Airtightness Testing & Air Permeability

An airtightness test (also called air permeability test or blower door test) measures how much uncontrolled air leakage occurs through a building’s envelope. A powerful fan is temporarily installed in an external door or window opening, creating a pressure difference of 50 Pascals between inside and outside. We measure the airflow required to maintain this pressure, which reveals the building’s air leakage rate. Results are expressed as m³/(h.m²) @ 50Pa—the lower the number, the more airtight the building.

Testing is mandatory for most new build dwellings and commercial buildings under Building Regulations Part L. It’s required for new homes, extensions over 50m² floor area, material changes of use creating dwellings, and most new commercial buildings. Testing may be exempted if you’re following an approved design detail route with evidence of similar constructions achieving required standards. BREEAM assessments also typically require airtightness testing to achieve energy credits.

Testing occurs at practical completion when the building envelope is complete and sealed. All external doors, windows, and vents should be installed. Internal fit-out doesn’t need to be finished, but the airtight layer must be complete—including sealing around services penetrations, loft hatches installed, and all external openings sealed. Many contractors opt for early “shell stage” testing to identify issues before applying finishes, then re-test at completion for certification.

A standard house test typically takes 1.5-2 hours including setup, testing, and report generation. Larger homes or complex layouts may require 2-3 hours. Commercial buildings vary significantly based on size—small units take 2-3 hours while large buildings may require 4-6 hours or multiple visits. We provide results and certificates on-site or within a few hours of completing the test, ensuring no delays to your project schedule.

If your building exceeds the required air permeability limit, we’ll identify the major leakage paths using smoke pencils or smoke machines and provide guidance on remedial measures. Common leak sources include service penetrations, junction details, window and door seals, and roof-to-wall junctions. After corrections are made, we’ll re-test the building. Many contractors choose pre-completion testing specifically to identify and fix issues before final finishes, reducing the risk of expensive remedial work and project delays.

Yes, proper preparation ensures accurate results. The building should be at practical completion with all external openings sealed, internal doors can remain open or closed (your choice—we record the configuration), all mechanical ventilation and extract fans turned off, and one external door or window opening clear for fan installation. Provide access to all areas for smoke testing. We’ll send you a detailed preparation checklist when booking to ensure the building is test-ready and avoid wasted visits.

Yes, we provide consultancy and guidance on achieving better airtightness. We can review design details before construction, advise on airtightness strategies and materials, conduct early shell-stage testing to identify issues, provide detailed leakage path analysis with thermal imaging and smoke testing, and recommend practical remedial measures. Many developers and self-builders use our pre-completion testing service specifically to optimize airtightness before final finishes, ensuring first-time compliance and superior building performance.