Modern sustainable buildings increasingly use mixed roof surfaces—combining solar photovoltaic (PV) panels with green (vegetated) roofing. While these designs improve energy efficiency and environmental performance, they present unique challenges for EPC (Energy Performance Certificate) modelling.

EPCs are designed to estimate a property’s energy efficiency based on building fabric, heating/cooling systems, and energy use. Standard EPC models assume conventional roof types and fixed thermal properties. Mixed roofing systems, however, introduce complex interactions between solar generation, insulation, and thermal performance, which can affect EPC accuracy.

Key Challenges

1. Variable Thermal Resistance

   • Green roofs provide excellent insulation and thermal mass, reducing heat gain in summer and heat loss in winter.

   • Solar PV panels can shade sections of the roof, changing the thermal performance locally. EPC software assumes uniform roof properties, so modelling mixed effects is difficult.

2. Impact on Solar Gains

   • EPC models include assumptions about solar heat gains through the building envelope.

   • Partial shading by PV panels over green roofs can alter solar gains, which is not fully captured in standard SAP/RdSAP calculations.

3. Combined Energy Effects

   • Green roofs reduce HVAC loads by improving insulation and thermal mass.

   • Solar PV panels generate electricity, reducing net energy consumption.

   • EPC software typically treats these separately, so the combined benefit may be underrepresented.

4. Complex Roof Geometry

   • Many mixed roofs have segmented or uneven layouts, creating non-uniform heat fluxes.

   • EPC tools assume simplified geometries, which can result in conservative energy efficiency ratings.

Practical Implications

• Buildings with mixed green and solar roofs may perform better in reality than EPC ratings suggest.

• EPC assessors may need detailed roof plans, material specifications, and system output data to improve accuracy.

• Supplementary energy simulations or advanced modelling can help reflect actual efficiency gains.

Recommendations to Improve EPC Modelling Accuracy

1. Provide full documentation of green roof depth, vegetation type, and insulation properties.

2. Include solar PV specifications, such as panel efficiency and shading patterns.

3. Work with EPC assessors familiar with sustainable roofing systems.

4. Consider Same-Day EPC services for fast assessment.

5. Explore our Residential EPC options for detailed modelling support.

For service details, see our Pricing Plan or Contact Us to schedule an assessment.

Conclusion

Mixed roof surfaces with solar PV and green roofing provide significant energy efficiency benefits. However, standard EPC modelling struggles to capture the combined effects of insulation, thermal mass, and energy generation. Detailed documentation, advanced assessment methods, and expert EPC services can help ensure your property’s energy performance is accurately reflected.