Hi Guys,

My first post so be gentle. We are about to buy a detached bungalow sitting half way up a hill in a village in North Yorkshire. The bungalow is circa 1970's and is a wreck. It is cavity wall construction with no cavity closers. The inner skin is cheap clinker type block and the outer skin is cement brick with a stone style riven finish giving an uneaven outer surface. South is 'up slope' & we will get a lot of wind hitting the house from the exposed North. Think of this as a shell. The oil fired boiler (a relic), the wet rad heating system, the old immersion heater, the rotten single glazed wooden windows, minimal loft insulation, etc are all going. This is a complete renovation job. While I have a history in new builds, renovations, conversions, etc this is the first time I have deliberately set out to 'go green'. I'll search & start other posts on heating, roof insulation, double glazing, etc but for the minute it is the wall insulation I'm scratching my head over.

First off I will be insulating & making air tight as much as I can. I do not want to dry line inside as this will cut room sizes too much. As I see things my options are:

1. Rebuild the outer skin in thermal block and wet render - not my preferred choice.

2. Batten, add a vapour barrier and clad with timber or other material (options?). If I do this how about additional insulation? Any material laid against the uneaven bricks will leave air gaps - what about condensation? The only thing I can think of that wouldn't is a mat material or applied foam. Due to the property shape I could end up with a really nice chalet style exterior as a by product of improving the insulation ( I am not a fan of the dated mock stone style finish).

3. I would also like to fill the existing cavity to improve insulation. Any reasons not to (especially if I clad externally and this then becomes a 'middle wall')? Blown rockwool, foam or any other suitable materials? What about sagging and or damp transferrance?

Any advice gratfully received - especially the technical point on condensation behind any solid external insulation.

Why don't you insulate externally with a lime render finish?

Nigel, never used lime render before - any good for insulation value? Cost of application, ongoing maintenance & weathering considerations? Could combine this with simply filling the cavity.

I guess there is no reason why you can't get a grant for cavity wall insulation? I think I'd get that done before anyone tries to make out you are a developer which might disqualify you - it will be the cheapest insulation you ever get! My preference would be for PU foam in the cavity.

Then I'd calculate the U-value of the wall and work out what thickness of insulation I needed to apply externally to get the U-value down to PassivHaus levels, say 0.1. If you search for "Build Desk" you can download some software that will do this for you. Look on the Kingspan website for Kooltherm. I guess you might have to render the walls to get them flat before fixing the boards.