Ventilation and airtightness

Key points

  • Having a healthy home that also has good thermal performance requires both reliable ventilation and good airtightness.
  • Ventilation is the intentional introduction of outdoor air into a building to maintain good air quality. Ventilation methods can be either natural (for example, windows) or mechanical (for example, fans).
  • Airtightness is avoiding the unintended introduction of outdoor air into a building or the loss of air to the outside (for example, through poorly sealed glazing or building envelope).
  • As building construction methods and airtightness of homes improve, good ventilation becomes even more important.
  • Improving airtightness can improve the thermal performance of your home by reducing the influx of cold air in winter and hot air in summer. Improving air tightness is one of the most cost-effective and easiest ways to improve your thermal comfort and reduce energy costs.
  • Careful design will be needed to ensure that the risk of condensation does not increase when improving airtightness.

Understanding ventilation and airtightness

What are ventilation and airtightness?

Ventilation is the intentional introduction of outdoor air into a building. All homes require ventilation to maintain good indoor air quality. It increases oxygen levels and dilutes and displaces carbon dioxide and airborne pollutants. It can also be used to increase thermal comfort and reduce humidity.

Airtightness is about limiting the unintentional introduction of outdoor air into a building, or limiting the loss of air to the outside. The greater the airtightness, the less unintended air movement occurs.

Windows and doors that have been designed to encourage ventilation and cool breezes when open are beneficial. But windows and doors that are poorly sealed, allowing cold draughts in winter or hot draughts in summer, are a problem.

Air typically leaks through:

  • unsealed or poorly sealed doors and windows
  • unsealed vents, skylights and exhaust fans
  • gaps in or around ceiling insulation and around ceiling penetrations (for example, downlights, pipes and cables)
  • gaps around wall penetrations (for example, pipes, conduits, power outlets, switches, air-conditioners and heaters)
  • gaps between building envelope junctions (for example, floor−wall or wall−ceiling)
  • poorly fitted or shrunken floorboards.

A diagram of a house shows potential sources of air leakage which include gaps between walls or floors and skirting boards, gaps between floorboards, and gaps where pipes penetrate walls. On the sides of the house, air can leak from gaps between windows, air vents, fixed air conditioners and heaters, construction joints between wall materials, as well as gaps up chimneys and around doors. Air can also leak through gaps between walls or ceilings and cornices, vented downlights and sky lights.

Common air leakage points

Source: Sustainable Energy Authority Victoria

Why are ventilation and airtightness important?

Good ventilation of your home is essential for your health.

Good airtightness (that is, reducing or eliminating air leaks) can improve thermal comfort and energy efficiency – air leaks can cause 15−25% of winter heat loss in buildings. Sealing your home is one of the simplest ways to increase your comfort while reducing your energy costs.

Note

An airtight house with inadequate ventilation may lead to condensation, mould and high internal levels of carbon dioxide. Build airtight for thermal comfort and energy efficiency, but not so tight that it compromises indoor air quality. Consult a qualified building professional on how to achieve this.

Increased carbon dioxide levels inside buildings can have negative health effects. Carbon dioxide levels are measured in parts per million (ppm) (Bonino 2016):

  • 350–1000ppm is the typical level found in occupied spaces with good air exchange
  • >1000–2000ppm is the level associated with complaints of drowsiness and poor air
  • >2000–5000ppm is the level associated with headaches, sleepiness, and stagnant, stale, stuffy air; poor concentration, loss of attention, increased heart rate and slight nausea
  • >5000ppm indicates unusual air conditions where high levels of other gases also could be present; oxygen deprivation or toxicity could occur.

If not paired with adequate ventilation, increased airtightness can also lead to the build-up of gases, toxins and pollutants, which can also trigger respiratory health issues. Toxic substances include:

  • carbon monoxide, sulfur dioxide and nitrogen oxide from heating and cooking
  • volatile organic compounds (VOCs) and formaldehyde emissions from furniture, carpet, finishes and building materials
  • airborne toxins from household cleaners
  • pollen, dust and dust mites.

