Thermal insulation of external elements of construction is
measured in terms of thermal transmittance rate, otherwise known
as the U-value. It is the amount of heat energy in watts
transmitted through one square metre of construction for every
one degree Kelvin between external and internal air temperature,
i.e. W/m^2 K.
U-values are unlikely to be entirely accurate, due to:
the varying effects of solar radiation, atmospheric dampness
and prevailing winds.
inconsistencies in construction, even with the best of
supervision.
`bridging' where different structural components meet, e.g.
dense mortar in lightweight blockwork.
Nevertheless, calculation of the U-value for a particular element of
construction will provide guidance as to whether the structure is
thermally acceptable. The Building Regulations, Approved
Document L, Conservation of fuel and power, determines
acceptable energy efficiency standards for modern buildings, with
the objective of limiting the emission of carbon dioxide and other
burnt gases into the atmosphere.
The U-value is calculated by taking the reciprocal of the summed
thermal resistances (R) of the component parts of an element of
construction:
U¼^1
R
¼W=m^2 KR is expressed in m^2 K/W. The higher the value, the better a
component's insulation. Conversely, the lower the value of U, the
better the insulative properties of the structure.
Building Regulations, Approved Document references:
L1A, Work in new dwellings.
L1B, Work in existing dwellings.
L2A, Work in new buildings other than dwellings.
L2B, Work in existing buildings other than dwellings.
Thermal Insulation, U-Value Calculations---1