February 2022

Knauf Technical Talk
U-VALUE CALCULATIONS EXPLAINED
Approved Document L (Part L) of the Building Regulations sets out how energy efficient
buildings and individual building elements must be. But how do you know whether the
build-up meets the required minimum standards? This month, Bradley Hirst, Technical
Services Manager at Knauf Insulation, looks at U-value calculations in more detail.
What are U-value calculations?
U-value calculations measure whether the
proposed build-up will meet the minimum
standards by determining how well it will
prevent heat transfer. They are measured in
W/m²K (watts per metre squared Kelvin) and the
lower the U-value, the more effective the build-up
is. This means less energy will be needed to heat
or cool a space, so a property with high levels of
insulation is more energy efficient.
Knauf Insulation says numerical
calculations (3D U-values) are
highly accurate, so they should be
used for rainscreen façade systems
rather than the simplified method.
Do I need a U-value calculation for every
project?
Not necessarily. Every project must meet the
limiting standards for fabric energy
efficiency, so architects usually
include a specification in their
plans. For insulation, they may list
a product name or the thermal
resistance (R-value) which has been
verified to achieve compliance. If you
follow the specification exactly, you won’t need to
undertake additional U-value calculations.
“If you change any
element, you will need
new U-value
calculations to
demonstrate
compliance”
But you may need to change the specification. For
example, on a rainscreen façade system build-up,
you may decide to improve the building’s fire safety
strategy by swapping combustible insulation for a
non-combustible solution such as Knauf
Insulation’s Rocksilk RainScreen Slab. If you change
any element, you will need new U-value
Left: Bradley Hirst, technical Services Manager at Knauf Insulation. Below: Knauf Insulation Rocksilk(R)
RainScreen Slab is non-combustible making it suitable for use on buildings of any height.
calculations to
demonstrate compliance.
For applications such as cavity walls
or flat roofs on extensions, many
installers draw on experience and use
the same build-up for every project. This
may save time but remember Part L changes in
June 2022, so check with your insulation
manufacturer’s Technical Team that your chosen
build-up meets the new minimum standards.
Are all U-value calculations the same?
How U-values are calculated is set out by BRE’s
BR443 (Conventions for U-value calculations).
There are two common methods: simplified
calculations and numerical calculations (3D U-
values). Which method is used depends on the
application and the accuracy required.
Simplified calculations are best for
straightforward build-ups
with few variables such as
brick and block walls. But
they aren’t recommended for
rainscreen façade systems
because the blanket correction factor doesn’t
accurately represent the system’s thermal
performance.
Instead, contractors and installers should insist
on 3D U-value calculations which consider the
complex interaction between the insulation,
façade, and rail and bracket system. This high
degree of accuracy often identifies solutions that
would have been ruled out by the simplified
method and ensures the thermal performance of
the final building matches its design.
Contact Knauf Insulation
www.knaufinsulation.co.uk
@KnaufUK
56 TC FEBRUARY 2022