design
mag |
91
Solar Reflectance, Solar Gain & the
‘Cool Roof’ Controversy
It’s well known that the colour of an object influences its surface
temperature and the amount of heat it will absorb.A black car
left in the summer sun will be hotter to touch than a white car.
The material being heated also influences its response to
temperature.
Recently, the City of Melbourne sponsored testing at The University
of Melbourne into ‘cool roofs’, the proposition that a roof treated
with a “cool roof paint” (CRP) will reduce heat gain into a building
and therefore reduce energy consumption.You can read the full
report at
/
CouncilActions/Documents/Cool_Roofs_Report.pdf.
Although the benefits have been simplistically touted as “paint your
roof white and your cooling bills will go down” the report is more
circumspect. For example, the use and location of insulation is very
influential:“If the insulation is moved from on the ceiling to under
the roof (i.e. from being between the roof space and the habitable
room to being between the roof and the roof space), it
dramatically reduces the roof space temperatures to the point
where the effect of the CRP is much less significant.” (page 36)
Although much of the general reporting highlighted the reduced
summer heat gain attributable to a ‘cool roof’, the report notes that
a darker-coloured roof may produce winter benefits outweighing
those of summer.“It should be noted that this component of the
research is not accounting for the difference in the amount of
cooling vs. heating required for a residential dwelling which skews
dramatically in favour of the heating conditions for the Melbourne
climate. Both graphs show that for a large part of the heating
season the roof space is below the typical indoor comfort
temperature range (and therefore would be drawing heat out of
the habitable space) and thus these houses would respond better
to higher roofspace temperatures (i.e. a darker roof).” (page 36)
In general terms it would appear that the greatest benefits from
cool roof paints will be on metal roofing, especially those that are
uninsulated, for example on industrial buildings, the report
concluding that “CRP products are best suited to older and/or
uninsulated [industrial] buildings.” (Page 40)
The final word from the report:“The single drawback of these
products [CRPs] has been shown to be this heat reduction
consistency between seasons as it is not particularly well suited to
the Melbourne heating climate.” (Page 41)
What does this mean for roof tiles? As mentioned previously, roof
tiles reflect more solar energy than a comparable metal product.
And roof tiles are available in wider range of colours than metal
products.Yes,‘cool roof paint’ may increase solar reflectance but
that is not the entire story and the benefit will vary with the
application and climate.
