Hi All,

I live in a late '50s semi that has some pretty big (and pretty bad) alu framed DG windows. Sizes range between 25-40% of the external wall area of any of the rooms.

Due to the "outboard" positioning of the windows we get a lot of thermal bridging in the reveals - the brick outer leaf is within the heated envelope of the house and is a condensation horror show, particularly on the north side of the house. This is being mitigated in the short term with foil tape to stop the emulsion going mouldy in the perpetual winter damp which, as you imagine, is not terribly attractive.

I have been considering some approaches to resolve the thermal bridging when we come to change the windows but they would lead to a reduction in the dims of the window openings and a cascade of issues relating to lead times for new windows and the requirement to temporarily block the openings with somekind of ply/PIR/plasterboard sandwich for some time. Which would probably not be acceptable within the household - even I think it's probably too much to live in the dark for X number of weeks/months!

Is such an approach ever practical or would it just be better to take the money allocated for any remedial work and spend on better windows in the original openings with some other lightweight strategy to mitigate the thermal bridging (aerogel)?

This website has some good pictures of the 'picture frame' external window approach:

http://www.fourwalls-uk.com/blog/category/windows-doors/


Plus, I've also just found this useful resource, the Retrofit Pattern Book, which may help to narrow down your options:

https://retrofit.support/category/IWI-W-RP/

Thanks for the replies DT and DJH. Apologies for any confusion from my initial post.

In answer to DJH first, I would be happy to reduce the window size to improve thermal performance and would think it necessary in order to remove (as far as possible/practical) the thermal bridging. Thanks for the suggestion about polycarbonate as temporary windows - definitely a better solution than the gloom!

In response to DT. Below *should* be a plan view of the construction of the walls showing the position of the window in the opening.

Thanks for the links to the fourwalls site and the Retrofit Pattern Book. The fourwalls property has some commonality with our house having finlock gutters. Interestingly though theirs were positioned above the windows with (I presume) a lintel against which the picture frame could properly seated. In my property the finlocks are directly over the window frame which may not prove to be great fixing point for the timbers of the picture frame. AIUI, the picture frame approach is only for EWI, right? In my limited experience I havent seen this approach on properties with brick/stone outer leaves.

I would love to EWI the house but that would be some way down the road for us. So maybe bringing the windows in towards the filled cavity would be the way to resolve the condensation on the walls without the complications of changing the size of the openings or requiring EWI. I appreciate it's not the long term thinking that is preferred but gives an option that enables us to remove the mildew defences (foil tape) currently in place.

Thanks for any further thoughts and suggestions!

We used Aerogel wall liner
https://www.roofingsuperstore.co.uk/product/spacetherm-a1-wall-liner-wl-insulation---600mm-x-1200mm-x-13mm.html?gad_source=1&gclid=CjwKCAiA0bWvBhBjEiwAtEsoW7YWcYXUbKuUTuRE7tN1nM7p5UVQgrTCsLcigZfZCmGESMDqapQZLxoCc3sQAvD_BwE

to line some narrow ventilation slits on our barn conversion. IWI so our insulation layer is on the inside with just plasterboard over it, the glazed units are near to the outer edge of the stone walls (about 3" to 4 " setback in a 500mm thick wall). Okay so far, but we do have MVHR so humidity is controlled.

Would it be worth costing it up? or trying it on one window and see if it resolves the problem?
I do even have a few offcuts, but probably not enough to do a whole reveal for one window.

I've never come across Finlocks before but the idea of using them as a lintel above a window strikes me as frightening How are the roof & ceiling supported above the window?

Assuming the structure is OK (not a given, definitely worth checking) I'd be tempted to put some EPS or phenolic insulation in the reveals with some 6 mm board over the top, even if it meant covering some of the glass area. I'm thinking of that as a short term fix, with the insulation bonded to the window frames. The plan for e.g. EWI and replacement of the windows as soon as circumstances permit. And fix the Finlocks at that time.

But I'm not a retrofit expert so others may have better ideas.

Posted By: djhI've never come across Finlocks before but the idea of using them as a lintel above a window strikes me as frightening. How are the roof & ceiling supported above the window?

AIUI, the roof and ceiling joists are supported by the finlocks - It's not uncommon in this village and most still have theirs in place (functioning as guttering). Only a handful, including my direct neighbour has had theirs cut off for purposes of preventing moisture ingress/aesthetics. The windows in that property were in replaced before he moved in so there was no remedial support works done during the finlock "removal". Not saying that things cant go wrong...just havent yet!

Posted By: djhI'd be tempted to put some EPS or phenolic insulation in the reveals with some 6 mm board over the top, even if it meant covering some of the glass area. I'm thinking of that as a short term fix, with the insulation bonded to the window frames. The plan for e.g. EWI and replacement of the windows as soon as circumstances permit. And fix the Finlocks at that time.

Thanks for the suggestion for the short term fix - something along these lines will have to be done this summer.

For a longer term supporting solution (and very much thinking out load here) could it feasible to resolve thermal bridging and support the gutters by extending the walls with 2 leaves of foamglass (or other material that has high compressive strength) and place a lintel on each leaf with an insulated cavity at the top of the windows. Having a "proper" lintel on the exterior face would mean the picture frame method of window installing could be used if ever the house was EWI'd. I imagine at this point we would be using an architect rather than dreaming up pie-in-the-sky solutions, however fun they may be!

(this is my first attempt at quoting...so bear with me if it's a mess)

Not having previously heard of or come across finlock gutters I goggled it and found this useful description which others might find useful.

https://www.guttermaintenanceuk.co.uk/what-is-finlock-guttering/#:~:text=A%20Finlock%20gutter%20is%20built%20from%20two%20troughs,cavity.%20This%20cavity%20sits%20on%20the%20internal%20wall.

Posted By: bxmanIt looks like a joint above each corner of the window and another in the center of it

I think there are seven blocks above the window.