Framed Openings and Cold Bridges
- Building Design Expert
- 9 years ago
How much thought does a building designer, or a contractor for that matter, put into the precise position of a window, or door frame, within the thickness of the wall in which the opening for it has been constructed? Is it even important?The original prescription was to set the front of the frame far enough back from the front wall face so that the thickness of the frame could coincide with the vertical DPC of the closed cavity, which had hopefully occurred on the back face of the external leaf. This usually resulted in a back set of around 25 – 40mm. But, in these days of supreme energy conservation designers must necessarily adopt a ‘war of attrition’ approach to previously glossed over details such as these.We put a hole in an external wall and then try and stop it letting the heat out whilst keeping the daylight coming in. It’s no mean feat. But by it’s very nature the glazed window frame is one HUGE cold bridge. Doesn’t matter where we put it; short of a solid metal wall, it’s still going to loose more heat than any other thermal element of the building. So the fractional savings we may be able to achieve by careful design and installation across a multi-windowed building could add up to something quite substantial.We now measure the U-value of windows on a ‘centre pane’ basis, and as a whole framed unit. The centre pane yielding the best results of course, because at that location there can be no cold bridge. The cold bridge is as a result of any physical connection between outside and in: The spacer bars that separate the glass panes, and indeed the surrounding frame itself.Can we cure cold bridging in window design? Play the following ‘pencast’ and find out.Turn up your sound and click ‘Play’.