Window fitment in old opening for sash boxes

[DJ Trixta]

Hi All! I'm new here but am attempting a renovation on my own of my old Victorian property in the South West. I wondered if anyone has any ideas how I should approach the fitment of a new window. The old sash window was removed a long time ago and so a modern UPVC unit looks to have been bodged in with a hardwood frame mounted on the inside of the brickwork. The walls are solid and have no cavity. The window itself is too small for the actual opening of the brickwork, so has been mounted in this frame. My intension is to replace the window but I am stuck on the fitment detail of the new window, and want it to actually fit the opening. I plan to build a stud wall inside along the external wall for insulation and services to be housed and I wondered if I could incorporate a frame into the stud that would support the window and hold just slightly off of the outer course of bricks to prevent any cold bridging. Any ideas on this or other methods? All help greatly appreciated, thankyou! 

[Big Jimbo]

Is the frame on show from outside ?

[DJ Trixta]

Hi mate, no it's covered with trim strips, looks bloody awful! Don't want to take them off just yet as haven't got the new window yet and don't want to cause a leak.

 

[Mr Punter]

You often see brackets used to mount windows.  The location in relation to the existing is part personal choice.  They would probably be best as per original.  If they are weighted box sash they need a bit of space at the sides.  Use insulated plasterboard for the reveals and insulation under the cill.

[DJ Trixta]

On 09/09/2025 at 08:45, Mr Punter said:

You often see brackets used to mount windows.  The location in relation to the existing is part personal choice.  They would probably be best as per original.  If they are weighted box sash they need a bit of space at the sides.  Use insulated plasterboard for the reveals and insulation under the cill.

Hi mate, no it's just going to be a replacement casement window going in. That is what is there now but I just want it replaced as the window is past its best and it's not fitted well at all. I think I will build a frame into the space left internally where the box sashes used to be and mount the window into that, leaving a small gap between frame and the brickwork. The window comes in two weeks now so I should be starting this build and removal of old window soon. 

 

Thanks to all for the advice!