To limit the probability that the overall thermal transmittance will be unacceptably high, which could lead to excessive thermal transfer through the building envelope, which could lead to excessive use of energy, which could lead to an unacceptable effect on the environment.
To supersede the requirements of Sentences (1) and (2) and require a more stringent overall thermal transmittance where thermal transfer could increase due to the presence of radiant heating cables, heating or cooling pipes or membranes.