Insulate Magazine Issue 11

Ignoring complexities in a proposed construction
for the sake of convenience, and trying
to represent them without using numerical
modelling, risks making the calculation result
unrepresentative of what will be achieved in
practice.
So what does ISO 6946 allow for?
It is best suited to building elements with layers
of consistent thickness and uniform thermal
properties (“homogenous layers”), but also
provides ways of dealing with inhomogenous
layers. It can include for the thermal properties
of air layers up to 300mm in thickness,
but can’t allow for metal components bridging
insulation layers.
There is a correction factor available for metal
fixings, however, as well as one for air gaps
in insulation layers; if these corrections total
more than 3% of the calculated U-value then
the result is amended to reflect the impact on
performance. Finally for the purposes of this
overview, there is an annex describing how
tapered insulation layers can be calculated,
providing a means of establishing the thermal
transmittance of cut-to-falls roofing schemes.
While ISO 6946 describes how to carry out
combined method calculations, it doesn’t offer
much guidance on what to include in them...
BRE Report BR 443
… which is where this free-to-download
document comes in, providing essential information
appropriate conventions to use when
calculating U-values. It should be familiar to
anybody who provides a calculation service.
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