An Energy Performance Certificate (EPC) is meant to be an objective assessment of a property’s energy efficiency. However, when specific details about your property are missing, the EPC software compensates by using assumed values—defaults that are often much worse than the actual performance of your home.
These assumed values can significantly downgrade your EPC rating, even if your property is well-insulated and energy-efficient. In this article, we’ll explain when EPCs use assumed values, why this happens, and how you can prevent your property from being unfairly penalised.
What Are Assumed Values in EPC Assessments?
Assumed values are default figures that the EPC software applies when certain data points are missing, unverified, or unknown. These defaults are deliberately conservative, reflecting a worst-case scenario to ensure the EPC does not overstate a property’s efficiency.
When Do EPCs Use Assumed Values?
1. Lack of Visual Evidence
If an assessor cannot see or physically verify an energy feature—like wall insulation, heating controls, or glazing type—they are required to mark it as ‘Unknown’. The software will then use a default assumption based on the property’s age and type.
2. Missing Documentation
Even if an upgrade exists (e.g., cavity wall insulation or a high-efficiency boiler), without proper installation certificates, manuals, or technical documents, the software applies assumed values.
3. Inaccessible Property Elements
For areas that cannot be safely accessed during the survey (such as sealed lofts, floor voids, or concealed systems), assessors must default to assumed values.
4. Non-Standard Materials Not in SAP Database
If your home features custom eco-materials like hempcrete insulation or imported glazing not listed in the EPC software database, it defaults to a standard assumption.
➡️ Understand how ‘Unknown’ entries penalise your EPC rating.
How Assumed Values Drag Down Your EPC Rating
a) Default U-Values are High
For unverified insulation in walls, roofs, or floors, the software assumes high U-values (i.e., poor insulation):
Example: Wall U-value may default to 2.10 W/m²K, even if actual performance is 0.30 W/m²K.
Result: The building appears far less thermally efficient, increasing calculated heat loss.
b) Heating Systems Get Downgraded
If a boiler’s make/model isn’t recorded, the software assumes it’s an inefficient non-condensing boiler (~65-70% efficiency), even if it’s a modern condensing unit operating at 90%+ efficiency.
c) Ventilation and Air Leakage Assumptions
Without evidence of mechanical ventilation systems (e.g., MVHR) or air-tightness test results, EPCs assume natural ventilation with high air leakage rates, reducing the property’s energy efficiency score.
d) Compounded Rating Drop
Each assumed value slightly worsens the EPC score, but when multiple defaults stack up, it can drop your EPC band by one or two levels (e.g., from C to E).
Real-World Scenario: Assumed Values in Action
| Feature | Verified Property A | Defaulted Property B |
|---|---|---|
| Wall Insulation | Verified cavity insulation (0.30 U) | Assumed no insulation (2.10 U) |
| Boiler Efficiency | Documented condensing boiler (90%) | Assumed old boiler (68%) |
| Glazing | Verified Low-E double glazing | Assumed standard double glazing |
| Ventilation | MVHR with documentation | Assumed natural ventilation |
| EPC Band C | EPC Band D/E |
Why Does the EPC System Default to Worst-Case Assumptions?
The EPC methodology is designed to avoid overstating a property’s efficiency. If evidence is missing:
The system assumes the building is underperforming.
This maintains compliance integrity, but can unfairly penalise well-maintained properties lacking documentation.
➡️ Find out how EPCrate ensures accurate assessments with minimal defaults.
How to Prevent Assumed Values from Hurting Your EPC Rating
1. Gather All Documentation Before the Assessment
Cavity wall and loft insulation certificates
Boiler manuals with efficiency labels
Window/glazing specifications
Ventilation system commissioning reports
2. Ensure Assessor Access to All Areas
Loft hatches
Boiler ID plates
Underfloor spaces (where possible)
3. Choose Assessors Who Minimise Defaults
At EPCrate, we:
Conduct detailed inspections to reduce ‘Unknown’ entries
Help you organise documentation to avoid defaults
Provide post-assessment recommendations to improve your rating
➡️ Book your EPC assessment with EPCrate
➡️ View our transparent pricing
➡️ Contact us for personalised guidance
Conclusion: Assumed Values Can Unfairly Lower Your EPC—But You Can Prevent It
Assumed values are not an error—they are built into the EPC system to ensure compliance when data is missing. However, with proactive preparation and a thorough assessor, you can ensure that your EPC reflects your property’s true performance—not worst-case defaults.
