Gone West

Why not glue in a looser fitting smaller dowel with epoxy wood filler. You'll use a lot of filler otherwise.

I would use 25mm dowel, glued in with PU adhesive.

Yes, we didn't use any tapes or membranes, just the exterior racking OSB3 and internal plasterboard.

https://cdn.shopify.com/s/files/1/2612/5402/files/Cool_Energy_CE-iH6_Pro_Inverter_Air_Source_Heat_Pump_Datasheet.pdf?v=1721657221

As far as I am aware a lot of people have completed this survey. Perhaps you should give your head a shake. I suppose it is a Sunday morning.

We had glue down LVT laid in our bathrooms. I fitted 10mm ply to the 22mm subfloor, filled all the screw holes, sanded then sanded and sanded again. When the chap turned up he looked at it and said I'll just prep the floor. By the time he was finished it was as smooth as a mirror. Our tiles were 2.5mm thick Amtico and the installer said that any tiny imperfection on the floor will show through the tiles. He did an amazing job.

It's not usually acceptable to use different foundation types on a house and an extension due to potential differential movement. There is an old thread about ground screws and a couple of links to Grand Designs that used ground screws. https://padstudio.co.uk/project/chestnut-farm-modern-sustainable-farmhouse/ https://www.arborarchitects.co.uk/projects/redbrook

You could use an endoscope. https://www.amazon.co.uk/SDYGDB-Endoscope-Borescope-Inspection-Waterproof/dp/B0B9T5MQ18?th=1

The Warmcel was slumping and as a result there were cold areas around the ceilings. All down to the installers I guess. What I liked about the Supafil Frame was that, at the time, they used a transparent membrane so it was possible to see how well filled it was.

Eleven years ago when I was considering what insulation to use, Supafil Frame was on my shortlist. I discounted Warmcel as a friend had used it in her house and had problems. In the end I used 350mm Icynene for the walls and roof of my timber I-beam portal frame. The only problem was trimming and cleaning up. I did use a similar product to Supafil Frame in the walls of the old bungalow I was living in, while building the house. When I dismantled the bungalow it was easy to see how well it had filled the old timber frame.

Perhaps they're trying to hurry you along. AFAIK that's not correct, with the Government increasing BUS funding to £295 million for 2025/26. https://energysavingtrust.org.uk/grants-and-loans/boiler-upgrade-scheme/

According to Wendy , that's just man cleaning, not proper cleaning! I'm keeping well out of it.

@DREG Hi and welcome to the forum. If you post details of your situation on the ASHP sub forum somebody will be along to help you. Unfortunately I'm not that person.

Yes we've got those in our wetroom, although we didn't choose them. Wendy curses them because they are so difficult to keep clean.

@BuildPentir Yes I have, and am still researching. I do have some 70s additions which are easier to deal with. I think @Roger440 has similar problems and may also be in Wales. You are not alone!!