Airtightness Guidance

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

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

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.