Excessive air leakage in our buildings is a problem

Draughts make us uncomfortable, and lead to higher heating costs and increased fuel poverty

Air leakage increases heat loss, making our buildings colder, and hence leads to condensation & mould occurring, resulting in damage to residents’ health – with consequent extra costs to, and stress upon, health services

Moisture is in the air moving through the walls or roofs of our homes and other buildings. This water can be deposited in the building fabric, often giving rise to major damage and shortening a building’s life

Contractually the most common immediate impact of poor airtightness is that all new buildings, and many refurbishment projects, include a specification that must be met – because of the above issues.  Failing to achieve the airtightness target, however described, means that the contractor will typically be liable to carry out improvement works to deal with the air leakage issues. Usually, they will also have to pay for extra airtightness testing, and may be on the hook for additional costs due to delays.

Where to start?

Airtightness shows quality in a building. Airtightness makes ventilation/heating systems work properly whilst improving insulation's effectiveness. This helps with reducing our fuel bills and increases health & comfort through the elimination of mould and condensation. 


We measure airtightness by temporarily mounting a calibrated fan in a window or door opening.  We then take a series of readings of imposed pressure differential against the airflow generated by the fan - which is a function of (a) how big the building is and (b) how airtight (or otherwise) the building is.



Airtightness testing in the UK is governed by ATTMA, the Air Tightness Testing and Measurement Association. This is part of BCTA, the Building Compliance Testers Association.

When carrying out airtightness testing, we use the following;

  • One or more calibrated fans, temporarily mounted in an open door or window.
  • Control equipment
  • Pressure gauges
  • Thermometers
  • Barometers
  • Tape measures
Large fan: Mounted in a bag door in an open doorway whilst depressurising an existing house during investigations.
Small fan: Positioned in a window board during acceptance testing for PH. Digital gauge shown bottom right

For an airtightness test to be valid, several conditions must be met;

  • The test equipment must have a current calibration certificate
  • The test engineer must be competent to test the building using the applicable standard
  • The zero-flow pressure differentials are low enough
  • The building must be ready to test
Site manager arriving to witness a preliminary airtightness test


Considering these conditions in turn:

Calibration: All test equipment - fans, pressure gauges, digital barometer, digital thermometer – must be calibrated

Competence: ATTMA has various levels of test engineers.

  • Level 1: Single-fan tests of ≤4000 cubic metres gross internal volume.
  • Level 2: Level 1 plus multi-fan tests ≤80,000 square meters external envelope area and/or up to 15 storeys tall.
  • Level 3: Complex and/or larger and/or taller buildings, but not currently enforced.
  • Passivhaus & Low Energy: Can test Passivhaus projects, in conformance with the TSL4 standard.

Zero-flow pressures: This is the pressure differential between the inside of the volume under test and the external environment, before the airtightness test fan or fans are operated. This must be no more than 5 Pa, either +ve (pressurisation) or -ve (depressurisation).

Readiness for Testing: Whether the building is ready to test can be a complex assessment, and fundamentally depends upon the type of test being carried out and the standard being applied.  This is discussed further in a separate document.

Key Risks associated with airtightness include:

Failing the airtightness test.  The most common problem for a contractor or designer.  Likely to mean extra sealing works for the contractor, possibly even design issues needing consideration (e.g. discovering that dot-and-dab plasterboard finishes cannot deliver the specified airtightness).

Poor airtightness.  This usually means that the property under test is too leaky.  But at the other end of the spectrum, ventilation can prove inadequate as the airtightness is improved. This can give rise to condensation and mould, with damaging health impacts.

For many years we have applied the slogan:

Build tight, ventilate right

These days, we try to avoid adding pollutants within a building, from paints, carpets and coverings, furniture.  But the biggest issue is usually water.  Water in the air leaking through the fabric of our buildings is likely to condense en-route, leading to damaging moisture in walls and roof spaces.  This can cause long-term damage to our buildings, as shown below.


Our homes and other buildings leak in a wide variety of places. Some are obvious (like windows that don't close properly), others are hidden behind skirting boards, in lofts or hollow floor spaces. Our resources are a distillation of over 30 years experience finding air leakage.

Where do buildings Leak



Paul Jennings

UK's most experienced airtightness tester○ AECB Expert Advisor○ Airtightness trainer & consultant
 ◦doorfanman@hotmail.com07866 948 200

Paul Jennings of Aldas is the most experienced airtightness tester and consultant in the UK, with over 30 years’ experience.  He has been an an AECB member for over 15 years and a Trustee for much of that time.  He is also a Director of the UK Cohousing Network.

He has presented at several AECB conferences and demonstrated the door fan test equipment at others. He has also given talks at Passivhaus conferences in Vancouver and Germany, as well as in the UK.  Paul has been involved in most UK PassivHaus projects in one or more capacities.

These schemes include:

the first UK certified domestic and non-domestic PH projects in Machynlleth, both built by John Williamson, initially tested in 2008 and then retested after a decade of operation in 2018
the Erneley Close (Manchester), Wilmcote House (Portsmouth) and Carlton Chapel House (North London) EnerPHit block refurbishment projects
the Agar Grove area regeneration scheme in Camden, where neglected 60’s social housing will ultimately be replaced by over 500 newbuild PH units
the Exeter Passivhaus swimming pool and leisure centre, currently under construction
numerous individual newbuild custom- and self-build Passivhaus schemes

Paul is an experienced trainer and originated the “Airtightness Champion” concept, as well delivering numerous on-site trainings and many CPD seminars to architects and builders.  He developed the 12 Steps to Airtightness approach to enable UK contractors to reliably deliver airtightness in Passivhaus and other low-energy projects.