Air Tightness in Buildings and Energy Usage

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While insulation is an essential element of low energy buildings, another crucial factor is airtightness. Draughty buildings tend to be uncomfortable to live in and expensive to keep warm.

The need for airtight buildings

Vast amounts of energy are lost due to airflow through buildings. Heating and cooling the air to maintain a comfortable environment consumes a substantial amount of energy, so it makes sense to control the airflow carefully. Incoming air also contains moisture vapour and pollutants, including exhaust fumes, microbes, and pollen. It is also essential to expel the toxins such as volatile organic compounds (VOCs) that build up in homes. Naturally, it is crucial to strike a balance - we need sufficient airflow to be healthy and comfortable yet not waste too much energy.

Air enters our buildings in a variety of ways. These include:

• Diffusion through the fabric of the building – we refer to this as uncontrolled ventilation.
• Ventilation through trickle vents, open windows, and forced ventilation systems - controlled ventilation

Airflow is a result of air pressure differences between the inside and outside of a building. Pressure differences have several causes, including:

• Wind pressure – although buildings are more draughty when it’s windy, the overall effect is complex. Wind tends to cause regions of both high and low pressure around the building envelope which can lead to complex airflow patterns
• Stack pressure – this refers to differences in air pressure between the top and bottom of a building. Naturally, tall structures such as blocks of flats are far more susceptible to this,

So how do we make buildings sufficiently airtight while maintaining adequate ventilation? How airtight should we make a new home?

Building regulations on air permeability

UK building regulations stipulate a maximum air leakage of 5m3/hm2 (5 cubic metres per hour per square meter of the building envelope) at 50Pa. The 50Pa figure represents the pressure difference between the internal structure and the external environment. The regulations concern uncontrolled leakage; they don’t include ventilation provided by trickle and other vents. These are sealed during airtightness tests.
In naturally ventilated homes, accepted minimum ventilation levels for healthy living are considered to be around 4-5m3/hm2. If natural ventilation levels are tighter than that, then some form of forced ventilation is usually recommended.

Uncontrolled and controlled ventilation

These regulations apply to uncontrolled ventilation, but what about controlled ventilation? You may wonder what the point of building an airtight home if we are going to provide additional ventilation anyway is? However, the fact is that there is no way to recover heat lost by uncontrolled ventilation. In contrast, in buildings with controlled ventilation, we can, at least in principle, use a heat exchanger to recover the heat from the air before it is expelled.

Where buildings leak

The significant leaks in homes occur at junctions between walls and between walls and floors; joints between windows and walls; around doors and other sealed holes in walls; and electrical equipment conduits. Altogether there are around 20 potential leakage sites in a typical new home.

Airtightness is measured by temporarily installing a fan in a wall and using it to pressurise the building to a set pressure, typically 50Pa. By measuring the airflow through the fan, we can calculate the airtightness of the building.

Energy savings provided by airtight homes

The critical question for both our heating bills and the environments is how much energy we save by making homes airtight. Studies have been carried out by several bodies, including the Building Research Establishment (BRE). The overall conclusion is that energy savings of 10 kWh per m2 of floor area a year are achievable. In the UK, 15% of the energy used to heat and cool homes can be saved by adopting current building regulation standards of 5m3/hm2.

Posted: June 2017