside will be damp - how damp will depend on the porosity of the wall.
When your house was built it probably had air bricks, open fires and windows without draught seals so damp in the external walls was not a problem. Seal the house up and there is no-where for the damp to go = wet walls, because the walls are wet evaporation makes the wall colder = condensation and that = mould.
Covering the walls with insulation makes the wall colder and depending on the insulation method used seals in the damp. The main problem is around the perimeter of the insulated wall, this will be the place where warm moist air condenses and this can have the effect that condensate (water) comes out at the bottom of the insulated wall.
I would not recommend directly insulating a solid external wall.
I would recommend that you form an insulated wall inside the existing solid wall with a ventilated gap between them.
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