Insulation and yet more insulation would be my first priority.

I daresay EWI has gone up in price since I last looked (it was about £80 per sq.m. pre-COVID) so £100 per sq.m. is probably par for the course now. IWI would be a lot cheaper and more DIY-able, but the work involved is substantial. I'm sure most of the posters on here will recommend EWI.

When you say insulation between the joists, I presume you mean the floor joists as you list your intention to top up the loft insulation as a separate item.

I would wait to see what effect the extra insulation has had before considering an ASHP.

Why are there two boilers? And what power/type are they? How many people live there?

Topping up loft insulation is probably a no brainer. Just do it unless there's any reason not to. 270 mm is the government target, I believe. 400 mm seems more reasonable on here.

I have never used Diathonite but I get the impression from some users that you have to sit down a long time once you know the price (esp. for spraying, which seems to be the way to do it.

EWI would be my first choice. JeffB, I think you were v lucky to find £80 pre-covid. Lots of contractors 'talk' £100, but once enabling works are added in the costs shoot up.The last EWI I supervised in about 2018 or 2019 was about £160 incl enabling works and scaff, for 100mm gEPS.

Agree with the retrofit plan. Plan for the Whole, even if you do it in parts.

Posted By: Nick ParsonsI have never used Diathonite but I get the impression from some users that you have to sit down a long time once you know the price (esp. for spraying, which seems to be the way to do it.

EWI would be my first choice. JeffB, I think you were v lucky to find £80 pre-covid. Lots of contractors 'talk' £100, but once enabling works are added in the costs shoot up.The last EWI I supervised in about 2018 or 2019 was about £160 incl enabling works and scaff, for 100mm gEPS.

Agree with the retrofit plan. Plan for the Whole, even if you do it in parts.

Wow, so I'm definitely out of touch then. Even at £80 it was out the question for our place (100 sq.m.), so at £160.....!!

Remember, much of the work/materials would be zero VAT rated, if you use VAT registered contractors (perhaps that's why you noted the budget ex VAT?

This HMRC link details the inclusions;

https://www.gov.uk/guidance/vat-on-energy-saving-materials-and-heating-equipment-notice-7086

As regards your list;
1. EWI - might be half your budget, but I wouldn't consider NOT doing it.

2. Insulation between joists - assume under your ground floors? various ways to do this, but I always use VPM to support the wool type insulation. This is a more DIYable task, esp is lifting floors, as can do one room at a time, at your own speed. Of course, you'll then have to pay standard VAT on the materials, but free labour.

3. Top-up of loft insulation - yes, after you've done any other rewiring etc, so it doesn't get trod over repeatedly. 3 layers, each perpendicular to the previous layer. Use 140mm, to give you 420mm, uValue of around 0.1 W/m2K.

4. Solar PV with batteries - likely £15k ish, so maybe not at this time?

5. Potentially replacing one boiler with an air source heat pump? - after insulation, you may find one is obsolete anyway. Would need to understand the reason for 2 boilers at the moment. Possibly due geographical pipe work, rather than total power output, but detail out how it runs right now, and then consider how to delete one. Also consider what additional minor changes may allow an ASHP to replace the remaining boiler in the future.

Posted By: Peter_in_HungaryBut any electrics should be above the insulation or against the plasterboard ceiling but not in the middle of the insulation i.e. between the new and the old.

I think GreenPaddy's comment was about the likelihood of the insulation being compressed below feet if the wiring was done after the insulation was placed. Not about the wiring itself.

Posted By: RenovationnoobThe retrofit plan sounds like a good idea. Does anyone have any recommendations of providers? There are so many options online and I assume more than half are cowboys...! We are based on the Surrey/London border.

Whilst I should be recommending my own profession (architect), I'd actually suggest you approach one of the charities that are likely to be a bit more impartial first.

https://retrofit.coop/

is probably a good place to start

Just to clarify, I always get the cabling fixed to the rafter undersides, about 500m above the ceiling ties, so it's not under the insulation. Drops then don't travel through much insulation. So you're both correct djh & PiH

Posted By: philedgeBedroom/upstairs socket circuits again will likely be carrying a fraction of the current that they are capable of so well over sized for the likely load.

The problem is when someone plugs in a fan heater, an electric kettle or an electric radiator etc. The usual way around the problem is to swap the MCB for one that suits the cable under the insulation.

If you are rewiring then not much extra in hanging the cabling on the rafters, although there are restriction on the drops and how much insulation the cable can go through before impacting the current carrying capacity.

A 3000w fan heater is 13A
If more than 500mm of the cable is in insulation then the derating factor is 0.5. So your 32A ring main is down to 16A. You don't much else on the ring main to take it over the top.