David Judd, Technical Specification Advisor at Ecological Building Systems, takes a look at building design to protect against moisture damage, whilst enhancing airtightness within Passive House dwellings.

Unwanted air leakage can significantly increase the space heating requirements of a building, as well as making it less comfortable, draughtier and less energy efficient. Air leakage can also lead to long term problems in the building fabric, which can be damaged by water vapour transfer, resulting in interstitial condensation.  

With airtightness being one of the essential elements of Passive House construction, the building envelope must be well sealed, therefore minimising uncontrolled air movement and maximising energy efficiency.

For designers and builders of Passive House dwellings, the gold standard is to achieve an air leakage rate of less than 0.6 air changes per hour (ACH) at 50Pa (Pascals of pressure). 

Ensuring airtightness

There are a number of approaches to take to ensure airtightness:

Firstly to identify the potential areas where air leakage can occur at the design stage. This includes service penetrations (such as electrical wiring or plumbing connections) and structural junctions, or a combination of the two. By proactively addressing and sealing these critical points, with products like Pro Clima* Aerosana Visconn airtight liquid membrane, a continuous and reliable air barrier can be created.

Secondly, an airtight membrane can be designed into the build, which responds accordingly to humidity. Airtight membranes, such as Pro Clima Intello Plus* have proven themselves for over 20 years. Intello allows up to 100 times more moisture vapour to pass through when humidity is high in a structure, protecting it against mould and moisture related issues.

Finally, high-quality air barrier tapes and sealants are crucial to ensure airtightness at joints, seams and penetrations. These specialist products are designed to withstand movement, temperature fluctuations and exposure to moisture, providing long-lasting airtightness performance. For example, Pro Clima’s core tapes have been independently tested with an expected lifespan of at least 100 years.

Quality control 

Ensuring airtightness in buildings requires a dedicated approach to quality control, both in terms of processes and materials.  Airtightness materials should have adequate certification from the Passivhaus Institute, BBA or other third-party test body. Early and frequent inspections are essential to identify and address any potential leaks, as well as clear communication and co-ordination among all trades involved in the construction process. 

The use of an intermediate blower door test can provide valuable insights into the building’s airtightness performance, allowing for remediation efforts to be made during the build process. 

Intelligent moisture management

As well as airtightness, a healthy living environment also needs to have careful control of indoor moisture levels and an effective ventilation system to prevent condensation and mould growth.  

There are many ways that moisture can enter a building from the inside. This includes convection, the installation of damp construction materials, vapour diffusion and flank diffusion (where water vapour bypasses a vapour barrier at a junction with a structural element).

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. Building in adequate drying reserves is therefore essential to prevent long term issues, such as mould and structural damage. 

Intelligent moisture management systems, such as Pro Clima’s Intello Plus, which has proven performance for over 20 years, are designed to provide humidity-variable diffusion resistance. These sophisticated internal membranes can adapt to changing environmental humidity levels, allowing the building fabric to dry out while preventing excessive moisture vapour intrusion in the winter months.

Effective ventilation

Airtightness and moisture management are closely aligned with the ventilation strategy of a Passive House. An effective ventilation system is, of course, essential for maintaining a healthy indoor environment and should regulate humidity levels and optimise energy efficiency.

Natural ventilation is not generally sufficient to meet the strict ventilation requirements of a Passive House, especially in colder climates.

Building designers are aware that planning ventilation into a home via mechanical ventilation with heat recovery (MVHR) or decentralised mechanical ventilation with heat recovery (dMVHR) is a very effective option. These systems use a heat exchanger to extract heat from the exhaust air and transfer it to the incoming fresh air, reducing the need for additional heating or cooling.

An effective ventilation system is one that strikes a balance between fresh filtered air supply versus unnecessary air supply causing excessive heat loss. It must also be able to control condensation through humidity regulation and provide heat recovery which lowers energy demand. 

The Passive House approach advocates the need to “Build Tight, Ventilate Right” reinforcing that air movement within the living space needs to be carefully managed and the building envelope to be meticulously sealed.

Summary

Achieving optimal airtightness and moisture control is essential for creating a high-performance Passive House that delivers exceptional comfort, energy efficiency and indoor air quality. By designing in airtightness and balancing this with active moisture control and effective ventilation, a building designer can minimise air leakage and maximise energy savings whilst providing a healthy living environment for the long term.

As a result of advances in products and technologies that enable advanced moisture control and airtightness, building a Passive House that meets the rigorous standards is more achievable than ever before.