Design for energy efficiency: airtightness

As we become increasingly conscious of our impact on the planet and our local environment, it’s essential that our built environment is constructed for minimal energy consumption. Previously we touched on Passivhaus, a higher quality of design and construction with a clearly defined standard for energy use in all building typologies. In this post we’ll explore airtightness, a critical requirement of achieving Passivhaus certification and why an airtight home has significant implications for a building’s occupants and the environment.

What is airtight construction?

In a nutshell, airtight construction means that the building envelope is draught-free. This is achieved by implementing an airtightness barrier which must be continuous and thoroughly joined across all junctions of the building envelope to form a complete loop. This continuous airtightness loop comprises a combination of interconnected materials and flexible sealed joints to ensure the building envelope has no unplanned gaps that would otherwise allow air to leak in or out of a building.

Why should we consider airtightness in our buildings?

The majority of the UK’s housing stock loses a lot of heat; approximately 20-30% of the heat generated to warm our homes leaks through the building fabric. This can be down to poor design, poor quality of building or mistakes during construction. Whatever the reason may have been in the past, we should strive to develop our built environment to be as energy efficient as possible. By building airtight, ventilation becomes an important consideration for the buildings’ occupants. Fresh air is brought into the building mechanically whilst stale air is exhausted outside. A heat exchanger warms up the incoming fresh air from the exhausted stale air without the two streams ever mixing. Mechanical ventilation is a very important consideration for airtight design and is a topic we will explore in a future post.

The benefits of airtight design

The airtightness of a building has important implications for our planet and the building’s inhabitants. An airtight building results insignificantly reduced energy demand & energy bills, reduced CO2 emissions, an increased level of comfort, and an overall healthier living environment. By achieving good levels of airtightness, you also achieve structural protection from mould growth, moisture, damp, rot, and condensation which in turn reduces maintenance required over a buildings’ lifetime.

How do we achieve airtight construction?

A clear airtight strategy is required to achieve a draught-free building envelope. An architect can help plan for airtightness during early stages of the design process by planning where all air penetrations should occur and detailing how the joints between materials are sealed. Early planning is critical to ensure careful consideration has been given for the various junctions that occur throughout a building. The airtightness of a building will be checked three times during the construction process to ensure the desired performance target is on track to be achieved. The first test is taken when the building has been made air tight. The next test is taken when all services have been installed. The final test is on completion. During these tests, any defects can be identified and resolved before construction is completed.

Project Focus

In the design of Wellington Lane, smartply sheathing was used to create the primary airtightness barrier. Airtightness has been engineered into this sheathing panel and the special surfacing provides an integrated vapour barrier with consistently high vapour resistance over the entire surface. The special coating also provides a smooth durable surface for excellent bonding of airtight tape at panel joints allowing the airtightness barrier to be continuous and joined up throughout the building. A photo taken during construction of Wellington Lane shows the airtightness barrier is located on the interior side of the wall. This allows the insulation and building envelope to breathe and wick any moisture away whilst keeping the interior spaces comfortable and airtight.