In 2022, building and construction Research Establishment (BRE) Group commissioned the project "Heat loss calculations through different windows using experiments and CFD" to address a defined challenge in the building and construction sector. The assignment combined computational fluid dynamics (CFD) and energy and thermal modelling with a decision-focused engineering study. Its purpose was to explain the governing physical behaviour, identify the variables controlling performance, and convert the findings into practical recommendations for design, operation and future development.

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.

The methodology centred on a conjugate conduction, convection and radiation model of the glazing and frame assemblies, linked to experimental measurements. The representation retained glass panes, gas cavities, spacers, frames, seals and surrounding reveals that determine the overall heat-transfer path. Boundary and operating conditions covered representative internal and external temperatures, alternative frame and spacer materials, cavity geometries and relevant installation details, with material, fluid and equipment properties assigned from the available design information. Resolution was concentrated in regions where steep velocity, thermal, concentration or phase gradients were expected, while the overall model remained efficient enough to compare several credible configurations. This balance allowed system-level performance to be linked to the local mechanisms responsible for it.

The assessment compared window constructions and detailing choices assessed against the test data. Performance was judged using overall heat-transfer coefficient, heat flux, interior surface temperature, condensation risk, local thermal bridging and uncertainty associated with the measurements. Results were reviewed through quantitative summaries and engineering visualisations, such as contours, vectors, streamlines, sections and time histories, selected to suit the physics. Important assumptions and operating uncertainties were considered so that the recommendations relied on repeatable comparative trends rather than a single nominal case.

For building and construction Research Establishment (BRE) Group, the principal value was standards-aligned evidence for selecting lower-loss windows and improving installation details. The final evidence linked each recommendation to the relevant model or process output, making it suitable for internal design reviews, supplier or contractor discussions and, where applicable, planning, safety or regulatory dialogue. The work also created a reusable baseline that can be updated as geometry, operating data or test results become available, reducing the cost and risk of later design iterations.