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 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.
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