Mr Punter

Welcome. Good progress and the elevations look smart. Be aware that zinc can be very costly. Any floor plans? Are the SIPs people leaving it watertight?

I am not convinced you will get your money back with UFH. I would go for 50mm PIR with chipboard overlay and rads. To dig it out is quite an undertaking. 150m2 is a large area. You could end up with 70 tonne of material removed.

If you can get a structural engineer to design something cheap, it would be your best bet. Having rain on it getting in all the gaps and behind the wall will be making it far worse.

I have mixed screed for a plasterer before in a normal cement mixer. He was very strict about how the mix was and I was not allowed to tip the mix until he gave his approval.

You can get joist pockets for built in joists. I have never used them so cannot vouch.

No, NHBC just recommend it as best practice. IMO it was included without any real evidence or thought and the risk outweighs any benefit. Can you point me to any evidence that having a sleeved vent through a cavity wall has ever caused any damage, let alone the "thousands" you mention?

If you gaffer tape a 400x400 piece of clear polythene on the screed where it won't get damaged you can see if any vapour condenses on the underside, or dark patches on the screed, indicating the floor is not dry.

The intumescent paints tend to go fairly mouldy unless they are covered. Best to give a quick brush of loose rust and a couple of coats of quality red oxide before you start with the intumescent.

No issue. It will cure under water. Rapid drying can lead to weakness and cracking. With temperatures above 5 degrees there will be no problems. They obviously want some of it to dry more quickly, hence the hot air. It still needs a really long time before you lay finishes.

You can use a fireproof barrier like Rockwool cavity sock. Stuff it in before you get the wall plate and trusses on. Get your roof insulation to meet with it so you don't have any gaps around the eaves.

It doesn't look too bad to me. No gaps between boards. I would be tempted to squirt in some low expansion foam or squash in some Rockwool to form a continuous barrier at this level and get them to fit the boards a bit snugger for the rest. Also, make sure that at the top the cavity is very well sealed.

Can't you have skylights are reversible that you can clean from the inside?

I never see them being built in as the build progresses and never see a tray put in above. The chances are the sparky or plumber will locate the duct or flue elsewhere in any case and they will not be dimensioned up on the construction elevation drawings, so no chance a tray will ever appear in the right place.

If the insulation is contiguous, with no gaps between the boards, there should not be massive amounts of outside air circulating between the boards and the blockwork. For the rest of the wall, make sure the blockwork is built up with wall ties and retaining clips and mortar cleaned off as it progresses. I think that dry sand may cause issues should you need to drill through the wall in future. Also, if the gap is quite wide you would need lots.

The point there was if you install the duct before the wall is complete it may gather mortar droppings. In reality the hole is drilled after and the link says is is not essential "Where a duct to an extract fan or boiler flue is installed through a core-drilled hole after completion of the wall it is not essential to provide a cavity tray although the provision of a tray during the construction of the wall is considered good practice."

There is. It has the sun motif between the deco style piers.

Tried and trusted.

The energy market is very odd. It is often best just to choose a supplier who will get the meters in, then change later.

No 1 recommendation in some of my rentals is fit a gas boiler!

If you have the money to do it, get it all done now. Living on a building site is a nightmare and life is short. Excavating the slab and adding lots of insulation is a good plan but expensive. Consider external wall insulation. For the roof, 370mm loft insulation is fairly cheap and simple to install. Between and under rafters PIR is tricky and more expensive. Forget the woodburner unless you want drafts and pollution. Consider solar PV instead of thermal as the energy produced is more flexible. Once you have decided, may want to get a specification and costings done. If it look horrendous you can trim some things back. Maybe get a few builders to quote for the main works and leave them to project manage.

I have found that once the blocks are in and grouted you have to smack the hell out of them to remove one.

These people https://urban-water.co.uk/sequential-and-exception-test/ are quoting £800-£2000. Why do you think you only have a 25% chance?

Well that's at least a year spent.

With masonry party walls we would use use the thin type A wall ties, medium density block plastered and 75mm cavity - either with or without insulation. For timber and metal frame walls and floors, look at the BG White Book and select what suits. You really need to go about it as if the rooms that want soundproofing are separate flats.

Fair enough. Some systems are more adaptable to different reinforcing etc so best to have someone who has worked with it before. It is so annoying when you don't get emails and calls returned, but commonplace I am afraid.