[Strawbale] window construction

[David Neeley]

Dave,

Isn't it more a function of priorities and return on investment? If
your other details are sufficient that having a thermal break in your
window bucks makes a noticeable difference, you are highly unlikely to
need it.

Structures in which such details are considered significant are very
likely to be deficient somewhere else, in my experience. By far the
worst problem in condensation is generally infiltration of outside
air, often because of insufficiently detailed structures in which
openings are not properly sealed. This is often true in conventional
structures, too, in cracks within walls and between wall members and
insulation.

If you take care to seal all openings well, then, and to insulate
properly in the roof structure, you should have little if any problem
from the window bucks themselves even though their thermal performance
is less than a full-width bale wall. Of more pertinence would be to be
sure that the entire window structure is fully sealed with no cracks
around it for air infiltration.

One strategy that can work quite well is to use very insulated and
sealed vents--or, perhaps, an energy recovery ventilator--for your air
exchange needs combined with fixed glass windows. This eliminates the
problems of air leakage around operable windows, so long as the buck
and the window panel itself is well and truly sealed. A non-glass vent
can be much more easily sealed and insulated than any window,
too...and the optimal locations for ventilation are often not at
window locations anyway.

David

On Fri, Apr 18, 2008 at 4:39 PM, Dave Howorth <dave at howorth.org.uk> wrote:
> 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.