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Calculating U-values of stick system curtain walls - Appendix
B
This section gives the reader all the necessary information
in order for them to calculate the U-value of a stick system curtain
wall.
Three methods are given to assess the overall U-value of
an assembly according to the accuracy of the result required.
•
Method one – approximate method. This method uses
simulation results (described in Appendix C) in order to calculate
frame U-values and psi
(Ψ)
values (generally available from system suppliers). The appropriate
values are then added together to give the overall U-value of the
zone. This gives a fairly close estimate of the overall U-value.
•
Method two – accurate method. This method simulates
the assembly as a whole and calculates a U-value of the assembly that
includes the edge effect. This method does not separate U-value
and linear thermal transmittance, and simulations need to be carried
out for each different glazing/infill panel option.
•
Method three – simplified method. This method is described
in prEN 13947 and should only be used at the early design stage as
it only gives an indicative overall U-value of the building element.
Frame U-values are calculated as described in Appendix C but
Ψ
values are taken as defaults that may differ from the exact
values.
Calculating
properties of framing members - Appendix C
This appendix
gives details on how to calculate frame U-values and
Ψ
values, especially the
Ψ
value of window insert, that are needed for the calculation of the
curtain wall zone. Information on the geometrical model used
for the simulations, and the boundary conditions used are given.
Calculating
U-values of rainscreen overcladding – Appendix D
The U-value
of a rainscreen may be calculated using either two-dimensional or
three-dimensional analysis. The two-dimensional method is simpler
and requires fewer simulations but the result can not be used for condensation
risk assessment. The three-dimensional method will be more accurate.
In the two-dimensional method the virtual frame is considered
continuous with discontinuous components such and brackets and fixings
‘smeared’ and given an equivalent thermal conductivity. This method
will result in an overall U-value higher than the realistic one.
In the three-dimensional method brackets and fixings are modelled
correctly with a point thermal transmittance (chi-value) calculated
for the thermal bridge formed.
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