U-What?!
Paul Trace from Stella Rooflight offers some guidance on the thermal performance
(U-values) of rooflights and what to look out for in the specification process
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.
Thermal transmittance (U-value) is
measured in units of W/m²K which stands
for Watts/meter square Kelvin. The lower the
U-value the more efficient the construction
is at keeping heat flow through the structure
to a minimum. It is worth noting that it’s not
just the building materials and products that
have an impact on the thermal performance
of a building, as both workmanship and
installation standards can strongly affect the
thermal transmittance. If insulation is fitted
poorly, with gaps and cold bridges, then the
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thermal transmittance can be considerably
higher than desired, no matter how good the
individual products are.
Thermal transmittance takes heat loss due
to conduction, convection and radiation into
account. The amount of heat conducted
through a material of a given volume, in a
unit of time i.e. the rate of conduction is why
the units are measured as W/K.
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 residents 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.
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