Civil Engineering Reference
In-Depth Information
Covering the bottom boxing with plywood
If covering the bottom boxing with ply it is advisable to fill the cavity with bulk insulation
to reduce the risk of a thermal bridge. One concern that I have with ply is the risk of
water pooling on the bottom boxing between the bales and the boxing in the case of rain.
To reduce this risk, reduce the width of the bottom boxing to 410 mm. The bales would
then over hang the bottom boxing so that water would be more likely to discharge cleanly
from the face of the bales rather than penetrating the junction of the bales and boxing.
The outer 20 mm or the bale that would overhang the boxing would have little if any
structural implication, as this is the loosest area of straw in the bale.
Filling the bottom boxing with crushed rock
If you choose to fill the cavity with crushed rock it is imperative that there be no fine
material in the crushed rock as there is in crushed rock used in driveways, for example.
The fine material would enable moisture to travel through the material to the underside
of the bales. I am not concerned about moisture from the ground penetrating the slab, as
adequate barriers should have been provided beneath the slab. I am, however, concerned
about moisture travelling from potential water spills in the house, for example, washing
machine overflows or a burst dishwasher hose. For this reason it is a good idea to provide
floor traps for water in the wet areas: these are also a legal requirement in some states. If
using crushed rock it may be necessary for any electrical cables within the bottom boxing
to be installed inside conduit to protect the cable from damage from the rock, particularly
if the rock has sharp edges.
Avoid the use of plastic
It has been suggested that the crushed rock should be covered with plastic onto which the
bales are laid. Cedar Rose, in her topic
The Natural Plaster topic
(2003, p. 42),
recommends that you avoid the installation of plastic against the bales. Plastic, rather
than preventing water penetration into the bales, actually tends to do the opposite. As an
experiment, place a straw bale on plastic under cover where no added moisture can get
on the bales or plastic. Leave the bale to sit undisturbed for a week or two and then check
the underside of the bale where it has been in contact with the plastic. You will find that
the bottom of the bale is quite moist, if not wet, due to condensation between the plastic
and the straw.
Knowing full well that moisture is the only enemy straw bales have, I would rather
not risk introducing plastic and the resulting moisture to the bottom of my lovely dry
straw bale walls. As plastic presents a very smooth surface that is quite slippery, I would
be concerned about sideways slip of the wall at the junction of the bales and the plastic. If
the plastic was to press into the crushed rock with the pressure of compression this would
to some degree negate the slippage concern. However this would indicate that there are
now dips and hollows in the surface of the plastic. If it were to rain and the water were to
run down the bales and in on top of the boxing, these divots would become little
reservoirs for water rather than flowing away, as is the case without the plastic.
