The thermal assessment of window assemblies, curtain walling and nontraditional building envelopes.
This document is intended for use by the building design team, building envelope
designers and building inspectors. It allows the early estimation of building envelope
U-values, details the information required in a specification, and describes methods
for showing compliance of the final building envelope design with the requirements of
Parts L-2A and L-2B of the Building Regulations.
The appendices show how detailed calculations should be undertaken to establish
component U-values and the properties of their associated thermal bridges, and how
these should be incorporated into a detailed calculation of the equivalent U-value of a
zone of the building envelope.
Finally the document shows how the equivalent U-values of a zone of building
envelope may be used as an input into software that models building energy use.
Estimating U-values of stick system curtain walls – Appendix A.
Appendix A of the document allows building designers to estimate the U-value of a
stick system curtain wall at an early stage in the building design.
The U-value of a stick curtain wall depends on;





Proportion of the wall that is glazed;
Mullion spacing;
Number of opening lights;
The U-values of separate components;
Heat transfer between components.
A series of curves have been developed to allow a quick way of estimating U-values.
They are based on mullion spacing with corrections to allow for opening lights,
additional opaque zones, and panel U-values.
A spreadsheet has also been produced that gives an alternative way to
estimate/compare the U-values of stick system curtain wall. Different elements
(including opaque panels and fixed and opening lights) are input into a grid up to 10
elements wide by 6 elements high. This allows any combination of curtain wall to be
assessed and allows quick comparison of different systems/different combinations to
ensure that it meets the envelope specification for that particular zone.
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 -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 Uvalues are calculated as described in Appendix C but -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 -values 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 threedimensional analysis. The two-dimensional method is simpler and requires fewer
simulations but the three-dimensional method will be more accurate and lead to
lower U-values.
The two-dimensional method the frame is considered continuous with discontinuous
components such and brackets and fixings are ‘smeared’ and given an equivalent
thermal conductivity. In the three-dimensional method brackets and fixings are
(-value) calculated for the
thermal bridge formed.
Input into building energy models – Appendix E
Building energy models such as SBEM require walls to be input as if they are
traditional walls comprising windows and opaque wall. It is therefore important that
non-traditional walls are correctly represented when using such models.
The individual U-values of the windows and walls do not need to be the correct
values as long as the overall U-value for the zone is correct. However the correct
area of glass must be used as this will be used to determine the total solar energy
transfer and the transmission of daylight.
The Appendix gives the method for ensuring that the correct values are used and
gives details on how to include extra heat loss due to thermal bridges.