I started posting elsewhere and realised that my task is about to become a saga, so have started a new thread. I’ll copy the two posts I’ve already done into this thread and then link to here.
The thermal camera has a narrower angle than the original photos but hopefully you can find your way around.
Well I still haven’t got the top board down but I did see that the plaster between board and ceiling was coming lose from my efforts. It was cracked previously, so no surprise. I ∴ decided that it would be less mess and danger to myself to remove the plaster myself rather than let it fall while I was below it. That would also enable my to see what was holding the board on place.
Oh dear, what a mess. Along the ceiling I see remnants of original plaster with sand and hair binding but the rest is a poor quality jigsaw of pieces stuck I by subsequent builders, window fitters and cowboys.
I have been able to excavate a space through to the replacement lintel fitted about 12 years ago. It seems it is only holding up the outer part of the (“solid” masonry) wall and a timber lintel, presumably the original, is holding up the inner part. The concrete lintel is deeper than the timber one. There was a concrete lintel before where the current concrete one is but it exploded due to iron reinforcements rusting and subsequently expanding. The job was done while we were on holiday, so I didn’t see the construction technique, but the replacement was only done from the outside.
I’m thinking that the packing to level out the top of the window was probably added in 1980, when previous owners replaced a door with a window. That window has since been replaced twice.
I am hoping that I can remove all the packing, remove the window board, replace the packing with insulation, insulate up against the concrete lintel and then again across the bottom to give a new flat surface that also insulates the top of the window frame. In between tasks a couple of coats of Blowerproof will, of course, be applied.
That is all more than I was anticipating but not too onerous.
Tim
I’m no expert but these photos from our build up might help, although its of a new build extension.
I would definitely put some type of cavity closure in to stop air from the cavity circulating.
Below is the first part of the build up which you can see below the opening which is Superquilt
This was covered with 50mm UniLin board between the battens and then sealed with Partel Vara plus.
Which was then protected with OSB boarding. The plan was to seal the spare membrane to the window.
Thanks. I can skip anything cavity related as the wall is “solid”. By which I mean no defined cavity but full of voids.
For the time being I’m just insulating the reveals. This gives me a chance to make window to wall joins airtight and at the same time treat the edges of the wall. I am hoping to get EWI and maintain thermal mass. Very important for A2W heat pumps but not for A2A.
My local council refused EWI a few years ago but now more local houses have them and rules are being relaxed so I may be trying again soon. The house is not listed and it’s only architectural merit is that it has stood for 90 years.
I know that this forum isn’t the best place for a quick reply, except perhaps from me 
but I am hoping for some prompt advice.
This is the thermal image of the top of my window this evening. Not particularly cold out but a good wind to show leakage.
As you can see there are two sources of leakage along the top. There are gaps in the foam sealant and another between the two lintels.
There is a gap of about 7mm between lintels. There were a few mortar snots from when the outer lintels was replaced. The gap seems a good idea from the perspective of damp penetration to the wooden lintels but there is a howling gale between them. My initial inclination was to retain the gap and seal across it but then I wondered about a strip of raised polystyrene pushed up between them and that air tightened relative to the room a quadrangular section of insulation would then be fitted below the inner lintel and abutting the outer one to give a flat surface before a layer of insulated plasterboard is attached below to help support the board I would use contact adhesive and support at each end from the newly plastered sides. Meanwhile the foam above the window would have been completed and also sealed with airtightness product. The one to hand, that I use for all my fiddly little jobs, being Blowerproof.
So, who will offer me a sanity check?