Can a building or a home be too airtight?

Some builders are concerned with building homes too airtight. Some reasons they give are:

1. Humans breathe. Your building also needs to breathe.
2. We don’t want people suffocating in our homes/buildings. You need to get fresh air from somewhere.
3. We don’t want to have to install a heat recovery (HRV) or energy recovery (ERV) ventilation system if we build too tight.
4. If I am going to naturally ventilate my building, why do I need to worry about air leakage?
5. We don’t want our homes or buildings to grow mould.

To begin with, each of these concerns arises from of lack of ventilation, not airtightness.  Air leakage is NOT ventilation. So let’s go back to basics: what is ventilation?

Ventilation is the intentional introduction of air from the outdoors into a building. It can be natural or mechanical.  In apartments, it is highly recommended that you consult a fire engineer, and consider a pressure relief strategy in the event of a fire outbreak inside the building envelope.

Natural ventilation is the flow of air through open windows, doors, vents, and other planned building envelope penetrations, and it is driven by natural forces.

Mechanical ventilation is the intentional movement of air into and out of a building using supply or exhaust fans.

For ventilation to be effective we need to consider the quantity, the quality, and the distribution of air into an occupied space. Can air leakage satisfy these three factors? Some things to consider:

• Air from leaks is not filtered. It may come from a mouldy, dusty, or vermin-infested building cavity.
• There is no way to easily temper the air coming in, so it brings the temperature, humidity or dryness of the outside with it.
• Air leakage leaves you at the mercy of the weather. On windy, hot, or cold days, lots of air exchange results, but on mild, calm days, there is very little transfer.
• Because you can’t locate or control all air leaks, it’s very hard to make sure each room gets the right amount of fresh air.

Mechanical ventilation is guaranteed fresh air from a strategically-located source, no matter what the weather is.

There is no way to compare air leakage with ventilation! Not only does infiltration fail to meet ventilation needs, it has additional harmful effects to your building and the health of its occupants.

Condensation in wall systems and inside internal surfaces in a building.

When infiltration brings air from outside, it can create localised cold spots in walls, floors, and ceilings. These have the potential to create condensation, which can lead to mould growth or stains on finishes. Unchecked, condensation can lead to rotting or corrosion of materials, causing structural damage. It also makes a more inviting habitat for a termite infestation.

These issues may lie undetected until it’s too late. The mould that forms inside walls, with infiltration, can distribute mould spores through your home, and you cannot clean or maintain these surfaces inside your building fabric.

For buildings that are designed for natural ventilation, the intentional openings should be fitted with condensate drains and be constructed using water-resistant materials. The location of these openings should be accessible for cleaning and maintenance. The most common example is mould that forms on leaky windows, which can be cleaned easily.

Reduces the performance of your insulation system.

When insulation materials are rated in the laboratory, they are under controlled conditions with no air moving through the test sample. In the real world, air leaks can pass air through insulation, which substantially reduces insulation performance in two ways:

• Direct bypass – air leakage can pass right around both rigid and fibrous insulation
• Wind washing – air can pass through bulk insulation material itself, disturbing air pockets trapped by the fibres and stealing the heat it is meant to retain

Unwanted distribution of pollutants throughout the building.

In leaky buildings, pollutants can distribute through a building via unintentional gaps and cracks. This can create problems ranging from minor nuisances such as odours to more serious issues such as carbon monoxide from a garage or car park. Most dangerously, they are a potential path of smoke in the case of a fire.

Other issues from air leakage

Air leakage in larger buildings can also contribute to noise infiltration and noise production from whistling under windy conditions.

Lastly, any hole to the outside, garage, underground car parks or other unconditioned spaces serve as easy paths for CO or other pollutants and ants, cockroaches, spiders, mites, mice, and other unwanted guests. Sealing up these paths is the cheapest and first method of pest control.

To address concerns for airtightness in homes:

1. When someone says that a house needs to breathe, ask them to close their eyes and… breathe. When they take a deep breath in, air only enters through their mouth or nose. They don’t need air to come in any other way. The same is true for a house. Breathe only through openings made for that purpose.
2. You cannot build a building so tight that it is dangerous when you include ventilation.
3. For typical homes, providing basic ventilation can be inexpensive and easy. A simple continuously running remote-mounted extract fan is quiet and energy-efficient.
4. There is no good argument for airflow through walls. Truly fresh air should be filtered, volume-controlled, and distributed by design. Getting the fresh air from a mouldy wall cavity is not healthy or reliable.
5. To reduce condensation and mould potential, add simple continuous mechanical ventilation and pay attention to thermal bridges in the construction of the home. Condensation is a complex phenomenon; increased ventilation alone may not solve the problem.  You can experience condensation and mould on a carport soffit that has no walls.