Neil Turner, UK Technical Manager at Ecological Building Systems, advises on designing airtight buildings with a focus on vapour openness and protection against moisture damage.
“Creating an airtight home is a fundamental part of modern building design, particularly for specifiers working on meeting the rigorous standards of Passive House construction.
Understanding airtightness
Airtightness refers to the ability of a building to prevent uncontrolled air leakage, in other words, eliminating leaks in the building envelope that shouldn’t exist. This is crucial for maintaining indoor temperatures, reducing heating demands and minimising energy consumption. Airtightness not only enhances energy efficiency, but with correct detailing and the use of appropriate materials along with controlled indoor ventilation, also supports moisture control, helping create a comfortable and healthy living environment for occupants.
There are several approaches to ensure airtightness.
Building designers should identify areas where air leakage could possibly occur at the design stage. This includes service penetrations (such as electrical wiring or plumbing connections), openings such as windows and doors and structural junctions, such as the interface between walls, roof and floors, or a combination of these. By proactively addressing and sealing these critical points with products like Pro Clima Aerosana Visconn airtight liquid membrane or Pro Clima Airtightness tapes, a continuous and reliable air barrier can be created.
A crucial approach to combining airtightness with moisture control is to incorporate an airtight membrane into the build, one that responds accordingly to humidity. Vapour control membranes, such as Pro Clima Intello Plus, offer a tried and trusted solution. Intello Plus allows up to 100 times more vapour to pass through when exposed to a humid environment compared to being exposed to a low humidity environment.
In the winter, when internal humidity is lower, the membrane behaves as a vapour control layer (high moisture vapour diffusion resistance). In the summer, when internal humidity is higher the membrane becomes more vapour open and allows any moisture in the insulation zone being driven from external to internal to harmlessly diffuse back to the room internally. This means that the moisture content in the building envelope insulation zone never reaches a critically high level.
Finally, high-quality air barrier tapes and sealants are essential for ensuring airtightness at joints, seams and penetrations. These specialist products are designed to withstand movement, temperature fluctuations and exposure to moisture, providing long-lasting airtightness. For example, Pro Clima’s tapes, such as Pro Clima Tescon Vana have been independently tested with an expected lifespan of at least 100 years.
Quality control
Implementing stringent quality control measures is crucial for maintaining airtightness throughout the construction process. This includes verifying that all materials used for airtightness carry appropriate certifications from recognised bodies, such as the Passivhaus Institute.
Airtightness testing is typically carried out using a blower door test, which quantifies the air exchange rate through the building envelope under controlled pressure. This provides a clear assessment of the building’s airtightness performance and demonstrates compliance with energy efficiency requirements. Conducting intermediate tests during the construction phase also enables targeted remediation of leakage paths, minimising the risk of defects and avoiding the need for extensive corrective work once the building is complete.
Intelligent moisture management
Airtightness must be balanced with effective moisture management to prevent interstitial condensation and maintain indoor air quality. Understanding how moisture vapour moves through different building materials can help to manage it effectively to maintain the integrity of the building envelope.
A careful balance needs to be struck between the moisture loading of materials and ensuring sufficient drying reserves. If the moisture loading of a build exceeds the drying reserves, structural damage can occur due to an uncontrolled build-up of moisture in the insulation zone of the building envelope. Building in adequate drying reserves is therefore essential to prevent long term issues, such as moisture induced mould.
Internal membranes like Pro Clima Intello Plus, used internally on walls and ceilings, and Pro Clima Dasatop, used in lofts and roofs applications, provide airtightness while managing vapour diffusion by adjusting to changes in humidity. They create a smart vapour control layer that reduces the risk of condensation and supports safe drying in constructions that involve wet materials, such as plasters or screeds. These membranes are often referred to as ‘intelligent’ membranes for this reason.
Specifying MVHR systems (mechanical ventilation with heat recovery) works particularly well with airtight homes, as they ensure a continuous supply of fresh air while extracting stale air, delivering a high level of indoor air quality. These systems also include a heat exchanger that recovers heat from the exhaust air and transfers it to the incoming fresh air, reducing the need for additional heating or cooling and improving overall energy performance.
Delivering Airtightness in Practice
To ensure airtightness of a building, it is essential that all parties involved in the construction process, such as electricians, plumbers and contractors, are trained in airtightness techniques and understand their importance. On larger projects, a dedicated airtightness specialist is often employed to oversee the implementation of airtightness measures and coordinate efforts among different trades.
Achieving effective airtightness together with appropriate vapour management is essential for delivering a high performance modern home with strong energy efficiency, comfort and indoor air quality. By integrating airtightness at the design stage and pairing it with well planned ventilation, designers can limit unintended air leakage and support consistent long-term performance throughout the life of the building.
As a result of advances in airtightness, membranes, tapes and ventilation technology, building a Passive House that meets the rigorous standards is more achievable than ever before.”