The term breathable should not be used when referring to building because it confuses the movement of air with the movement of water. For example, it is possible for a building membrane to be vapour-permeable and air-tight, i.e. it allows for passage of water vapour, but not for air. Hence it is better to refer to building materials as vapour permeable, rather than "breathable".
EPC - Energy Performance Certificate
A certificate created when a building is analysed using the UK Standard Assessment Procedure
RdSAP - Reduce Data Standard Assessment Procedure
A government procedure for analysing the performance of buildings that uses a reduced data input, it does this by making a range of assumptions about the building fabric
SAP - Standard Assessment Procedure  
-   The UK method of assessing the energy performance of domestic dwellings. The SAP method produces the Energy Performance Certificate (EPC) that many of us a familiar with. The system has long been due a revamp and a new standard is due to be released in later 2022.
PHPP - Passive House Planning Package
The simulation software developed by the Passive House Institute for modelling the energy performance of buildings. This method is used in the design of Passivhaus buildings, but can also be used to assess existing buildings and is acknowledged as being a more accurate method than the UK SAP procedure.
Vapour Open
This term refers to the transport of water in it's gaseous form. A vapour open structure will allow water vapour to move through it.
Vapour Closed
A vapour closed structure does not permit the transport of water vapour through, so accumulations of vapour must be dealt with in other ways, i.e. using ventilation, if a vapour closed construction is used. Failure to do this could allow condensation, damp and mould to form on the surface or within the structure.
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