We quantified the thermal performance of alternative glazing and frame constructions to support energy efficiency decisions in building envelopes. Experimental measurements were combined with computational fluid dynamics and energy and thermal modelling to resolve conduction, convection and surface radiation through window assemblies under representative boundary conditions. Parametric variants covered frame materials, spacer configurations and internal air gap geometries, while surface properties and film coefficients were set consistently with test data. Metrics included overall heat transfer coefficients, interior surface temperatures to assess condensation risk and sensitivity to installation details such as reveals and blinds. Mesh and solver sensitivity checks ensured stable trends across options, and uncertainty from measurement inputs was propagated to provide decision ready ranges rather than single numbers. The analysis highlighted configurations that reduce heat loss while maintaining comfort at the occupied surface, and identified where marginal gains are dominated by installation detailing. The outcome was a clear evidence base to support specification and standards aligned guidance, together with a lightweight model and configuration pack to enable future re-runs as product data evolves.