Clearview National January 2020 - Issue 218 | Page 66

ROOFLIGHTS& LANTERNS
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
thermal transmittance can be considerably
higher than desired, no matter how good the
individual products are.
66 » JAN 2020 » CL EARVI E W- UK . C O M
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 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 rooflight 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