A one-dimensional model describing the steady-state dynamic and diffusive behaviour of a three-layer building envelope element is developed with the objective of elucidating the physics of simultaneous heat and vapour transport through dynamically insulated building envelopes. The equations are simple to program on a spreadsheet, enabling architects to build tools which will allow them to design "breathing" envelope constructions. The variables at the designer's disposal are air and vapour permeability, thermal conductivity and thickness of the layers comprising the envelope. Users wishing to consider more complicated constructions will find that they can readily extend, by inspection, the equations from the three layers presented here to any number of layers. Whilst mass transfer has been discussed in terms of water vapour transport, the equations are very general and can be applied to the transport of any gas through a permeable wall.