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4 months ago

Chloe Finan

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Airtightness in Modern Construction: Best Practice for Timber Frames & Weather-Resistant Barriers

Introduction

It’s been an extremely Airtightness is now one of the most crucial objectives on any building project. It is a key factor for energy efficiency, maintaining a comfortable and healthy living environment as well as protecting the building’s structure. In this article we consider the importance of airtightness in modern building design, focusing on the principles of airtight weather-resistant barrier (WRBs) detailing at timber frame junctions and the practicalities of achieving this. and productive year at Partel, so we thought it would be fun to look back and revisit some of the interesting, varied, and sometimes challenging projects, news, and product stories from the past year.

Why Airtightness Matters

Airtightness is the key to building energy efficient buildings. Avoiding heat loss is important as it means less uncontrolled air movement in and out, ensuring heating systems work more effectively and thereby reducing heating bills and energy wastage. It also contributes to maintaining thermal comfort both in winter and in summer and with the added benefit of improved sound insulation.
Buildings with effective airtightness strategies improve the health and well-being of occupants by preventing substances such as pollutants, allergens, and bacteria being carried into the building via air leakage. This is vital for buildings in the health care sector where pollutant control is essential.
Building durability is also improved through airtightness by preventing damage caused by moisture leaking into the building envelope which can cause condensation, damp and mould growth, which can impact the health of occupants, as well as potentially damaging the fabric of the building.
A robust airtightness strategy requires all stakeholder and trades to focus on an improved build quality and workmanship which in turn prolongs the life of the building.

Achieving Good Airtightness

The first part to achieving good airtightness is firstly in the design of the building with the consideration of developing simple details that are buildable, avoiding complex junctions where possible.
The second part is in the construction itself. Operatives on site who are working with the airtightness layer must be trained in airtightness products to ensure they are installed correctly. This is the responsibility of the whole supply chain to ensure high quality installation to achieve the desired results.
The final part is the compulsory blower door testing which establishes the performance of the airtightness layer. Air tightness testing is normally carried out toward the end of construction, but as soon as is viably possible in order for any leaks to be picked up and rectified. Air tightness testing is required by the Building Regulations for the construction of new buildings. The results of the tests are used within the energy efficiency calculations (SAP or SBEM).

Principles of Weather Resistant Barriers

A building’s exterior is effectively it’s primary weather barrier with cladding serving as the first line of defence in protecting a building’s occupants from the outdoor elements. Regardless of how tightly sealed the cladding is, moisture can often seep through gaps and small crevices, which can cause issues that can affect the durability, indoor air quality and thermal efficiency of the building.
After the cladding, the WRB is the second line of defence against moisture entering a building, so it is vital to install an integrated robust system to assist in protecting the building from air and moisture intrusion. The important features of the WRB are – Continuity, Structural Support, Air impermeability, and Durability.

Continuity

To ensure continuity, each component must be interconnected to prevent air leakage at the joints between materials, components, assemblies, and systems and penetrations through them, such as conduits and pipes.

Structural support

For effective structural support all components of the WRB system must be able to resist the positive or negative structural loads that are imposed on it by wind, stack effect, and HVAC fan pressures without rupture, displacement or undue deflection. Design consideration must determine adequate resistance to these pressures by fasteners, tapes, adhesives, with a system approach the preferred option.

Air Permeability and Airtightness

Part L states the maximum permitted air permeability for a new dwelling is 8 m3/hr.m2 @50Pa. However, the ‘notational dwelling’ described in Part L, against which an actual building design is compared, has an air permeability of 5 m3/hr.m2. The Future Homes Standard is aiming for significantly higher airtightness of 5 m3/hr.m2, with mandatory testing of all new homes rather than the previous sampling approach. While FHS 2025 sets high standards, in a typical dwelling shape, a passive house would be hitting well below 1 m3/hr/m2.

Air permeability is the physical property used to measure the airtightness of the building fabric. It is defined as air leakage rate per hour per square meter of envelope area at a test reference pressure differential across the building envelope of 50 Pascal (50 N/m2).

Durability

Materials selected for the air barrier system must perform their function for the expected life of the structure.

Key Strategies for Airtight Timber Frames

Although with careful design detailing and workmanship it’s possible to make any build method airtight. Timber frame does not necessarily have any integral advantages but this off site build method has airtightness as a key objective, often achieving Passivhaus Standards. Here are some specific build details to consider.

Continuous barrier (AVCL): Install a continuous vapour control layer on the warm side of the insulation, ensuring it is not punctured by electrical sockets or plumbing. Use specialised tapes and to seal all membrane overlaps, joints, and junctions.
Airtight panels (OSB): Use OSB or plywood structural sheathing, ensuring all joints are taped or glued to form a rigid, airtight barrier.
Window and door sealing: Apply EPDM rubber membranes or specialised sealant tapes around window and door frames to create a flexible, durable seal, avoiding reliance on expanding foam alone.

Service penetrations: Seal every pipe, cable, and joist penetration through the airtight barrier using dedicated grommets or airtight tape.
Junction detailing: Pay special attention to the ground floor (slab/floor-to-wall joint), the first-floor joist zone, and the ceiling/roof connection.

Specific Considerations for WRB and Mechanical Interfaces

As stated, a proper mechanical interface, including detailing at penetrations, window-wall junctions, and transitions, is crucial for long-term performance.

Continuous configuration: The WRB must be installed as a continuous, uninterrupted, and properly lapped layer around the entire building envelope.
Window/door interfaces: WRBs should be integrated with flashing using a “shingled” method, ensuring that any water that penetrates the cladding is directed down and out, rather than into the wall assembly.
Mechanical fixings & fasteners: When using mechanically fastened barriers, ensure they are securely fixed but that all penetrations (nails, staples) are properly sealed.
Joints and transitions: All seams, joints, and transitions (e.g., roof-to-wall) should be sealed, typically using specialised sealing tapes or liquid-applied membranes that are compatible with the primary barrier material.
Material compatibility: Ensure that sealants, tapes, and accessories are compatible with the WRB material to prevent premature degradation.
Service Void: Install a 38mm or 50mm service void (battens) inside the airtight membrane to allow for plumbing and electrical installation without breaching the air barrier.
Protection during construction: Water-resistive layers must be protected during construction to prevent damage from following trades.

Retrofit

It’s generally regarded that much less ambitious airtightness targets should be set for existing buildings, given the limitations posed by existing junctions and the requirement to make meaningful improvements for that building.
It can be a misconception that existing building stock is not airtight. The fabric of older homes can be remarkably airtight, such as those built with mass concrete and wet plastered walls. An Energy Action study found six Dublin homes built between 1920 & 1940 averaged below 5 m3/hr/m2 whereas the remaining 14 homes in the study built between 1960 & 1980 average 9 – with the worst three all being the newest.)

Conclusion

Airtightness is a key component in modern building construction, and the correct installation of high-performance weather resistant barriers is vital. The use of weather resistant barriers in construction has increased in recent years and these materials have become a crucial component to the building’s exterior wall system. WRBs protect the building structure itself and well as the well-being of occupants and ensure a modern and futureproof construction.