Achieving good ventilation and airtightness

Ventilation

There are various strategies for ventilating your home:

  • natural ventilation (that is, the house openings such as windows and doors)
  • mechanical ventilation (for example, fans, heat recovery ventilation systems)
  • a combination of natural and mechanical methods.

Natural ventilation

Natural ventilation relies on moving air through a building in either or both of these ways:

  • wind applying a pressure differential from one side of the building to the other
  • an internal to external temperature difference, which generates a pressure differential from inside to outside, and thus a draught.

Adequate ventilation using only windows and doors requires the occupants to open sufficiently sized windows at all times of year, which may be less likely during winter. In cooler climates, the use of openable windows for ventilation will also significantly increase the heating energy required in the home.

Mechanical ventilation

Mechanical ventilation generally uses fans to introduce air from the outside and distribute it in the building. Examples of mechanical ventilation systems include exhaust air ventilation, positive pressure air replacement and mechanical ventilation heat recovery systems.

As airtightness increases, so does the necessity for reliable ventilation. Opening doors and windows is not seen as reliable ventilation because it depends on manual operation by the householder, which is difficult to achieve consistently.

Note

There is no scientific agreement on when mechanical ventilation is required in a building. There is some consensus that if your home is achieving below 3 to 7 air changes per hour at 50 pascals (Pa) (known as ACH50), mechanical ventilation is definitely required. Find out more on measuring airtightness later in this chapter.

Exhaust air ventilation

Mechanical extractor fans are a reliable way to control moisture build-up and humidity in kitchens and bathrooms. These fans should be vented to the outside and not into roof spaces or other cavities. Ensure that adequate draught stoppers are fitted to the fans to prevent unwanted air exchange when not in operation.

Exhaust air ventilation may draw its replacement air either through open windows in a well-sealed home or through air infiltration in more leaky homes. Exhaust air ventilation can be used to provide fresh air for occupants, usually in conjunction with windows. In these cases, it is important to ensure that the air being drawn into the building is as clean as possible.

Continuous exhaust ventilation (for example, a fan running 24 hours a day, 7 days a week) may be coupled with trickle vents. This can offer a more reliable continuous ventilation strategy for airtight buildings than occasional purging, which relies on occupants opening windows when triggered by odours or when outside air will improve thermal comfort. Caution should be used because the continual addition of untempered outdoor air may increase discomfort and energy use.

Mechanical ventilation heat recovery

Mechanical ventilation heat recovery (MVHR) systems provide a total ventilation system for well-sealed homes. These systems may work in conjunction with an existing heating system. They draw in external air and extract heat from outgoing, humid air with a heat exchanger and use it to temper fresh incoming air. This recovers the heat from outgoing air to minimise losses, while maintaining fresh air replacement at levels required to maintain health and amenity. The air is filtered as it enters and leaves the home; this is especially useful in areas and during periods of high pollution and pollen. The system operates continuously to ensure optimum indoor air quality.

These systems require expert design, but when installed and commissioned correctly are quiet and require minimal maintenance. These systems are used to provide ventilation in houses built to the highest standard of airtightness.

A diagram of a two storey home shows how mechanical ventilation heat recovery works.

MVHR supplies filtered outdoor air to living rooms and bedrooms while extracting air from bathrooms, kitchens and laundries

 

Airtightness

Qualified technicians can conduct air leak audits or offer a full retrofit building sealing service. Blower door tests are used to measure the airtightness of the building, with results measured in air changes per hour or ACH at 50 pascals (Pa; known as ACH50). In such a test, a calibrated fan is mounted in a flexible panel positioned in an external door. The fan measures airflow into and out of the building as it is pressurised and depressurised. In addition, thermal imaging cameras and smoke tests may be used to locate the leaks.

A more detailed energy assessment can also be obtained where, in addition to an air leak audit, the technician examines all aspects of your home’s thermal performance. This includes glazing, insulation, thermal bridging, thermal mass, airtightness, and passive heating and cooling. You may find a list of technicians in your location by searching online for ‘home energy audit’.

