ca s e st u dy
Smart choice of window
films can reduce heat loads
and increase customers’
comfort
Michaela Tymichova ,
PhD
3M Application Development Engineer,
Renewable Energy Division
W
e are all familiar with the heat
issues caused by the sun shining
through windows into our homes,
work offices, retail outlets, fast food outlets,
service stations and also into large shopping
centres with skylights. During the hot
summer months, the air-conditioning systems
work hard to keep the temperatures at
comfortable levels. This of course increases
the energy consumption and puts high
demands on HVAC systems. The solution
is simple – reduce the amount of solar
radiation coming through the windows.
The market offers a large variety of high
performance glazing that will do exactly that.
New buildings and homes are designed with
glazing that complies with BCA regulations
for heating and cooling requirements for
specific locations around Australia. What
about existing buildings? There are older
homes and buildings, many of them with
single glazing, where replacing the glass may
16 Australian Window Association Summer 2015
not be practical or economical. This is where
retrofitting window films can provide the
best solution. There are many different types
of films on the market so it is important to
select the film that is most suitable for your
application.
This case study describes one approach
3M took to investigate the performance
of Prestige 70 (PR 70) film at the 7-Eleven
Stores Pty Ltd in Robina, Queensland.
3M PR 70 film was selected for this trial
because of its ability to reduce a substantial
amount of infrared radiation (which is the
main cause of heat gain inside buildings)
while allowing a large amount of natural
light into the facility and maintaining good
visibility through the glass. This nonmetallised film has very low internal and
external reflectivity which makes it hardly
noticeable when installed. The external view
into the open store is preserved during the
day while the night visibility from the store
to the external service area and carpark is
clear. This ensures continuous safety of staff
in case of any unsocial events taking place
outside at night.
After the initial film selection criteria
was satisfied, the effectiveness of PR
70 in reducing the heat load had to be
determined. This was performed by
monitoring the heat flux through the filmed
and unfilmed glazing using TES-132 Data
Logging Solar Radiation Meters. These were
installed behind the filmed and unfilmed
glass panels. The difference in the heat
flux between the filmed and unfilmed glass
was significant, as shown in Figure 1. The
plot shows that in the afternoon, when the
western façade is exposed to direct sun, the
maximum heat flux on the existing façade is
about 930 W/m2 (at maximum peak). This is
reduced down to 400 W/m2 (at maximum
peak) on the façade with 3M PR 70. Based
on the collected data, it was determined that
the heat load reduction is approximately
60 per cent during the afternoon period
(calculated by comparing the area under the
curve).
It is worth noting that this trial was
conducted during June 2014, a cooler winter
month. Based on data from the Bureau of
Meteorology (BOM)1, the average solar
exposure was 11.4 MJ/m2, data is shown in
Figure 2. In the summer period, the average
solar exposure was 22.1 MJ/m2 in 20142 which
is much higher than the June average. Both
numbers are based on data from BOM.
Therefore, during hot summer months, the