Civil Engineering Reference
In-Depth Information
droplets of water until they reached overload in my general vicinity, whereupon they
dumped on me.
How condensation runs
The water from condensation will run until it hits an obstruction, and then fall.
Condensation that forms below the iron will run down the underside of the iron until it
strikes an obstacle such as a roof batten, or until the droplets gather together, thereby
increasing their weight beyond their cohesive capacity whereupon they drop like rain. It
was once considered that a roof with a pitch in excess of 22 degrees did not have a
problem with condensation, however my experience proves this to be untrue. In
Heathcote I have three-metre wide verandahs with a 25-degree pitch. The condensation is
clearly evident, as are the lines on floor where the water drops from the roof battens. This
is not an issue in this application, but it is if there is the remotest chance that the water
will drop onto bale walls.
The solution for condensation
The solution for condensation is the installation of sizalation either above the roof
battens or below the roof battens. Either application is acceptable, although I personally
prefer to have it installed below the roof battens. When it is fitted on top of the battens it
is essential that the material be tight enough to prevent pools of water forming behind
the roof battens and yet it must not touch the underside of the iron between the battens.
Whereas with the installation below the battens, while it is still essential that it be
stretched out, there are no physical barriers behind which the water can gather until you
get to the fascia or tilt batten. At this point you are past the danger point for the straw
bale walls. The sizalation installed on top of the roof battens will have a number of
penetrations through it where the roofing material is screwed to the sizalation and into
the roof battens. When the sizalation is fitted below the battens it will only be penetrated
by the fixings for the battens. Any water lying on the sizalation will tend to flow away
from the highest point, which of course is at the rafters into which the battens are fixed.
Extent of the sizalation
Extend the sizalation beyond the external line of straw bale walls. If your house has
verandahs and you do not wish to fit sizalation over the verandahs, ensure that the
sizalation is extended beyond the external line of the straw bale wall. The water that comes
from inside a roof is not always clean, so any discharge down the face of your wall may
well stain the wall. If you extend the sizalation about 300 mm or more past the external
wall line you should avoid this problem. To achieve this, fit a roof batten in line with the
end of the outer edge of the sizalation. The sizalation can then be fixed up to the underside
of the roof batten thereby providing another barrier for vermin entry into the roof cavity.
When choosing the appropriate sizalation for your house, avoid a decision based
purely on price. Use a heavy weight sizalation or Air-Cell rather than a cheap lightweight
material. Sizalation is there to protect your walls, so don't sell out for the initial savings, as
it could be a very costly decision. Also consider the likelihood of birds damaging the
sizalation. If this is likely it may direct you toward Air-Cell or you could protect the
exposed area with bird wire.
