Skylights are a popular architectural feature in modern homes and renovations. They flood interiors with natural daylight, enhance aesthetics, and reduce the need for artificial lighting. But when it comes to Energy Performance Certificates (EPCs), too many skylights can unexpectedly drag down your property’s rating.

At EPCrate, we often encounter properties where extensive glazing, particularly in roofs, leads to EPC scores lower than expected—even in energy-efficient homes.

How EPC Software Assesses Skylights & Roof Glazing

EPC assessments evaluate skylights based on:

  1. U-Value (thermal transmittance of the glazing)

  2. G-Value (solar gain coefficient)

  3. Glazed Area as a Percentage of Roof Surface

  4. Frame Material & Air Leakage

While skylights improve daylighting, they also represent potential points of heat loss in winter and solar gain in summer, both of which are factored into EPC heat loss and cooling load calculations.

Why Excessive Skylights Can Lower Your EPC Rating

1. Higher Heat Loss through Roof Glazing

Even high-spec double or triple-glazed skylights typically have higher U-values than well-insulated solid roof areas.
EPC algorithms penalize this increased heat transmittance, especially in dwellings with a large percentage of roof glazing.

2. Default Assumptions May Apply Unfair Penalties

Without exact manufacturer specifications (U-values, G-values), EPC software defaults to conservative figures, further penalizing the rating.

3. Summer Overheating Risks Increase Cooling Loads

Large skylight areas can significantly increase internal temperatures in summer. If not paired with shading devices or specialist glazing, EPC calculations may assume higher energy usage for cooling, reducing overall efficiency scores.

4. Frame Material Thermal Bridging

Metal-framed skylights, if undocumented, may default to high thermal bridging factors, further worsening the EPC result.

When Skylights Help EPC Ratings

Skylights can contribute positively when:

  • They are specified with ultra-low U-values (≤1.2 W/m²K).

  • They incorporate solar control coatings to manage G-values effectively.

  • They significantly reduce artificial lighting demand (documented in the assessment).

  • The overall glazed area is balanced relative to the roof’s solid sections.

How to Avoid EPC Penalties for Skylights

  1. Provide Manufacturer Specifications for All Skylights
    Ensure documentation includes:

    • U-value certifications

    • G-value (solar gain) data

    • Frame thermal performance details

  2. Incorporate Shading Solutions
    Roof blinds, external shading, or integrated smart glass can mitigate overheating risks and should be documented.

  3. Limit Skylight Area Relative to Roof Surface
    Excessive roof glazing (>20% of the total roof area) should be carefully evaluated, as EPC penalties become more pronounced beyond certain thresholds.

  4. Use SAP Assessments for Complex Glazing Designs
    For extensive glazing designs, especially in new builds or deep retrofits, full SAP calculations allow for more accurate inputs than RdSAP, which often relies on defaults.

  5. Work with Assessors Familiar with Architectural Glazing
    EPCrate’s assessors ensure proper input of glazing specs and advanced features to minimise EPC rating penalties.

Book an EPC Assessment for Skylight-Heavy Properties with EPCrate

Planning a skylight-rich renovation or concerned about an existing property’s EPC rating?
EPCrate specialises in assessments for architecturally complex homes.
Book your EPC assessment today or explore our pricing plans for bespoke glazing-heavy projects.