Notional and Actual Alpha values can be found in the Building Global Parameters section of Compliance documentation:
Calculated Alpha values for your Actual building could be substantially higher than for the Notional building.
Alpha values are not set directly in Bentley Hevacomp's PartL program, but are determined by the SBEM calculation engine using the information provided for Thermal Bridging on the "Misc." tab in the PartL program:
This provides an option to manually enter Psi values. The calculation method used by SBEM thereafter is determined by the National Calculation Methodology as set out in the Modelling Guide. An excerpt containing guidance on Thermal Bridges can be found below:
For queries regarding Alpha value calculations, you should consult with your accreditation Scheme Provider.