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CONSERVATOR IES & ORANGERIES
U what?
Paul Trace from Stella Rooight oers some guidance on the thermal performance (‘U’
values) of rooights and what to look out for in the speci cation process.
“the pitch of the roof will change the
thermal performance of your rooight”
www.ggpmag.com January 2020
One of the most important
elements of modern
building materials is
thermal performance,
which is measured in terms of heat
loss. In the construction industry
this is commonly expressed as a
‘U’ value or even sometimes an
‘R’ value. ‘U’ value calculations
will invariably be required early
on in the building strategy stage
as it provides an indication as to
how much heat loss a building is
likely to suffer upon completion.
The products used in the build are
normally required to be tested and
a figure for each component given.
One example of where this is an
essential requirement is rooflights.
There are guidelines in the UK,
set out in Building Regulations
Approved Document Part L
(Conservation of Fuel and Power),
that give the maximum ‘U’ value
that materials and structures are
allowed to have in a range of
buildings, including domestic
properties. It sets a national standard
to ensure that homes must be built
to a certain performance level
of energy efficiency for both the
reduction of carbon emissions and
the reduction of heating bills.
A ‘U’ value is one of the most
difficult thermal measurements
to calculate and so it is important
that any figures are produced using
reliable software from a bona fide
source. When it comes to rooflight
suppliers providing ‘U’ value figures
for their products, we all want the
lowest possible number to prove that
our rooflights give the best thermal
performance, which ultimately
reduces heat loss for our customers.
In the rush to be the best, it is not
inconceivable that figures get a little
massaged so it is always best to ask
for a copy of the test performance
report to ensure that a) the figures
are genuine and b) that the figures
were produced in the correct way.
Pitch perfect
Despite them being used in their
millions across the country, you
might be surprised to find out
that there is no specific test for a
rooflight. Instead rooflights are
tested to BS EN ISO 10077-1:2017,
which is a thermal performance test
for windows, doors and shutters.
So does this actually matter?
Well, actually yes it does because
most rooflights cannot be used in
the same way as windows and this
will result in a change to the ‘U’
value. For example, the pitch of
the roof will change the thermal
performance of your rooflight.
However, the testing of rooflight
performance is based on either
a vertical (above 60 degrees) or
horizontal (0 degree) pitch. Both of
these positions are usually outside
the maximum and minimum
pitch that rooflight manufacturers
recommend for their products.
There is quite a bit of difference
in the ‘U’ values given to rooflights
at both ends of the scale. For
example, our own Stella rooflight
can achieve a ‘U’ value of 1.5 W/m²K
in the horizontal position but this
improves to 1.1 W/m²K when used
above 60 degrees. Now when you ask
most companies what the ‘U’ value
is for their rooflight which figure do
you think you will be given? This
is why it is important to either ask
for both figures or to request data to
support the figures quoted.
The design of a rooflight is also
critical to the thermal performance.
Most modern rooflights are
produced from thermally broken
aluminium, which is then clad
internally with wood to provide very
good ‘U’ values. Whilst making the
frames more thermally efficient, this
approach also makes the rooflights
chunky, which often results in them
sticking several inches above the
roofline and providing poor frame to
glass ratios. It is not uncommon for
some rooflight frames to make up
over 40% of its overall size, which
ultimately means 40% less light
entering a building.
One of the easiest ways to lower
a ‘U’ value on a window or rooflight
is to increase the thickness of the
glazing. A decent double-glazed
unit will give a centre pane value of
1.0W/m²K and a triple around 0.5W/
m²K. However, it should also be
noted that by reducing the ‘U’ value
in this way, you will be significantly
increasing the weight of the product,
the cost and also the depth of the
rooflight profile.
If you are working on a Listed
Building or in a Conservation area
then you will require a conservation
rooflight design, which should sit
completely flush with your roofline.
If you are looking to use quad
glazing or modern bulky rooflights
to improve thermal performance
then this will most certainly be
to the detriment of the buildings
aesthetics.
There are a few exceptions relating
to the thermal requirements in
certain Listed buildings and those
in conservation areas, as well
temporary buildings and carports,
so it is always a good idea to take
professional advice before beginning
work on your project.
In conclusion, despite the fact that
the testing methods for obtaining
a rooflight’s thermal performance
could do with an overhaul, a ‘U’
value figure is very important when
both choosing products and in
the overall construction of your
property. The lower the figure, the
more thermally efficient a product or
building is and this can contribute
to reducing heating costs and
ultimately helps in the fight against
global warming.
It is also important that the
figures provided are genuine and
it is advisable to request proof that
‘U’ values being quoted are not
only correct but are applicable for
the application that you are using
that product. Nowhere is this more
important than rooflights because
of the current way in which these
products are tested.
Paul Trace
/www.ggpmag.com
