Understanding airtightness, moisture control & ventilation

Dave Judd, Technical Specification Advisor at Ecological Building Systems, explores airtightness, active moisture control and effective ventilation in sustainable homes, including Passivhaus projects.   

Unwanted air leakage has been proven to significantly increase the space heating requirements of a building, as well as making them less comfortable, draughtier and less energy efficient. Air leakage can also lead to long-term problems in the building fabric, resulting in interstitial condensation.  Airtightness is therefore one of the essential components of modern sustainable homes. 

For designers and specifiers of Passivhaus 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). This stringent target ensures that the building envelope is highly sealed, therefore minimising uncontrolled air movement and maximising energy efficiency. 

Ways to achieve airtightness 

There are a number of approaches that can ensure airtightness. The first step is to identify potential air leakage areas at the design stage. These include service penetrations (such as electrical wiring or plumbing connections), structural junctions or a combination of the two. By proactively addressing and sealing these critical points, for example with products like a Pro Clima Aerosana Visconn airtight liquid membrane, a continuous and reliable air barrier can be created. 

Secondly, an airtight vapour control membrane can be designed into the build that responds accordingly to humidity. Intelligent airtight membranes such as Pro Clima Intello Plus have proven themselves for over 20 years, allowing over 100 times more moisture vapour to pass through when humidity is high within the structure to guard 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.  

Quality control  

Ensuring airtightness on-site 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 bodies. 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 a 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 

Understanding how moisture vapour moves through different building materials can help to maintain the integrity of the building envelope through intelligent moisture management of the building fabric.  

We expect to have to prevent moisture from entering a building from the outside, but 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 (eg, an unplastered chimney breast). 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.  

Intelligent vapour check membranes 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 when needed, while preventing excessive moisture vapour intrusion in winter months. 

Effective ventilation 

As well as airtightness, a healthy living environment also needs to have careful control of indoor humidity levels and an effective ventilation system to prevent condensation and mould growth. Natural ventilation is generally not sufficient to meet the strict ventilation requirements of a Passivhaus, especially in colder climates.  

An effective ventilation system strikes a balance between fresh filtered air supply versus unnecessary air supply causing excessive heat loss. It must also control condensation through humidity regulation and provide heat recovery which lowers energy demand. Achieving optimal airtightness and moisture control is essential to create a high-performance Passivhaus that delivers exceptional comfort, energy efficiency and indoor air quality.  

www.ecologicalbuildingsystems.com