When an Energy Performance Certificate (EPC) assessor inputs ‘Unknown’ values into the software, it’s not about guesswork—it’s a default system designed to ensure consistency when certain building details are missing. But what does this mean for your property’s rating?

In this article, we’ll decode EPC ‘Unknown’ values, focusing on U-values, heating systems, and ventilation defaults. We’ll also explain how these defaults impact your final EPC score and how to avoid them.

What Are ‘Unknown’ Values in EPC Assessments?

During an EPC survey, if the assessor cannot verify specific construction details, such as wall insulation thickness or boiler efficiency, they select ‘Unknown’ in the software. This prompts the software to apply default assumptions, which are often pessimistic estimates to err on the side of caution.

Common Areas Where ‘Unknown’ Defaults Apply:

  • U-Values of walls, roofs, and floors

  • Heating system efficiency

  • Ventilation type and air permeability

  • Window glazing specifications

How ‘Unknown’ U-Values Affect Your EPC Rating

A U-value measures how well a building element (like a wall or roof) resists heat loss. Lower U-values mean better insulation.

If your property’s wall construction is unknown, the software will assume the worst-case scenario for that building’s era. For example:

  • Solid brick walls with no insulation (pre-1930 default)

  • Uninsulated cavity walls (1930-1980 default)

This can drop your EPC rating by one or two bands, even if you have had insulation retrofitted but lack documentation.

Heating Systems: The Impact of Default Efficiency Values

When assessors can’t identify your boiler make or model, they must select an ‘Unknown’ option. The EPC software then assigns a default Seasonal Efficiency of Domestic Boilers in the UK (SEDBUK) rating, which is typically lower than the actual efficiency of modern boilers.

For example:

  • Unknown gas boilers default to ~65%-70% efficiency

  • Unknown heat pumps might be treated less favourably

Result? You could lose critical points in your heating system’s contribution to the EPC rating.

Ventilation Defaults: Overlooking Mechanical Systems

If your property has mechanical ventilation with heat recovery (MVHR) but no documentation or evidence (like commissioning certificates), the assessor will default to:

  • Natural ventilation for older homes

  • Assumed air permeability rates which are higher than actual values in well-sealed buildings

This could lower your EPC’s assessment of air-tightness, a key factor in energy efficiency.

Why ‘Unknown’ Defaults Hurt Your EPC Rating

EPC software is designed to avoid optimistic assumptions. When details are missing:

  • It assumes older, less efficient materials and systems

  • It applies conservative performance values to avoid underestimating energy use

  • The overall EPC band can drop from C to D or even lower

How to Avoid ‘Unknown’ Defaults in Your EPC

  1. Gather Documentation: Insulation certificates, boiler manuals, window glazing specs.

  2. Provide Installers’ Reports: For heating systems, MVHR units, and retrofit works.

  3. Book a Detailed Survey: Hire experienced EPC assessors who investigate beyond surface-level checks.

➡️ Book your EPC assessment with EPCrate now to ensure all verifiable details are recorded and defaults are minimised.

Why Choose EPCrate for Accurate EPC Assessments?

At EPCrate, our certified EPC assessors specialise in reducing the impact of ‘Unknown’ defaults by thoroughly inspecting properties and verifying materials and systems wherever possible.

Conclusion

‘Unknown’ values in EPC assessments are not mere placeholders—they directly affect your energy rating by applying worst-case assumptions. By proactively preparing documentation and choosing a thorough EPC service like EPCrate, you can ensure your property gets the rating it deserves.