External insulation

[Paul Cumbria]

I am considering having my house re-rendered (1929 semi detached ex council 3 bed, currently rendered top half only). One of my neighbours works in the building trade and is having his done soon, when one of his trusted sub-contractors has a gap in his schedule - he is very busy at present with new builds. As all the local builders arrive on horseback and ride off in to the sunset the earliest opportunity, I am minded to go with my neighbour's recommendation and wait until this guy becomes available. I will probably opt for new render over the whole house as the lower brickwork is looking tired and could do with a makeover. My neighbour's advice is to render over the existing upper floor render (using a mesh I believe) as knocking off the old stuff could cause vibration damage etc to the old walls - I can go with that also. Any advice on the above would be welcome.
I have a couple of questions - there is a proposal that external insulation be fitted before being rendered over. First of all, would this be breathable? One of my main reasons for re-rendering is that the existing render, although in fairly good condition, has many small cracks allowing moisture ingress and the underlying brickwork here is the soft porous type (presumably to keep construction costs down - the lower half brickwork is decent quality waterproof brick) so it is imperative that any existing moisture be allowed to evaporate out; I am assured that the new render will be breathable. Secondly, would EWI make any real difference if fitted to the outside of a cavity wall? I can see the point in a a single skin wall but don't think the extra cost involved would pay back in terms of reduced heating bills in a reasonable timescale. (I had the cavity wall insulation removed several years ago due to worsening damp problems, but that in another story.......)

 

[Mathieu31]

Do you have a vmc in your house. The more you insulate, the less you prevent the air from circulating. It is however absolutely necessary that the air circulates to manage the problems of moisture

 

[Paul Cumbria]

Hi Mathieu, and thanks for your reply. There is no vmc in the house, other than extract fans in the kitchen and bathroom. I agree entirely on the question of air circulation, this is one of the reasons I had the cavity wall insulation removed - with no air movement in the cavity the walls would never dry out. I could write several pages on the issues I have found with damp, but I think I have addressed them all now, and if moisture is no longer ingressing in the volumes it has in the past, then the walls should dry out nicely.

My main question was to the efficacy of fitting insulation on the outside of a cavity wall - I can't see it making that much difference in terms of heat insulation, and it will be an extra thickness for existing moisture to wick or evaporate out of.