At the start of each building elemental section, information is provided on the functional unit. A functional unit provides essential information about the general attributes of each specification, including the unit of comparison and its performance characteristics. An example of the functional unit for insulation is shown below.
Functional unit insulation: 1m² of insulation with sufficient thickness to provide a thermal resistance value of 3 m²K/W, equivalent to approximately 100mm of insulation with a conductivity (k value) of 0.033 W/mK
The functional units were developed by BRE and put out to consultation to ensure they covered the relevant aspects and typical situation for each element.
All specifications included in the Green Guide will need to comply with this functional unit in order to ensure fair comparison. Whilst specifications are generally comparable, there are, inevitably, instances where comparisons are not exactly like for like: e.g. some partitioning systems have lower acoustic transmission than others. Specifiers are expected to use their expert judgement in such cases.