How do you work out how experienced a builder is?

Our neighbours are having work done and the builders appear good. We have asked them for quotes including insulation of ground floor suspended timber floor above cellar. Specification appears good and they have mentioned thermal bridges and that they install solar panels but haven’t asked about cellar that has an intermittent water issue when water table high nor mentioned thermal cameras. Should I expect a builder to say what about that or do they tend to respond to what they are asked which might be the wrong thing to do first

They are strongly in favour of cellutex rock is too messy plant type gets ruined if it gets wet

Builders cannot be expected to do/request surveys or design solutions outside their given specification. Those things are down to the householder or their appointed retrofit coordinator (for which a qualification is required).

How is the typical householder meant to know this? I don’t know.

You could ask the builder whether they work with a retrofit coordinator and if so check their qualifications. If it isn’t someone with a known track record you need to shadow them like a wolf and ask them to justify recommendations. Unfortunately the role of retrofit coordinator has only recently been defined in law and with that the required qualifications. This means that there is a shortage of suitable professionals, which in turn pushes up the price.

Most Passivhaus designers are qualified and there is an AECB list of those who have passed their CarbonLite Retrofit course.

It looks as though you need a free draining breathable insulation such as Rockwool, particularly round the perimeter, all topped off with an airtight layer.

Does the cellar have adequate ventilation?

My first house (1975) had a half cellar with cobbled floor and back wall and permanently wet front wall (and to a lesser extent side walls). The only ventilation was a coal shute. I added a piece of concrete one brick thick to the bottom of the shute to stop rain running straight in, it then drained down and gradually came through the wall. It made a huge difference when I used an epoxy based paint designed for dairy walls. It soaked well into the bricks before curing, so probably didn’t flake off for years, if ever. I had the house for 2 years and never risked using the cellar.