Window Reveal angle affecting light levels


In creating vertical wooden supports around the window, we intended to provide a surface to attach shuttering, and subsequently attach wooden window jambs and architraves. We automatically decided to create these at a right angle to the face of the window frame, but after seeing the verticals in situ I am not completely happy with them.

The current placement of the vertical timbers is ‘chopping off’ the angle of light which will affect the way light spills into the room & reduce light levels. This effect is one which I am very aware of as a photographer of buildings and someone who has an acute awareness of light quality and the cause/effect relationship of objects and their interaction with light. It is something many people seem to largely underestimate the impact of.

Something I see regularly is how much a small increase in the thickness of window frames and reduction in glazed area can reduce light levels in a room dramatically. Even 5mm all around can make a big quantitive and qualitative difference.

I have been lucky in that, whilst the lovely original wooden sash windows have sadly been replaced with UPVC, these are of a type and dimension that mimics the originals, so not much light should be have been ‘lost’ that way. Whilst the area of glass will be remaining the same in this room, the increase in the depth of the wall by ~10cm will mean that in effect a short ‘tunnel’ will be created with the right angle of that edge, cutting off a chunk of the light traveling into the room. Additionally this will completely change the proportions & character of the attractive bay window.

The victorians were very good at creating room and window proportions that were carefully calculated and inter-related to be aesthetically pleasing, creating functional rooms that avoided overly dim corners, reduced the need for artificial lighting, prevented eye-strain and just feel better than more modern room & window dimensions.

In old cottages with thick walls, the window reveals are often splayed or sometimes curved. The georgians also often had splayed window reveals which allowed better penetration of light into the room and there was often wooden shuttering that folded into this area.

In order to balance the desire for extra wall insulation and moisture buffering capacity, with the desire for good quality and quantity of light in the room, I can foresee two solutions.

One is to angle the jambs at approx 45 degrees from the two side window frames (which would be approximately 90 degrees in relation to the middle window, or the front wall of the house). This would require moving the vertical timbers across which may be awkward for placement of dochens and will take extra time. alternatively, those timbers may be able to be ripped in a mitre along their length.

The other is to leave the current verticals in place for holding the shuttering to enable the installation of hempcrete, then they can be removed and the hempcrete molded into a curved, rounded edge before it has set. This is currently my preferred solution, although it has the downside that it would mean foregoing an attractive architrave detail around the window. However, since this is likely to be hidden behind curtains, it probably doesn’t matter so much! Additionally, there will be two curves close together (the bay corner will be slightly curved to mimic the beautiful original plastering) which may look a little odd – again, the curtain may hide this somewhat anyway.


One Comment

  • Margery Slater

    You’re absolutely right, this makes a massive difference. Suggest you find a friendly architect who can use PHPP (passivhaus planning software). You can calculate the influence your choice of window reveal will have fairly simply in an excel spreadsheet using PHPP.

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