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Eco-friendly homes are designed to minimize environmental impact through energy-efficient technologies, innovative construction materials, and sustainable living strategies. However, when it comes to Energy Performance Certificate (EPC) assessments, many of these unique, high-performance homes receive disappointing ratings. The culprit? Default values used in EPC software that fail to account for bespoke, eco-conscious design choices.

In this article, we’ll explore why default assumptions in EPC calculations can unfairly penalize eco-friendly homes, and how homeowners can mitigate these issues.

What Are Default Values in EPC Assessments?

EPC assessments are governed by standardized software that models a property’s energy performance. When specific information about building components, heating systems, insulation, or air permeability is unavailable, the software inserts default values—pre-set estimates that often represent average or worst-case scenarios.

While designed to provide a consistent baseline, these defaults can severely underestimate the actual efficiency of bespoke eco-homes.

Common Scenarios Where Default Values Penalize Eco-Friendly Homes:

1. Unconventional Wall & Roof Constructions

Homes built with cob, straw bale, hempcrete, or other alternative materials are often misclassified as “unknown wall type,” triggering default U-values that don’t reflect their true thermal performance.

2. Lack of Airtightness Test Data

If a property lacks a formal air permeability (airtightness) test result, the EPC software applies a generic leakage rate. Eco-homes designed for passive house standards may therefore be penalized with inflated air leakage assumptions.

3. Renewable Energy Systems with Missing Documentation

Solar PV systems, solar thermal panels, or heat pumps require detailed specifications (efficiency ratings, system size, installation data). In their absence, the software assigns conservative default values, reducing their credited efficiency impact.

4. Underfloor Heating and Low-Temperature Emitters Not Properly Accounted For

Unless explicitly documented, the software may assume standard radiator systems, which lowers the perceived efficiency of heat pumps or low-temperature heating solutions.

5. Ventilation System Assumptions

If a property lacks documented ventilation strategies (e.g., MVHR systems), default assumptions regarding air quality and ventilation energy use may be applied, often to the detriment of the rating.

The EPC Reality Gap for Eco-Homes

Eco-homes often achieve real-world energy consumption far below that of standard houses. Yet, because EPC software is built on generalized assumptions and averages, unique design features can be overlooked or undervalued, leading to lower-than-deserved EPC ratings.

How to Avoid EPC Downgrades from Default Values

  • Supply Detailed Documentation: Ensure U-values, system specifications, renewable energy performance data, and airtightness test results are available and presented to the assessor.

  • Highlight Sustainable Construction Methods: Make sure non-standard wall and roof constructions are explained with accompanying performance data.

  • Choose Assessors Experienced with Eco-Homes: Assessors who are familiar with sustainable building techniques are better equipped to navigate EPC software limitations.

  • Consider a Full SAP Assessment (for New Builds): For new eco-builds, a full SAP assessment allows for custom inputs and a more precise reflection of your property’s performance than a simplified RdSAP assessment.

EPCrate: Specialists in Eco-Home EPC Assessments

At EPCrate, we specialize in EPC assessments for unique, eco-friendly properties. Our EPC assessment services ensure that your home’s sustainable features are properly accounted for, avoiding unnecessary penalties from default values.

Explore our pricing options and schedule your expert assessment through our online booking system.

Book an EPC Assessment That Understands Your Eco-Home

Don’t let default assumptions downgrade your property’s true performance. Contact EPCrate to book an EPC assessment designed for unique, sustainable homes.