INSULATION Project1_Layout 1 07/05/2013 IOur values, your values and U-values
Rob Firman, technical and specification manager at RAVATHERM UK, highlights a growing concern around misleading U-value calculations and
why the construction industry needs to be more vocal about higher standards
t’s an unfortunate truth that despite their
importance, U-values and their calculation
are not as well understood as they should be
and that can lead to misleading calculations.
As an insulation manufacturer, raising
this issue can be tricky; any suggestion that
calculations produced by others are unfair can
sound like sour grapes.
However, accuracy and adherence to standards
is an issue that cuts right to the heart of the built
environment. U-values resulting from inaccurate
calculations feed into energy assessment
calculations (either SAP or SBEM, for domestic
and non-domestic respectively) that are directly
responsible for demonstrating compliance with
In many cases, a miscalculation is the result of
an honest mistake or lack of understanding, but
alarmingly, there is also growing evidence that
standards aren’t being adhered to.
Turning a blind eye
A scenario when this may occur is when the
insulation specification is switched during
construction and after project design stage
compliance has been achieved. There is often
an assumption that the new product will have
the same declared thermal performance as the
one it is replacing. The manufacturer will have
to supply a calculation to prove this. If the result
is not identical, difficult and costly, remedial
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measures will be required to make up the shortfall
Sadly, in an effort to avoid that time and
expense, it is not unknown for a revised calculation
to be issued that says the same as the original.
Calculation issues and errors
There are several important factors to consider
in a U-value calculation, particularly for inverted
roofs such as the building’s location, the effects
of rainwater cooling and ensuring the product’s
‘design’ thermal conductivity is used.
There are additional variables, any of which
could cause a difference in result between two
apparently ‘correct’ versions. Specific issues we
have encountered recently include:
Rounding up R-values: The thermal resistance
(R-value) of a construction material is its
thickness divided by its thermal conductivity.
An Extruded Polystyrene (XPS) product with a
conductivity of 0.034 W/mK, at 200mm thick,
has an R-value of 5.88 m2K/W or 5.85 m2K/W if
rounded down in accordance with BS EN 13164
to the nearest 0.05 m2K/W. We’ve seen examples
of calculations where it has been rounded up, yet
the R-value cannot be any higher than that without
breaking the laws of physics.
Correct thermal values for airspaces: U-value
calculation software allows the dimensions of
an airspace to be entered to ensure the correct
thermal resistance is used.
Where one, or both surfaces, either side of the
airspace has a low emissivity, ISO 6946 includes a
formula to work out an increased airspace thermal
resistance. We have seen examples of calculations
where a greater airspace resistance has been
included, despite the surfaces either side being
standard high emissivity surfaces. There has also
been no justification as to why the higher resistance
has been claimed in the airspace description.
Average UK rainfall figures: U-value
calculations for inverted roofs account for the
cooling effect of rainwater. The lower the rainfall,
the lower the impact on the result. If the location
of the building is known, rainfall data for that
location, or one nearby, should be used (ISO 6946
refers to ‘data relevant for the location’). However,
we have seen the UK’s average rainfall figure be
used, even when the building’s location is known
to have greater rainfall.
Misleading calculations call into question the
construction industry’s ethics and values, as well as
its commitment to delivering a quality end product.
As a responsible manufacturer, transparent
about the calculations we produce and willing to
talk through the results with any customer, it’s
frustrating to know that inaccurate calculations
make their way into the marketplace.
Unfortunately, there is insufficient knowledge in
the industry, especially regarding relevant standards,
which makes it very difficult for this issue to be
adequately policed. This is exacerbated by the fact
that training in calculations isn’t up to scratch.
Initiatives like the British Board of Agrément’s
competency scheme for the calculation of
U-values are supported by a minority of insulation
manufacturers and companies, but not enough to
really put the spotlight on accuracy and competency.
A rigorous approach
Until this situation changes, we call on the
specifiers and purchasers in the construction
industry to join us in being more vocal about
Test the manufacturers you work with. Obtain
more than one calculation for the projects you are
specifying and compare and question the results.
Seek clarity and be confident that a
manufacturer does not simply sell you
the thinnest solution with the biggest
margin, but a product with an accurate
U-value which will ensure a building
– and our industry – performs to the