It is important to understand that the Ψg value is not an actual intrinsic physical property, but a compensation value for all effects that cannot be described with the simple U-value model. Therefore the Ψg (as any Ψ value) is dependent on the dimensional reference. Using internal measurements you will get a different value as for using external measurements. However when performing the whole building calculation using U-values and Ψ-values you will always get the same total energy values – if you have consistently remained in the same reference system.
What are the correct reference lengths? The correct dimensions for your Ψ calculations are always the ones that are defined and used in your building energy performance calculation. Unfortunately these vary quite significantly depending on the geographic region as well as on the type of calculation (e.g. passive house, energy pass,…). If you are doing the building energy calculation yourself you will certainly know the correct dimensional references, otherwise you will have to get this information of the person responsible for doing this calculation.
To provide an example some commonly used reference levels for the bottom of the wall are:
- grade level (ground surface)
- top level of floor
- top level of floor slab
- bottom level of floor slab insulation
possible references for the horizontal measurement are:
(this value will define the edge of your B’/2 model)
- exterior surface of the wall
- interior surface of the wall
- exterior edge of the floor slab