Hi

Just having a head scratch about how I’d tackle the cavity wall considerations.

So we obtained planning to add 100mm EWI to whole of the uninsulated walls of our property.

We are terraced so no brainer for main part and front of property which is solid stone, but the rear we’ve some complications as we’ve 2 old brick extensions 2 stories, then a modern single story flat roof off those two. So it’s what has become internal or is still an external wall for those.

1. First old extension is solid brick - no cavity so that will need to be EWI only.
2. Second old extension - we have cavities for these and most is now internal except for above the new flat roof - only about 4sqm as there’s a small bedroom window, and just a bit above a stairwell. Then we’ve a downstairs loo but there’s no external access to 2.5m wall as 100mm gap (can that be filled?) between us and neighbour all
Other loo wall is internal now but that’s less than 1m and has an old window . Upstairs above the loo is the small bedroom and its wall would be accessible above neighbours extension from outside, but bedroom has sloping ceiling and so it’s only about 1.8m height of wall to access by 2.5m

In the loft there is some access to the top of the cavities.

So in the areas with difficult access it is possible to just knock a few bricks out here and there for internal access and sort it all that way given the small areas if need to fill and if so what is best material? I’m pretty sure I’m best to fill cavities that are now internal walls?

Or is it still best to get a professional company in?

I’m trying to make it all work in order in my head and if I know I can sort either myself or otherwise with remediation from inside, then I can proceed with EWI across whole of rear second storey.

Welcome, @Just_Breathe! It is very gratifying even at this stage of GBF's run to get new members.

Definitely need a pic so that we can understand the specifics of the issue, and so that consultants (if there are any) and contractors can understand what the issue is and what you are asking them to achieve. CWI on walls to be EWI'd is often more about stilling air movement - or potential air movement - which might strip away heat via the cavity than it is about adding a bit more insulation. In terms of stilling air movement you could arguably (Oh! It sounds so simple!) just block up any 'ins' and 'outs' feeding air to or discharging air from the cavity and Robert is a filial relation of your Mother and you don't *need* the CWI. In reality it probably is not that simple.

Drawings in 2 planes will help, and if we can engineer a solution then a lot of future EWI-ers will be helped too.

Not a recomendation, just what I did over time. 4 bed cavity wall 1963 build, with CWI fluff in - and I wanted to EWI it (150mm platinum EPS). Moving windows out showed locations where there was missing fluff around them, especially just under - I filled with similar stuff from our loft.
Adding a porch, discovered a lack of fluff in the CWI around there - probably as that wall was originally rendered. Filled with Silvapor - it's a bit like poly beads, just flows better.
Removed cladding - no fluff in that CWI section either (!). Filled with poly beads.
Top of walls - no cavity closer. Foamed XPS into location, to form a continuous barrier to loft insulation, and arranged so that water from a broken tile would hopefully drip down the outside of the house, and not into the cavity. Better safe than sorry!
If I thought I could convince a pro to do it, I would have used fluff, keeping similar to existing. We have wires in the cavity in some places, hence avoiding poly beads in that porch area.

The EWI is great btw.

Here’s a rough plan…