Civil Engineering Reference
In-Depth Information
sodden and soft. The render would have been above the flashing and therefore against the
underside of the window sill which would no doubt lead to the early demise of the
window sill. All in all, this was not an ideal treatment of the window area. It was
admirable that he had taken the time to investigate the installation of windows in
conventional construction, but unfortunate that he did not understand all the
implications of applying this method to straw bale construction.
Window flashing in conventional construction
In conventional construction builders are required to seal the bottom and sides of all
windows to prevent water penetration. The conventional method of sealing is by using
aluminium or plastic flashing. While in the window replacement business we developed a
method to seal the windows without the use of such flashing down the sides of the
windows. The conventional method of flashing the sides of the windows to be installed in
a completed house would require the removal of the external cladding, as flashing has to
be fixed not only to the window, but also the face of the house frame.
In many instances this cladding was in the form of brickwork, the removal of which
was not financially viable, to say nothing of being extremely undesirable. The opening for
the window in the timber frame of the house was always greater than the overall
dimensions of the window to be installed, so we had little difficulty sealing this space
with foam fill once the window was fixed in place. This achieved the same end results as
the flashing with the added benefit of preventing heat loss and drafts around the window.
In the majority of houses, if you hold your hand against the architraves around the
window on a windy day you will actually feel the cold air forcing through the cavity
between the window and the frame.
Conventional building flashing in straw bale construction
There are several conventional flashing products on the market. You can purchase rolls of
black plastic flashing in various widths. This is the same product that is used between
courses of bricks as a moisture barrier. There is also aluminium flashing which is silver, or
a plastic flashing with a core of aluminium within it. This enables you to bend it to the
required shape so it holds its position. With straight plastic flashing there is little you can
do to alter the position that it naturally sits in.
The flashing is to be installed across the width of the window without joins, as joins
would provide an avenue for water to bypass the flashing. The flashing is fixed to the
internal face of the window with a turn up of approximately 10 mm (see Figure 10.3A).
The side flashing will be fixed to the internal face of the window in the same manner as
the sill and then brought through the cavity between the window and the window buck
or frame to the external face of the buck (see Figure 10.3B). The lower end of this flashing
is to be enveloped by the sill flashing so that any water captured by the side flashing is
discharged into the sill flashing, which is in turn discharged out of the structure (see
Figure 10.3C).
Discharge of water
When using conventional flashing its discharge point must be onto the external surface of
the render, not beneath the render. This will allow any water to flow freely away from the
