How to prep this bare brick window opening for a window?

[nightyard]

I'm currently in the midst of renovating my 1970s house and have just ordered new windows for the living room. I've stripped the room back to bare brick, presenting an excellent opportunity to ensure the new windows are installed according to the best practices.

The house features a cavity wall with a 3cm gap between the outer brick and the inner wall, which is quite narrow compared to standard cavities. This gap is currently exposed. I'm looking for advice on the best way to close this cavity. I'll be adding a row of bricks to the internal wall below the window to bring it back to level with the outter brick. Most of the cavity closers I've seen, such as those available at Screwfix, seem designed for wider gaps (50-100mm) and are sold in packs of 10 at screwfix for £120+. Does anyone have recommendations for dealing with a cavity of this size?

Another consideration is whether I should use a membrane to seal the window frame to the brick. Ensuring a tight, weatherproof seal is crucial, but I'm unsure if a membrane is necessary or if there are better alternatives.

This is my first experience installing windows and working with cavity walls, so I'm eager to do this correctly. I plan to start by installing a plasterboard sill and later, possibly upgrade it with a wooden or quartz sill for a more refined finish.

Any insights, tips, or guidance on ensuring a seamless fit and finish would be greatly appreciated!

photos for context:

 

[23vc]

You could get a 50mm cavity closer and trim the foam down so it fits. But that would involve clearing out the cavity which has already got a vertical dpc in it. So I’d probably just foam/put bits of PIR in the gaps and crack on

 

[noseall]

I'm currently in the midst of renovating my 1970s house and have just ordered new windows for the living room. I've stripped the room back to bare brick, presenting an excellent opportunity to ensure the new windows are installed according to the best practices.

The house features a cavity wall with a 3cm gap between the outer brick and the inner wall, which is quite narrow compared to standard cavities. This gap is currently exposed. I'm looking for advice on the best way to close this cavity. I'll be adding a row of bricks to the internal wall below the window to bring it back to level with the outter brick. Most of the cavity closers I've seen, such as those available at Screwfix, seem designed for wider gaps (50-100mm) and are sold in packs of 10 at screwfix for £120+. Does anyone have recommendations for dealing with a cavity of this size?

Another consideration is whether I should use a membrane to seal the window frame to the brick. Ensuring a tight, weatherproof seal is crucial, but I'm unsure if a membrane is necessary or if there are better alternatives.

This is my first experience installing windows and working with cavity walls, so I'm eager to do this correctly. I plan to start by installing a plasterboard sill and later, possibly upgrade it with a wooden or quartz sill for a more refined finish.

Any insights, tips, or guidance on ensuring a seamless fit and finish would be greatly appreciated!

photos for context:
View attachment 325896View attachment 325897View attachment 325898

The new window position should be further back (into the room) so that none of the external masonry is visible beyond that vertical DPC.