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What is Air Tightness Testing

Air tightness testing, often known as air leakage testing or air permeability testing, is the process of measuring the amount of conditioned (heated or cooled) air leaking from a building. 

In practice the effect of this can be felt as a draught under pressurisation and indicates that the internal space is open to the external atmosphere. By reducing uncontrolled air leakage, and reducing draughts, CO2 emissions are reduced as less energy is required to condition that internal space while also providing a more comfortable environment for occupants.

Buildings are measured against a target rate, known as Design Air Permeability, set out by SAP (L1a Domestic) and SBEM (L2a Commercial) calculations. These allow for different target rates depending on the construction or presence of renewable technology. However, the building regulations minimum requirement is an air permeability rate of 10m3/hr1/m2 or less. 

As a general rule, a “standard” domestic dwelling with current contemporary specification is more often than not required to achieve a design air permeability of 5m3/hr1/m2.

Air testing is a requirement on all new residential (L1a) and commercial (L2a) buildings to meet building regulations compliance and demonstrate the ability to meet the expectations of a predicted EPC. 

Additional information on air tightness testing is available from The Air Tightness Testing and Measurement Association (ATTMA).

How is an air test carried out?

The building to be tested should be in an appropriate state of completion to undergo testing; see our ‘Preparing for Testing’ section, with all openings, internal penetrations and mechanical ventilation sealed. 

Specialist calibrated airtightness testing equipment is then placed in an opening in the building envelope (usually an external door) and forces air into (Pressurisation), or extracts air out of (Depressurisation) the structure creating a pressure differential which in turn enables the technician to identify air leakage pathways. 

The calibrated equipment is used to calculate air flow through the structure. Simply put, the amount of air passing through the building while under pressure is the amount of ‘air leakage’ and the result is known as the building’s ‘air permeability rate’. 

The higher the resulting score (measured on a scale of 1-10), the greater the amount of uncontrolled air leakage and the greater the amount of energy is required to heat or cool the building and the greater the C02 emissions of that building.

When should an air test be conducted?

We recommend you contact us as soon as possible so we can guide you through preparation to ensure the process is as smooth as possible, however the optimum time for a test to be conducted is prior to completion when all plasterwork and sealing is complete with plumbing works and electricals installed and operational, but before carpet/flooring is laid. 

Importantly all penetrations through the plasterwork, such as toilet wastes & stacks, open plasterboard bottoms, electricals, cavity closers, and thresholds are sealed with silicon, or similar, to prevent air leakage behind the plasterboard and into the wall cavity. We would advise that skirting boards are sealed with silicon/caulk at both the top and bottoms onto the floor to ensure no leakage behind plasterboards.

If the building is adequately prepared for testing the process is very easy and stress free and shouldn’t take any longer than an hour per plot to conduct from start to finish.

Can I seal my building too much?

As building air tightness target levels reduce (low air permeability results typically less than 5m3/h1/m2) we often get asked if it’s possible to “over seal” and make a building too airtight leading to building sicknesses such as damp and discomfort for the occupant through lack of ventilation. 

A high design air permeability (greater than 5m3/h1/m2) gives a wider choice of ventilation systems as the building fabric will allow for greater air changes and adequate air quality naturally. 

A lower design air permeability (less than 5m3/h1/m2) will need greater care to be taken when installing mechanical ventilation systems to ensure they meet Part F requirements.

Buildings with advanced ventilation systems, such as continuous extraction or heat recovery systems, can use more energy than designed if they do not meet designated air permeability rates as the systems operation can draw in external air leading the system thermostat to think the building’s temperature is different to intended and therefore condition the internal environment continually when not required.

It is important to assess the ventilation strategy of buildings with low target rates and match air permeability with adequate ventilation. If the building’s designed ventilation (bathroom extractors fans, fire vents, trickle vents, cooker hoods, etc.) is installed in accordance with its manufacturers’ instructions and meets the ventilation rates specified by Part F Ventilation regs. (see our Extract Fan Testing page), then it is difficult to “over seal” a building. However, to ensure adequate air change and air quality it is important to test the ventilation meets the requirements set out in Building Regulations Part F: Ventilation (Book an Extract Fan Test today).

Best Practice Design Air Permeability vs. Ventilation Strategy


Ventilation System

Target Air Permeability


Target Air Change Rate

Trickle vents & intermittent extractor fans 3.0-5.0
Passive Stack 3.0-5.0
Continuous Mechanical Ventilation 2.0-4.0
Continuous Mechanical Ventilation with Heat Recovery 1.0-2.0
Other Seek Specialist Advice
Passivhaus Standard 0.6*
Air Tightness Test Progress Internal
Air Tightness Blower Door Test

Air tightness Tests start from just £175 +VAT

Significant discounts on multiple air tests on the same site with FREE same day re-test where applicable.

Prior to arrival on site we will require FLOOR PLANS, SECTIONS, & ELEVATIONS in order to calculate the building envelope for the test.

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