Insulate Magazine Issue 12 - November 2017

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4 Bridging details
Calculating U-values is easiest when a construction
comprises simple, uninterrupted material layers of consistent
thickness - but layers often have to be interrupted
by other materials. Accounting for bridging is important,
especially when it’s an insulation layer and the disparity
in thermal performance of the two materials is significant.
This section states what percentage of the layer is
bridged and by what material. But it might not mean anything
to the reader. It’s not obvious what “12.5% timber
bridging” represents in reality, but if the layer description
says the material is bridged by 50mm wide timber joists
or studs at 400mm centres, then the world suddenly
makes more sense!
5 Result and corrections
And so we reach the headline of the whole document! A
figure for the predicted loss of heat energy (in Watts, W),
per square metre of the construction fabric, per degree
of temperature difference (in Kelvin, K) between inside
and outside.
But what are the numbers that accompany the result?
Upper and lower limit thermal resistances refer to the
two methods of calculating heat paths through the construction,
as described by the Combined Method. These
two values are averaged, then a reciprocal taken, to give
the U-value.
Within the confines of mathematically modelling construction
build ups, certain corrections can be applied to
reflect the realities of installing insulation. In any calculation,
you may see values associated with the following:
Uf - corrections for mechanical fixings penetrating the
insulation layer.
Ug - corrections for air gaps in the insulation layer
Up - correction for compression of insulation in built up
metal roofing and cladding.
Ur - correction for rainwater cooling on inverted roofs.
Urc - corrections for rails and/or brackets supporting
rainscreen cladding.
6 Application-specific information
In the last issue of Insulate we looked at standards that
define how U-values are calculated. The Combined
Method is fairly limited in scope, so other standards
supplement it with calculation methods for ground
floors, anything involving steel components etc. Where
these other standards are employed by the calculation
software, the relevant information is displayed in this
section.
One point worth noting for ground floor constructions:
while it’s tempting to think that perimeter upstand insulation
provided to screeds and slabs would be classed
as edge insulation, for the purposes of a U-value calculation
it isn’t. It is there to treat the thermal bridge, so
could be referenced in the layer description to show it
has been thought about (e.g. ‘Reinforced concrete slab
with perimeter upstand insulation to address thermal
bridging’).
In Conclusion
Anybody calculating U-values should be armed
with as much information as possible about
the intended materials, and should ask lots of
questions where relevant information is missing.
Judging the veracity of calculations is difficult for
many, but communication is key. Knowing that
a design has been accurately calculated, or that
what is being constructed reflects what a calculation
shows, can go a long way to helping address
performance gap issues.
We don’t have the space to delve into these corrections,
but if they impact on the calculation sufficiently then they
cause the result to be changed. It’s important to ensure
that comparable corrections have been included when
judging one calculation against another, otherwise the
comparison is not a fair one.
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