[Strawbale] window construction

[Dave Howorth]

Hi guys (and any ladies who've remained quiet),

Thanks for all your answers.

Bill, Derek,
To give some background to my questions, I'm thinking in the context of
the Passivhaus standard. It's normal there for the windows to be triple
glazed and for the centre-pane or glass U value to be better than that
of the frame. For that reason, the timber frame in at least some of the
designs commercially available has a thermal break to improve its
performance to be closer to that of the glass. It seems to me that a
timber/plywood buck next to straw is likely to be a short-circuit.

If the wall construction is timber frame, a double wall construction is
often used to prevent the timber studs forming thermal bridges.

Rob,
Thanks, I see what you mean about the problem being the same as with
other wall constructions. I'll have to brush up on the solutions used
elsewhere.

Jeff,
I agree that the cross-sectional area of the buck is small but there are
a couple of points that made me think it might be significant enough to
consider. Firstly, the opening reduces the effective depth of the straw
for some distance either side and straw isn't the best of insulators -
it achieves its value by large depth. It's not an 18" depth we're
looking at but just the depth of the window frame. Secondly, even good
window frames are considered significant enough by the likes of Amory
Lovins and others to consider covering them with extra insulation
post-installation. Put the two together and I was hoping to find some
well-proven techniques to optimise this detail. I guess as Rob suggests,
I need to look at other wall construction techniques.

Rob, Jeff,
As regards cutting out openings, it's clear you two don't agree! I'll
have to keep looking and make up my own mind. Thanks for sharing!

Cheers, Dave