The building envelope is a major factor determining energy use in buildings along with factors associated
with usage patterns. The building envelope contributes to thermal comfort and energy efficiency through its:
A further consideration is the avoidance of condensation resulting from thermal bridging.
- Control of solar radiation
- Control of air permeability
- Control of long wave radiation
- Thermal mass
Part L of the Building Regulations (England and Wales) covers the energy efficiency of buildings. The 2006
Regulations require the design and construction of the building envelope and building services to result in
sufficiently low carbon dioxide emissions from the building. Compliance can only be shown by considering all
aspects of the bulding use and servicing as well as the envelope performance.
It is no longer be possible to state that a zone of the building envelope complies with the Building Regulations.
In whole building energy models, specifiers must now state what U-values, g-values and air permeabilities have been assumed for a particular zone of the building envelope.
Whole building air leakage
Air leakage is a major contributor to energy loss in buildings. From 2002 the Building
Regulations (England and Wales) have required whole building air leakage of larger buildings, from 2006 this
will apply to most buildings other than dwelings. Whole building air leakage of the finished building has to be
less than the air permeability assumed in energy calculations and less than 10 m3/hr/m2 at
High levels of air leakage are normally associated with the interfaces between elements, particularly between
different cladding contract packages of work. CWCT gives guidance on whole building air leakage testing in
Technical Note TN44.
Environmental control glass and shading devices
It is predicted that global warming will lead to an increase of several degrees in summertime temperatures
over the next few decades. This has the potential to increase the demand for energy for cooling. To reduce the
need for cooling energy the amount of summertime solar gain has to be limited.
Solar transmission into buildings can be controlled by shading devices such as blinds and brise soleil or
with environmental control glasses. These can reduce solar transmission whilst giving a smaller reduction in
transmission of daylight. It is also possible to reduce the area of window but this wil reduce the incoming
daylight and increase any need for artificial electric lighting.
Thermal performance guidance
CWCT, jointly with the Council for Aluminium in Building, has developed 'The thermal assessment of window assemblies, curtain walling and non-traditional building envelopes' to support the 2006 Building Regulations. This gives advice on how to specify, design and analyse building envelopes that will, when considered at a whole buiding level, allow compliance with Parts L-2A and L-2B of the Regulations.
CWCT also publishes guidance on U-values and condensation.