Proctor Breathable Roof Membranes by Dynamic Composite Technologies
rain in the roof - mould in the walls
Each winter we hear stories about leaking roofs or "rain in
the roof" even though there have been no clouds in the sky
for several days. These so called "ghost leaks" and instances
of rotting or corrosion of bottom plates are more often a result
of the building getting wet from the inside. As buildings are
increasingly being designed as air tight and well insulated,
such problems are becoming more widespread.
Most methods for keeping out rain and wind involve wrapping
the building with vapour tight materials. This stops the water
from getting in, but in the winter such materials prevent vapour
from getting out safely resulting in the cold roof space
and walls can get wet due to condensation.
The water vapour created in the building naturally diffuses
through the plasterboard into the roof space and wall cavities,
also finding its way through openings behind power points
and light fittings. The moisture laden air passes freely through
the bulk fibre insulation dropping in temperature until reaching
cold surfaces within the roof or walls, releasing condensation
in the same way as seen on the outside of a cold beer glass
or the inside of a window in winter.
If insulation is not installed correctly in relation to vapour
barriers and breather membranes, it can lead to structural
damage and health problems for occupants due to mould
spreading unnoticed within the building fabric. If moisture
gets trapped in the insulation it stops it from doing the job
it was put there to do. A report from the CSIRO highlighting
these exact same concerns was published in 2001.
Fortunately if you are aware of the potential problems,
expensive and embarrassing rectification can easily be
avoided at the design stage for a fraction of the cost of
fixing a problem once the building is occupied.
Contrary to basic rules of building physics - foil, perforated foil
or reflective bubble foil type sarkings continue to be installed
behind the external wall claddings, brick veneer or under roof
tiles. Vapour barriers should instead be used on the warm
(interior) side of the insulation.
A breathable membrane should be used on the cold
(exterior) side of the insulation. This does the same job as
foil sarking such as keeping rain, snow and dust from getting
in, but also lets moist air escape.
To allow condensate to drain safely out of the wall and help
wall assemblies dry out, common practice overseas is for a
vented cavity between the external cladding and breather
membrane. This was a key measure introduced in New
Zealand in response to the leaky building syndrome and is
increasing being adopted by good builders in Australia.
Typically, breather membranes are textile products that work
much in the same was as Gortex fabric and have a very low
vapour resistance of less than 0.3MNs/g. These should not
be confused with some perforated foil type products labelled
as "breathable." Although well intentioned, these perforated
products do not really breath enough and their vapour
resistance is 10 to 70 times the standard for a breather
membrane set by the National House Building Council
in the UK.
The building industry is starting to recognise the need for
real breather membranes as regulation drives increases
in insulation. If you've not already come across breather
membranes, Weathertex, Austral Bricks Terracade and
VM Zinc all insist such products are used behind their
cladding. The range of Proctor Breathable membranes
from Europe have been used recently in ACT at the ANU
and under the roof of the Cox Humphries Moss designed
AIS swimming pool installed by Delnas Metal Roofing.
Due to the increasingly high levels of insulation required in
ACT, insulating and ventilating the roof is becoming more
complex. The building designer is responsible for ensuring
that the materials specified and installed in your project do
not lead to condensation problems. If using a foil faced
blanket under a metal roof be aware that by adding more
insulation at ceiling level could potentially bring the foil below
dew point. Also a foil faced blanket needs to be well installed
and sealed to prevent vapour from reaching the underside
of the cold metal roof and condensing.
High humidity projects such as swimming pools, museums
and libraries are particularly at risk and we recommend you
carry out a condensation risk analysis. For further information
on these issues, visit www.dctech.com.au/mba
where an on-line presentation and a list of related resources
are available.
For further information contact:
Dynamic Composite Technologies
Address: Unit 8, 171-175 Newton Road, Wetherill Park, NSW, 2164
Phone: 02 8788 9....
Fax: 02 9604 7....
Web: www.dctech.com.au
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