Most Australian houses would benefit from improved airtightness. Some old houses are very leaky, with total air changes in excess of 30 per hour at 50 pascals (Pa). The current average new Australian home has an airtightness of 15.4 air changes per hour at 50 pascals (Pa) (that is, 15.4ACH50) (CSIRO 2015). By comparison, a certified ‘Passive House’ achieves a maximum air leakage of 0.6 ACH50.

Airtightness regulations and testing

The National Construction Code (NCC) includes acceptable construction practices for building sealing. These practices are listed in Part 3.12.3 of NCC Volume 2.

The NCC does not currently quantify an air leakage rate for new buildings or major renovations. Instead, new single dwellings that are constructed to NCC performance requirements and assessed using Nationwide House Energy Rating Scheme (NatHERS) software are assumed to achieve a permeability rating of 10m3/hr.m2 (a measure of air leakage through a square metre of building envelope). This is broadly equivalent to 10 air changes per hour at 50 Pa when applied to detached homes.

You may choose a different pathway to comply with the NCC, by using a blower door test. Compliance is verified when a building envelope is sealed at an air permeability of not more than 10m3/hr.m2 at 50 Pa reference pressure when tested in accordance with Australian Standard AS/NZS ISO 9972 Method 1.

A blower door tester is installed in a door way which consists of a frame and a flexible panel fitted in the doorway with a fan and an air pressure gauge.

Blower door tests are used to measure the airtightness of the building

Photo: Wikimedia (Ket555)

Retrofitting for airtightness

Airtightness should be considered when you are buying or building a house, but airtightness can also be improved through simple renovations.

Care should be taken when improving airtightness to ensure that the risk of condensation does not increase.

Building envelope

Use airtight construction detailing, particularly at wall−ceiling and wall−floor junctions. Standard cornices set with cornice cement may achieve this while allowing for building movement. Traditional quads and timber mouldings do not, unless joints are filled with sealant before fixing.

Junctions and gaps

Seal junctions and gaps between building components with durable, flexible caulks and seals:

  • at the junction of window and door frames, walls, floors and ceilings, skirting boards, plumbing pipes, exposed rafters and beams, inbuilt heaters and air-conditioners
  • between dissimilar materials (for example, masonry walls and timber framing).

In cavity wall construction, internal sealing is more effective. Seal larger gaps with expandable foam. Seal gaps between the window and door frames and the structural building frame before fitting architraves.

It is possible to retrofit insulation into existing wall cavities although it is preferable to be able to install vapour-permeable membranes behind the cladding to reduce the likelihood of moisture wicking through the cladding. It is critical that the cladding reduces water absorption; well-maintained paint or a clear sealer on brickwork is recommended.

While vapour-permeable membranes can prevent air from entering around wall penetrations such as switches and power points, there is usually a gap left around windows. When architraves are fitted, they often do not create a good seal. This allows cavity air to enter under pressure.

This gap should be sealed by:

  • taping the membrane into the frame
  • using expanding foam (take care not to bow the reveals)
  • pushing strips of bulk insulation into the gaps around the frame.

When using high-performance membranes, ensure that all joints are taped with compatible high-performance tape products. As with all building products, a warranty life corresponding to the product lifespan is best.

A photograph of a persons hands installing a weather strip around a window frame.

Use adhesive weather strip to seal gaps around windows and door frames

Photo: Getty Images

Airlocks

Provide airlocks at all external openings. Airlocks can be double-purpose rooms (for example, laundries and mud rooms). Seal wood storage areas if wood heating is used.

Floors

Use tight-fitting flooring and insulate the underside of raised floors with insulative membranes in cooler climates. Various insulating sheet and roll membranes are available and simple to install. Consult the manufacturer’s technical information and installation guide to manage condensation risks. Seal to the edge of floors to prevent penetration by air pressure from subfloor vents. Do not block subfloor vents or wall cavities.

