As sustainable building practices gain momentum, alternative wall construction methods like cob, straw bale, and hempcrete are becoming popular for eco-conscious homeowners. These natural materials offer exceptional thermal mass, breathability, and low embodied carbon. However, when it comes to Energy Performance Certificate (EPC) assessments, these innovative building methods often face challenges that can lead to lower-than-expected ratings.

Why Non-Traditional Walls Pose a Challenge for EPC Assessments

EPC methodologies are designed around conventional construction types—brick, block, timber frame, and cavity walls with standard insulation. Alternative materials like cob, straw bale, and hempcrete don’t fit neatly into these categories, resulting in:

  • Misclassification during data input.

  • Default conservative U-values being applied.

  • Underestimated thermal mass benefits.

How EPC Software Assesses Cob, Straw Bale & Hempcrete Walls

1. Default U-Values for Unknown Wall Types

If specific thermal performance data isn’t provided, EPC assessors may have to select a generic ‘other’ wall type with default U-values, which are often worse than the actual performance of well-built natural walls.

2. Thermal Mass Benefits Overlooked

Cob and hempcrete provide excellent thermal mass, stabilizing internal temperatures by absorbing and releasing heat. Unfortunately, EPC software does not fully credit thermal mass effects, focusing primarily on insulation thickness and U-values.

3. Breathability & Moisture Regulation Ignored

One of the main advantages of natural wall systems is their breathability, which helps maintain healthy indoor air quality and reduces condensation risks. EPC assessments do not consider this attribute, missing out on recognizing indoor comfort improvements.

4. Airtightness Assumptions May Penalize

Many natural homes are hand-built and may not have formal airtightness test results. In such cases, default air permeability values are applied, which can overestimate air leakage rates, penalizing the EPC score.

5. Lack of Recognition for Low Embodied Carbon

EPC ratings are based on operational energy efficiency, not the environmental impact of construction materials. Thus, the carbon-saving advantages of cob, straw bale, and hempcrete walls are not reflected in the EPC outcome.

Real Efficiency vs EPC Rating Discrepancy

Although natural wall homes often perform exceptionally well in real-world energy consumption and comfort, EPC ratings can fall short due to software limitations and lack of standardized input categories for these materials.

How to Ensure Fair EPC Ratings for Non-Traditional Wall Constructions

  • Provide Detailed U-Value Calculations: Get a professional thermal performance assessment done for the wall system and submit the calculated U-values.

  • Conduct Airtightness Testing: Supply empirical air permeability data to avoid harsh default assumptions.

  • Document Thermal Mass Strategies: While not always weighted heavily, well-documented design strategies (e.g., passive solar design) should be noted.

  • Work with Assessors Familiar with Eco-Buildings: Choosing assessors who understand alternative construction methods is critical for accurate input and interpretation.

EPCrate: Accurate EPC Assessments for Eco-Friendly Homes

At EPCrate, we specialize in EPC assessments for non-traditional, eco-friendly homes, including cob, straw bale, and hempcrete constructions. Our EPC assessment services ensure that your property’s true thermal performance is fairly represented.

Review our pricing and schedule an assessment through our online booking system.

Book Your EPC Assessment for Alternative Construction Homes

Don’t let software limitations under-represent your eco-friendly home. Contact EPCrate to book an EPC assessment that understands sustainable wall construction.