Civil Engineering Reference
In-Depth Information
If this is a problem for conventional construction, how much more of a problem is it
for straw bale construction?
Prevention of condensation around windows
The solution to preventing condensation around windows is to fill the cavity to reduce
the potential marriage of the moist air and the cold air, which brings about all those little
baby droplets of water. This can be done to some degree by installing bulk insulation,
such as slivers of fibreglass insulation, however I would recommend the installation of
a foam fill. There are numerous manufacturers of spray-in foam products, which are
supplied in pressure cans and are available from most good hardware stores. You will
be surprised how far a can of foam will go, but it still warrants checking up on the cost
of different products. With most brands of foam, you will have greater growth if the
area to be filled is dampened prior to the installation of the foam. It is also important
that the can is warm in order to gain the potential yield from the can. Read the
instructions prior to using the product, as incorrect use can reduce your yield by up to
50 per cent.
I have seen two different manufacturers with cans that yield very similar quantities of
foam; one retailed at just over $5 while the competitor's product was over $20 (2005).
Apart from the insulation benefit, foam has other attributes that I shall cover shortly.
Filling the cavities caused by the rounded end on the bales
If the foam is used to fill all cavities between the window frame and the window buck of a
straw bale house, this will prevent the possibility of moist air condensing in this area.
Needless to say, the connection of the straw to the outside of the window buck is the next
most vulnerable place for this problem. The ends of a bale of straw are not perfectly flat,
but to some degree rounded. As a consequence, the bale will be tight against the timber in
the middle section of the bale, but it is likely to have a small gap at the upper and lower
sections of the bale end. This can be easily overcome, either by trimming the end of the
bale to allow the bale to partially encase the window buck where you have a centrally
located buck that is narrower than the distance between the twine, or by filling the cavity.
The cavity can be filled by forcing straw into it, and/or by filling with cob: a mixture of
straw and wet clay. (Refer to the chapter on straw bale walls for further details on cob.)
Control of liquid water on or about the windows
House design to reduce water penetration at windows
The control of water should be the primary objective when installing windows and
external doors in straw bale walls. Ideally a straw bale house would have verandahs
protecting the walls from rain, however, this is not always practical and often does not
meet the aesthetic design requirements. While it might not be possible to build with
verandahs, I believe that it is inappropriate to build a straw bale house without significant
roof overhang (known as eaves) to protect the walls. A common verandah width is 1.8
metres; I have heard it said that this will negate any issues with water around windows,
however this is far from the truth. Even if the verandahs are three metres wide, this alone