Windows and doors

Choose high-quality windows and doors with tight air seals. Australian Standard AS2047 Windows in buildings — selection and installation, allows a maximum air infiltration rate of 5.0L/s.m2 for a closed window or door. This is the volume of air that can infiltrate a square metre of window per second at a positive pressure of 75Pa. Window manufacturers are required to have their products tested to this standard and register them with the Window Energy Rating Scheme (WERS).

However, the standard allows too much leakage for good energy efficiency, particularly in cool climates and high-wind areas. In these areas, you can improve the performance of existing windows and doors by using draught-proofing strips, ensuring they are appropriate for the window style (that is, maintain easy operation). Fit retractable draught seals at the bottom of hinged doors.

Overlapping brush seals can allow full movement of sliding or double hung windows while making an acceptable seal. Self-adhesive neoprene pillow or foam strip seals are effective on hinged doors and casement and awning windows.

Fit automatic door closers to external doors and doors leading to unheated areas. Avoid using cavity sliding doors in critical air-leak paths because they are hard to seal and acoustically poor.

A number of different styles of draught proofing products for doors are illustrated. They include three different types that can be fitted to around the door cavity, with varied surfaces. Six different types of stopper are illustrated that can be fitted to the bottom of the door itself.

Many products are available for draught proofing

 

Vents, skylights, hatches and penetrations

Avoid or replace open-vented downlights that penetrate ceiling insulation because they contribute to both air leakage and additional heat flows. In some situations, you can replace in-ceiling downlights with surface-mounted downlights, thereby avoiding penetration of the ceiling and insulation.

Lighting that emits high levels of heat (for example, halogen) requires clear space around it so it does not come in contact with the roof structure or insulation. These should be replaced with appropriately rated LEDs, allowing insulation to cover the full area of ceiling.

Electrical outlets (switches and power points) are a common and problematic source of air leaks. They can be difficult to trace because construction air spaces become pressurised under wind loads and transfer draughts long distances from the source. This is especially a problem in cavity brick construction. Sealing around switches and power points can help, but this makes them difficult to remove for maintenance.

Draught-proof vapour-permeable building membranes in cavities or under cladding are ideal to reduce the risk of condensation. However, these are difficult to retrofit unless recladding.

Exhaust fans and range hoods commonly open into the roof space, allowing uncontrolled movement of air. This raises water vapour levels and fire risk. It is therefore important to duct exhaust fans to outside and install nonreturn baffles.

Do not use permanently ventilated skylights and seal off permanent air vents. Use windows and doors or heat recovery systems for ventilation as required.

Insulate manholes or roof space access hatches (which are often poorly sealed and uninsulated) with rigid insulation fixed to the inside of the hatch. Fit air seals to all sides.

Air leakage from heating and cooling

Unflued gas heaters should be avoided. If used, they require fixed ventilation to prevent the build-up of toxic gas. Mechanical systems can provide adequate ventilation but fail in blackouts when gas heaters will continue to operate.

Flued gas heaters are fine from an indoor air quality perspective because the toxic gases are directed to the outside via the flue.

Open fireplaces should be avoided. They are inefficient and draw in large volumes of air through the gaps in the house – both cold air to fuel combustion and hot air in summer. Fit dampers to chimneys and flues and ensure that they are closed when not in use. Insulation regulations in Victoria require dampers to be fitted to all new fireplaces.

Ducted air-conditioner penetrations through ceilings and floors are a common source of air leaks. Floor registers should be sealed with spray foam, compressed bulk insulation or self-adhesive sealing strips. Ceiling registers are generally not airtight and are often displaced during cleaning; seal with self-adhesive sealing strips.

Evaporative coolers often lack automatic dampers and therefore extract air from the house when they are not running, increasing heating bills and allowing outdoor heat to enter. They use large amounts of water and should only be used where a reliable, sustainable water supply is available. Generally, high-efficiency reverse cycle air-conditioning is more effective and efficient.

References and additional reading

Learn more

  • Explore Insulation and Glazing for more ideas on how to slow heat transfer through your home
  • Read Condensation for information on how to prevent condensation and mould in your home
  • Review Materials for ideas on methods and materials for new home construction

Authors

Original author: Chris Reardon 2013

Updated: Andy Marlow 2020