32 | DECEMBER 2019
Products
Read online at www.proinstaller.co.uk
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 im-
portant 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 some-
times 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.
Thermal transmittance
takes heat loss due to con-
duction, convection and ra-
diation into account. The
amount
of heat conducted
through a material of a
given volume, in a unit of
time i.e. the rate of con-
duction is why the units
are measured as W/K.
There are guidelines in
the UK, set out in Build-
ing Regulations Approved
Document Part L (Conser-
vation of Fuel and Power),
that give the maximum
U-value that materials and
structures are allowed to
have in a range of build-
ings, including domes-
tic 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 heat-
ing bills.
A U-value is one of the
most difficult thermal
measure-
ments 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 sup-
pliers 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 custom-
ers. In the rush to be the
best, it is not inconceivable
that figures get a little mas-
saged 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 roof-
light. 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 roof-
lights 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 ther-
mal performance of your
rooflight. However, the
testing of rooflight perfor-
mance is based on either a
vertical (above 60 degrees)
or horizontal (0 degree)
pitch. Both of these posi-
tions are usually outside
the maximum and mini-
mum pitch that rooflight
manufacturers recommend
for their rooflight prod-
ucts.
There is quite a bit of
difference in the U-val-
ues 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 roof-
light 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 thermal-
ly 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 roof-
light frames to make up
over 40% of its overall size,