One of the most overlooked yet critical factors influencing Energy Performance Certificate (EPC) ratings is air tightness. While insulation, heating systems, and glazing get the spotlight, uncontrolled air leakage through gaps, cracks, and junctions can drastically undermine a building’s energy efficiency.

A professional air tightness test (blower door test) quantifies these air leaks, providing data that can significantly impact your EPC score. But when is it necessary? And when does it actually make a difference?

At EPCrate, we help property owners understand when air tightness tests are worth the investment for their EPC objectives.

What is an Air Tightness Test (Blower Door Test)?

An air tightness test involves:

  • Fitting a temporary blower door fan to a property’s main entrance.

  • Pressurizing and depressurizing the building to measure the rate at which air leaks through gaps and penetrations.

  • Providing a measured air permeability result (m³/h.m² @ 50Pa), which is then used in EPC software calculations.

How Does Air Tightness Affect EPC Ratings?

EPC assessments include air permeability as a key input affecting:

  • Heat loss through uncontrolled ventilation.

  • Heating demand calculations.

  • Overall carbon emissions estimation.

If no actual air test result is available, EPC assessors must use default air leakage rates, which are conservative and often penalize older properties or untested new builds.

When Does an Air Tightness Test Make a Difference?

1. New Builds Aiming for High EPC Ratings (A or B)

  • New properties designed to meet high efficiency standards benefit immensely from proving their air tightness.

  • A good test result can bridge the gap between a B and an A rating.

2. Retrofit Properties with Air Sealing Measures

  • If you’ve invested in air sealing (draught-proofing, membrane installations, careful detailing), a blower door test provides evidence to justify improved EPC inputs.

3. Large Detached Properties

  • Larger properties suffer more from volume-related heat loss. Demonstrating low air permeability can mitigate penalties from default assumptions.

4. Properties with Complex Junctions (e.g., Extensions, Dormers)

  • Complex builds often default to high assumed leakage rates in EPC software unless proven otherwise.

When Air Tightness Tests May Have Limited EPC Impact

  • Poorly Insulated Properties: If walls, roofs, and floors are uninsulated, air tightness improvements alone won’t significantly uplift the rating.

  • Small Apartments in Block Developments: Flats often benefit from “sheltered” assumptions due to surrounding dwellings, reducing the EPC impact of air tightness testing.

How to Ensure Your Blower Door Test Benefits Your EPC Rating

  1. Conduct the Test Post-Improvement Works
    Air tightness tests are most effective after draught-proofing, insulation upgrades, or window replacements.

  2. Work with Assessors Who Integrate Test Results Correctly
    At EPCrate, we ensure air permeability data is entered accurately into the EPC model for maximum rating benefit.

  3. Use Tests Strategically for MEES Compliance
    If your property is borderline EPC E (or C under proposed 2025 updates), a blower door test could be the decisive factor in reaching compliance.

Book an EPC Assessment & Air Tightness Consultation with EPCrate

Considering an air tightness test to improve your EPC rating?
EPCrate offers integrated assessment packages that ensure blower door results are correctly factored into your EPC.

Book your EPC assessment or explore our pricing options for combined testing and certification services.
For tailored advice, contact EPCrate today.