IWI in Conversion Leasehold Flat

[rflynn]

Hi folks

Any help/advice is much appreciated

I am currently in the process of buying a leasehold Edwardian terrace Flat conversion in London.

Naturally I want to make the property as efficient as possible and I am currently looking into internal wall insulation. The property is of solid brick conversion

The issue is the outside of the property (owned and the responsibility of the freeholder) appears to have a terylene render. Internally the walls appear to have modern cement plaster as well

My question is do I go for vapour permeable insulation materials or as everything in place already is not as permeable do I continue in the same pattern. As it is leasehold I can't remove the external resin and I worry by using vapour permeable materials on the internal side it's going lead to moisture being trapped behind the cement resin

Thanks for any help

 

[aockenden]

If you’re going to install an insulation material against a cold wall there is the chance of condensation forming between the insulation material and the cold surface. That’s essentially an unavoidable risk in the real world. So once you’ve accepted that, now you need to provide a way for that condensed water to get back out and not get trapped (and start to grow mold / progressively saturate the wall etc).

The water that condenses between the cold wall and the insulation material has THREE general routes to escape:

1. Ventilate the area between the insulation and the cold wall with relatively dry air. This is technically challenging in a retrofit scenario like yours and uses more floor space.

2. Wick out through the wall itself and evaporate out of the building to the exterior. This is not going to happen with a cement internal render AND K-rend on the outside. So barring removing all that and replacing with clay or lime we are left with option 3.

3. Wick out through the insulation material itself, through the plaster, through the paint and then evaporate out into the interior atmosphere in the room.

If you use an impervious insulation / plaster / paint internally AND you leave the internal cement / external k-rend in place AND you don’t create a thoroughly ventilated gap between the insulation and the wall, then you should expect condensation to build up, soak the wall, and start to mold.

 

[paulc]

If you’re going to install an insulation material against a cold wall there is the chance of condensation forming between the insulation material and the cold surface. That’s essentially an unavoidable risk in the real world.

To a certain extent, you can prevent moist warm air getting behind the insulation by using a vapour control membrane. However, it is difficult to get a perfect seal along the top & bottom and around any openings when retrofitting.

 

[aockenden]

To a certain extent, you can prevent moist warm air getting behind the insulation by using a vapour control membrane. However, it is difficult to get a perfect seal along the top & bottom and around any openings when retrofitting.

Thus my statement: in the real world there is ALWAYS a risk. We don’t live in laboratories. And when you are a fallable human being and don’t get a perfect seal now your membrane serves to trap moisture even more than any gypsum or cement or plastic paint.