What the calculation tests
SAP compares your dwelling with a notional dwelling of the same size and shape built to a reference specification. Four things must pass: the Dwelling Emission Rate against its target, the Dwelling Primary Energy Rate against its target, the Dwelling Fabric Energy Efficiency against its target, and the limiting standards for individual fabric elements and building services. The fabric measure is the one that catches people out, because renewables improve carbon and primary energy without improving fabric efficiency at all.
What we need from the drawings
Accurate geometry matters more than a finished specification. We take dimensions off scaled plans, elevations and sections, so a set that is missing a section or has no scale bar slows everything down. Orientation matters because solar gains and losses are modelled per elevation, so we need a site plan with north on it.
- Scaled floor plans, elevations and sections for each dwelling type
- Site plan showing orientation and plot positions
- Window and door schedule, or dimensions on the elevations
- Fabric build-ups, or a note of what is assumed
- Heating, hot water and ventilation strategy
- Air permeability target, if the contractor has one in mind
Why the timing changes the cost of compliance
At the point where drawings are still being developed, a failing type can usually be fixed by adjusting insulation thickness, reducing a glazed area slightly, improving a junction detail or changing a glazing specification. Once the fabric is ordered and the frames are on site, none of those are available, and the remaining routes are services and renewables, which cost more per unit of carbon and often need coordination with the electrical design. Commissioning the model early is not about compliance paperwork; it is about keeping the cheap options open.
Thermal bridging and junction details
Heat lost at junctions between elements is modelled through linear thermal transmittance values, and the default values in SAP are deliberately pessimistic. Using accredited or calculated details for the junctions that repeat most across your scheme, usually eaves, window reveals and the wall-to-floor junction, can move a marginal dwelling type into compliance without changing anything you buy. We flag where that is worth doing rather than applying it everywhere.
Air permeability: choose a target the site can hit
The design-stage model uses a design air permeability figure, and the as-built model has to use the tested result. Specifying a tight figure that the build cannot achieve simply moves the problem to completion, when the only fixes left are expensive. We would rather model a figure your team has actually achieved on comparable work and find the remaining margin elsewhere.
