Clearview National January 2020 - Issue 218 | Page 67

ROOFLIGHTS&LANTERNS
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. Then of course
there are some that offer quad glazing, which
reduces this figure further. 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 buildings and
it is always a good idea to take professional
advice should your project fall into one of the
following categories:
i) Where the replacement roof windows are
unable to meet the requirements because of
the need to maintain the external appearance
of the façade or the character of the building,
replacement windows should meet a centre
pane U-value of 1.2 W/m2K.
ii) Buildings which are; Listed in
accordance with section 1 of the Planning
(Listed Buildings and Conservation Areas)
Act 1990; In a conservation area designated
in accordance with section 69 of that Act;
or Included in the schedule of monuments
maintained under section 1 of the Ancient
Monuments and Archaeological Areas Act
1979; Carports.
iii) Buildings which are; Listed in accordance
with section 1 of the Planning (Listed Buildings
and Conservation Areas) Act 1990; In a
conservation area designated in accordance
with section 69 of that Act; or Included in the
schedule of monuments maintained under
section 1 of the Ancient Monuments and
Archaeological Areas Act 1979; Used primarily
or solely used as places of worship; Stand-
alone buildings with floor area less than 50m 2 ;
Carports; Temporary building with a planned
time of use less than two years.
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.
If you have any questions or require further
technical information concerning the use of
rooflights in your own project, please contact
one of the Stella Rooflight team on 01794
745445 or visit www.stellarooflight.co.uk
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