Why still cavity walls ?

[omph]

Correct.

Do you have any data on this ?

I do find rendering longevity to be an interesting topic - why is the no-fines stuff outliving the more modern renders on block work. I am not entirely convinced whether it is constrained to plasiticiers, or even beading, although I am sure they may be a component in render degradation.

Bare no-fines walls are very rough and provide a lot of key for the render, whereas the block work may have a few key lines but no way near as a no-fines wall. I've put a chisel to some block render and entire chunks just fall off - Its a lot like paint that is sitting on the plaster - It's there but its not really adhering.

However, with no-fines you can't take the render off, it's almost as if its a part of the wall and trying to hack it will take chunks off the wall instead. This may explain why most of these systems are still going 75 years on (even with cracks) and it is not unreasonable to infer that they could probably do another 50 years or more.

There is also the cement to sand ratio - no-fines aggregate is a lot harder than a standard block so a stronger mix can be used. The weaker mix for blocks, coupled with not enough key seems to be a major cause of disintegration of render in my experience.

It does evoke an interesting question, if this is the case then perhaps using dense blocks coupled with a stronger mix will last longer, and a 1:1 cement:sbr slurry on the wall before the first coat will ensure the coat adheres properly.

 

[phatboy]

Our house was externally insulated nearly 9 years ago, and we have in total 0 problems to date. We also extended recently, and build solid 215mm block, then externally insulated it, too.

Window replacements are much the same (We replaced some windows 7 years ago). We moved the windows to the edge of the concrete wall, so that there was no need to insulate the reveal separately. Inside we have deeper window reveals, but thats fine. The outside reveal is the thickness of the insulation (100mm)

It resolved damp issues the original building had from driving rain, coming through the 1950s pebble dash. We had no rising damp, as there was an existing DPC, and the base rail for the EWI was matched to this level, and waterproof EWI boards fitted below and into the ground 100mm.

We haven't seen any cracks, maybe because the entire building is covered in mesh in the base coat of render.

Having relatives in Europe, they build new builds with EWI, and most houses are being upgraded with it. I think a lot of the opinions against it here are from people much more familiar with cavity builds.