You’ve invested in insulation, upgraded your heating system, and even installed new windows—yet your EPC rating comes back disappointingly low. Why? The culprit is often buried in a little-known aspect of the EPC process: ‘Unknown’ defaults.
In this deep dive, we’ll uncover how ‘Unknown’ entries in an EPC assessment silently drag down your rating, and what you can do to ensure your property gets the score it truly deserves.
The ‘Unknown’ Defaults: A Silent EPC Killer
Energy Performance Certificates (EPCs) are designed to be evidence-based. If an EPC assessor cannot see or verify specific details about your property’s construction or systems, they are required to select ‘Unknown’ in the assessment software.
But here’s the catch:
The EPC software automatically applies conservative, worst-case assumptions whenever a detail is marked ‘Unknown’.
This is not a glitch—it’s a compliance mechanism built to avoid overestimating a property’s energy efficiency.
Why Do ‘Unknown’ Entries Appear?
1. No Documentation Provided
If you can’t supply insulation certificates, boiler specifications, or glazing reports, the assessor is forced to select ‘Unknown’ even if the upgrade exists.
2. Non-Visible Installations
Upgrades like cavity wall insulation or underfloor insulation that aren’t visible without invasive inspection are often marked ‘Unknown’.
3. Non-Standard or Eco Materials
If your home uses materials like hempcrete, triple-glazing not listed in the SAP database, or bespoke systems, the assessor may have no option but to apply defaults.
4. Inaccessible Systems
Heating systems or mechanical ventilation units (MVHR) that are installed in concealed spaces with no accessible identification plate or commissioning certificate will default to ‘Unknown’.
How Do ‘Unknown’ Defaults Affect Your EPC Rating?
a) Default U-Values Are Brutal
For walls, floors, and roofs where insulation can’t be verified, the software assigns default U-values that reflect an uninsulated structure. This drastically increases the calculated heat loss.
Example: An insulated cavity wall (U-value ~0.30 W/m²K) might default to 2.10 W/m²K—7x worse than reality.
b) Heating Systems Are Penalised
If a boiler’s make or model is unverified, the software assumes it’s an old, inefficient non-condensing unit (~65-70% efficiency), regardless of actual performance.
c) Ventilation Efficiency Drops
Without proof of mechanical ventilation systems like MVHR, the system defaults to natural ventilation with assumed high air leakage, reducing the air-tightness rating.
d) Cascading Rating Drops
Each ‘Unknown’ entry adds a conservative estimate, causing a compounding effect. A Band C property could easily slip into Band D or even E purely because of missing documentation.
Real-World Impact of ‘Unknown’ Defaults
Lower EPC ratings affect property value and marketability.
Mortgage approvals, rental compliance, and green grants often require minimum EPC standards.
Properties in London with EPC ratings below Band E may face rental restrictions.
How to Avoid Defaults Dragging Down Your EPC Rating
1. Gather Your Documentation
Ensure you have:
Cavity wall insulation certificates
Boiler installation reports
Window glazing specifications
Mechanical ventilation commissioning papers
2. Hire Detail-Oriented EPC Assessors
Not all EPC assessors go the extra mile. At EPCrate, we specialise in detailed surveys to identify hidden upgrades and minimise ‘Unknown’ defaults.
3. Book a Pre-Assessment Documentation Review
We offer a pre-assessment consultation to help you prepare the necessary documentation before the site visit.
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Conclusion: Your EPC Rating Deserves Accuracy
‘Unknown’ defaults in EPC assessments can be the hidden reason behind your lower-than-expected rating. By understanding how they work and preparing accordingly, you can ensure your property’s true energy efficiency is reflected.
At EPCrate, we help London property owners reclaim lost EPC points by ensuring every possible detail is verified and correctly recorded.
