I've found this interesting paper [1] that compares different window positions relative to the cavity and their affects on condensation and thermal bridging. Six different positions were compared (modelled) and conclusions drawn:

1. Aligned with the outer face of the façade.
2. Fixed into the external leaf and overlapping the cavity 30mm.
3. Fixed into the external leaf and overlapping the cavity 70mm.
4. Aligned with centre of the cavity.
5. Fixed into the inner leaf overlapping this leaf by 50mm.
6. Aligned with the inner face of the room.

 

Whilst position 2 is the most common form constructed as it is "a much more robust detail" but doesn't perform as well from a thermal bridging (best: 5, 4, 3) or condensation perspective (best: 3 and 4).

 

My Take: Given that (3) is a position near to (2) it probably doesn't reduce robustness of installation too much and so represents a good balance. The paper acknowledges that it only considers the thermal impact, not other factors (e.g. maintenance and drainage). I'm using Thermally broken lintels so suspect that would change things a bit. My gut feeling is it would reduce the benefit of 4 over 3 over 2.

 

What choice have most made here?

 

 

[1]  https://www.sciencedirect.com/science/article/abs/pii/S0378778816316383

Edited June 9, 20215 yr by MortarThePoint

Keystone Lintels has a PSI value calculator which says "Frame to overlap cavity min. 30mm". Probably still better to have more overlap.

 

Edited June 9, 20215 yr by MortarThePoint