How Important is Airtightness in Building Design? -

20, May, 2024 By Peter Kinsella Posted in Part L

Airtightness in buildings is a subject that has moved from the sidelines into the mainstream over the last few years, and has recently been codified into the building regulations. As builders and home-owners seek to make homes more energy efficient, airtightness has become a key part of any design strategy.

Residential architect Philip Benton explains the main points to consider, with a focus on home design:

What is the meaning of airtightness in buildings?

Air permeability, or airtightness is the extent to which a building allows air from outside to get into the building, or vice versa. In older buildings we may experience this as noticeable drafts, especially around windows and through floors, but even newer buildings will have some level of air permeability, where heat escapes from the building via uncontrolled ventilation through the building fabric.

Whilst heat-loss is a bad thing for energy efficiency, air permeability does actually perform a useful function of allowing in fresh air to alleviate stuffiness, smells, or moisture within the home. Because of this, when buildings become increasingly airtight architects need to think more about ventilation systems, such as Mechanical Ventilation with Heat Recovery (MVHR).

What is MVHR?

Mechanical Ventilation with Heat Recovery is a system that extracts moist and stale air from within the home, and replaces it with fresh air from outside. In appearance, MVHR systems look similar to a bathroom extractor fan, but unlike an extractor they operate at a low level, quietly and continuously. Air is generally extracted from wet rooms – kitchens and bathrooms, and fresh air inputted to living rooms and bedrooms. Crucially, the outgoing and incoming air pipes pass through a heat exchanger that transfers the majority of the heat from one to the other, – so warm outgoing air is cooled, and as it does so it pre-heats the cold incoming air. In summer conditions this function can be bypassed to aid cooling the house.

What is good airtightness?

Generally, a new-build house that is not specifically designed with airtightness in mind can achieve a score of less than 10.

A score of 3 or less is considered to be good airtightness, and this number represents the point at which MVHR becomes necessary.

What is the airtightness of building regulations?

The most recent revision of the Scottish building regulations has added requirements around airtightness – codifying what had previously been informal advice.

These new regulations offer 3 options to the architect:

Low airtightness (any score above 5). Homes that fall within this range are required to have natural ventilation through trickle vents (usually provided as a small strip vent at the top of a window frame), and also have extractor fans in the wet rooms.

Medium airtightness (any score above 3). Homes with a score of between 3 and 5 are required to have continuous mechanical extract ventilation.

This is a type of ventilation system but is not the same as MVHR because it does not have the heat-recovery function. I expect that this option may not have wide popularity as it offers most of the costs of installing a ventilation system but misses out on many of the benefits for energy efficiency and running costs.

High performance (any score). Homes with a score of less than 3 are required to install a ventilation system that both supplies and extracts air – normally in the form of MVHR.

Why should I choose an airtight home?

Airtight homes that employ MVHR have improved energy efficiency, creating lower running costs and carbon footprint. In addition to knowing that you are doing good for the environment, there are also sound financial reasons for choosing improved airtightness.

As part of the design process your chosen airtightness score will be inputted into the SAP calculation for the house, with lower scores making it easier for the house design to achieve a pass in SAP. This lower score can reduce the need to spend on other energy measures, such as insulation or renewables.

Anecdotal evidence from architects suggests that as insulation levels have improved, houses with better performance have reached the point where adding further insulation makes little difference to the energy efficiency, but still adds cost to the construction. At this threshold, better gains can be made by reducing air-infiltration instead.

What is a SAP Assessment? 🔗

Whichever airtightness score you choose to include in your SAP calculation, it is important to note that the building will be airtightness-tested and needs to achieve the chosen score. Proper planning should be made during the design stage to ensure that a building is not caught short as it is difficult to remediate the airtightness of a building once it is complete.

Builders are advised to have the building tested as early as they can so that if there are any issues these can be fixed.

How is an airtightness test done?

To complete a test, a large fan is sealed in to the open doorway of the home, and the building is pressurised to 50 pascals. The tester will then observe how quickly the building depressurises as the air leaks out, and assign a score based on this.

How can I make my building more airtight?

The first step is to form an airtightness strategy. An airtight membrane in the walls, roof and floor does the job of keeping air in, and the strategy shows how this membrane fits within the construction, and its main joints.

The airtightness membrane can be positioned towards the inside or outside of the structure, or mixture of both. Whichever you choose will have implications for design details and aesthetics.

If the membrane is towards the inside (just behind the plasterboard) think about how sockets and switches will penetrate this, and consider including a service void to allow cables and pipes to run inside of the membrane to avoid puncturing it. Recessed downlights in ceilings present a similar issue, so should either be omitted or designed around.

If the membrane is to be located towards the outside (just under the cladding and inboard of the cavity, remember that this is prone to damage on site during construction, so must be monitored and repaired with airtightness tape. In this location you must specify a Breather Membrane that can also function as an Airtightness Membrane, as not all membranes can do both.

Whichever location you choose will need to work with the details of your design. Overhanging eaves, sprockets, and balconies will typically pierce an outer-type membrane, resulting in a tricky task for builders to seal the membrane around every structural member as it passes through. Floor joists and roof trusses will pass through an inner-type membrane as they connect to walls. Architects need to think these through when they choose the airtightness strategy.

Tips for achieving airtightness in construction:

Form a strategy that works for your design, and make sure that your builders understand it and its implications in each part of the building.

Tape and seal around windows and doors to ensure that they connect seamlessly to the walls.

Ask your window and door manufacturers for the air permeability of their products – especially if choosing sash or traditional windows, or sliding doors.

Remember to ask the window manufacturer to omit trickle vents if required.

Letter boxes are a known weak point in high airtightness houses, so look at options for an external box or a sealed flap.

Remember to seal around services penetrations, such as the electricity, gas and water mains as they enter the building, drainage pipes as they go out, and connections for vent grilles, flues, or outside lights or taps,

If chimneys or flues pass through the roof, a heat-proof and air-tight seal will be needed.

Open fires are not possible, of course because they open the room to the chimney, but wood-burning stoves can still be used. Stoves have an air inlet to allow in air for combustion, so check that your chosen stove comes with a ducted air inlet that draws air from outside rather than from the room – otherwise your chimney will not draw properly.

Lastly, remember that air-tightness is not a bolt-on that can be considered in isolation, so needs to be thought through at every stage for a successful outcome. If you get it right it can make a real difference to your bills.