Airtightness & Breathability – How do They Differ?

Ryan Gerrard / 03 March 2023

In the construction industry, airtightness and breathability have been buzz words for several years now, but what exactly do these hot topics refer to and how do they differ?

Here at illbruck, we are well versed in the concepts of airtightness and breathability, the differences between the two, as well as the important role our solutions play in adhering to ever evolving building standards and regulations. Read our latest blog article to find out more.

What is Airtightness?

Airtightness / (ˈɛəˌtaɪtnəs) / noun – the quality of being airtight.

Due to various changes in building regulations, we are now in the age of making buildings airtight to reduce the impact of convection heat losses. Enhancing the thermal retention qualities of a building subsequently enables structures to become more energy efficient and cost-effective for builders and homeowners alike.

To achieve airtightness around the building envelope, a seal is applied at the point of uncontrolled air leakage, thus reducing convection energy loss and increasing thermal performance. These problem points occur around any unsealed gaps in the façade, especially the windows and doors. There are a number of remedies that can be used to provide an airtight seal, including both liquid and sheet membranes, impregnated foam tapes, pressure sensitive tapes, as well as sealants and foams.

What is Breathability?

Breathability / (ˌbriːðəˈbɪlɪtɪ) / noun – ability to let air to pass through so that perspiration can evaporate.

Air is like a sponge that absorbs moisture. Inhabited buildings create significant amounts of warm moist air over the course of their lifetime, much of which is produced through spikes in the build up due to changing human activity. In older buildings, condensation can present less of a problem as the internal air is exchanged regularly through air bricks, chimney flumes , and unsealed windows and doors. This in effect removes or suppresses the moist air, allowing this moisture to pass through the wall build up, avoiding the creation of a dew point – the threshold where condensation forms.

Whereas airtightness looks at mitigating air loss through points of uncontrolled leakage, breathability strives to control moisture build up within the wall to prevent damp and mould issues from arising. In the UK, air tends to be cooler outside as opposed to inside, resulting in a positive pressure gradient that causes the internal warm, moist air to try and migrate through the walls to the exterior of the structure. This pressure gradient is increased when we make our buildings airtight.

In modern Steel Framing System (SFS) construction, a significant proportion of the wall build up consists of absorbent Class A1 mineral wool. The moisture passing through the wall to the outside is absorbed by the insulation creating cold spots in the wall, subsequently reducing the thermal insulative performance of the wall by up to 25%. Where the warm moisture from the building’s interior meets the colder air from the exterior, condensation forms potentially stagnating and creating an ideal environment for organic material to grow, including mould and fungi, contributing to diseases such as asthma and a phenomenon called Sick Building Syndrome.

The Centre for Window and Cladding Technology (CWCT) uses the building principle of “inside tighter than outside” when referring to the concept of breathability. Adopting this principle works to ensure the moist air created internally is not allowed to migrate uncontrollably through the wall build up but is retained within the internal space it was originally formed. This can be achieved through the use of non-breathable or vapour impermeable membranes, such as vapour control layers or vapour barriers, air and vapour tight membranes, wet sealants, and liquid membranes. The build-up of internal moist air is then evacuated or suppressed from the building mechanically through the following means:

  • Heating, Ventilation, and Air Conditioning (HVAC)
  • Mechanical Ventilation with Heat Recovery (MVHR)
  • Positive Input Ventilation (PIV)

However, because buildings move and settle over time creating potential microscopic leaks and cracks, it is virtually impossible to permanently seal a building envelope. Additionally, during the construction of the dwelling, the outer skin is exposed to the elements for a significant period of time. Therefore, any residual water vapour created by concrete curation means there is water present in the insulation zone of the wall build up from the beginning of the construction process.

In order to effectively control moisture build up within the wall, a breathable membrane (vapour open) should be applied to protect the façade prior to the outer skin’s erection. A breather membrane needs to be weather/water and airtight to prevent increasing moisture levels during the build phase. However, it should also be vapour open to allow the controlled migration of moisture already within the wall build up, to slowly release into the atmosphere.

How Can You Achieve Airtightness & Breathability With illbruck?

Architects and contractors must now take into consideration new legislation that mandates the use of compliant solutions and practices to maintain airtightness and ventilation within buildings. With a UK target of achieving net zero emissions by the year 2050, outlined in the recent changes to Approved Document L is the requirement for all new buildings to be built to a minimum standard of total energy performance. The new metric for doing so is Target Primary Energy Rates (TPER), which is the maximum primary energy use for the building in a year (measured in kWhPE/m2).

Looking towards a fabric first approach to construction will help architects, contractors, and developers to prioritise airtightness and breathability by homing in on the materials used and maximising their performance. Whilst it is important to achieve air and water tightness, implementing adequate vapour control is also crucial to minimise the risk of moisture build up. This can be done through adopting a full system that covers the internal, external, and insulation sealing, such as illbruck’s BBA certified i3 system, which strives to seal OUT the weather whilst sealing IN comfort and energy savings. This externally weatherproof system also prohibits the formation of condensation and prevents thermal bridging around the frame.

Additionally, solutions such as illbruck’s ME010 Façade UV and Fire Membrane can be used as a breather membrane, which is applied directly over thermal insulation or to the sheathing boards to provide protection to the built-up wall layers in a closed, open, or partially open façade. Having been tested in accordance with EN 13501-1 achieving Class B-s3,d0, ME010 provides the dual advantage of being both breathable and airtight.

A Team You Can Trust

Here at illbruck, we have a team of technical experts who share many years of industry experience between them, putting us in pole position to advise you on the correct solutions for constructing airtight yet breathable structures. As a member of the Tremco CPG UK family of brands, we have access to products for all six sides of the building envelope along with the knowledge and support that go hand-in-hand across each of our ranges.

Also available to you is the CPG Training Academy. This is your gateway to construction excellence and provides a whole host of training from webinars, RIBA approved CPD seminars, site visits, toolbox talks, and much more.

Find out more about illbruck today and for further information, please get in touch.