Smarter Homes: Ventilation

With good ventilation, your home will be drier, healthier and more comfortable.

Ventilation is about helping air to circulate in your home. It allows moisture and airborne pollutants to escape, and fresh, clean air to be drawn into your home. Well-designed ventilation will provide cooling in summer. In winter, it will let stale air out but keep warmth in.

Effective ventilation depends to a significant extent on the size, placement and type of windows, doors and other openings in your home. With good design, you can control the circulation of air, rather than having draughts.

With good design, you can use windows, vents and other openings for most ventilation – this will save on your energy costs.  However, you may need some mechanical (active) ventilation, for example, extractor fans to expel moist air from the kitchen, bathroom and laundry outside.

Does ventilation matter?

Yes. A 2005 BRANZ survey of the condition of New Zealand homes found that many were damp and poorly ventilated.

Most bathrooms relied only on windows for ventilation. Only half of kitchens vented moist air to the outside. And 40% of timber-framed homes had poor or seriously inadequate subfloor ventilation.

Poor ventilation allows moisture and airborne pollutants to build up inside your home. This can cause health problems such as asthma for you and other members of your household. Moisture can also make your home uncomfortable to live in and damage its structure.

When should you think about ventilation?

Planning a home or renovation

If you're building or renovating, ventilation should be considered early in the design process.

Good design should strike a balance between the need to introduce fresh, healthy air into your home and the need to maintain comfortable temperatures, so ventilation should be considered alongside passive heating and passive cooling options. If you consider heating without ventilation, you may end up with a home that's warm but not as healthy or comfortable to live in as it could be.

During and after construction

During the construction process and for a few weeks afterwards, you'll need to provide good ventilation to minimise your exposure to airborne pollutants such as formaldehyde from new building materials. See Unhealthy air for more.

In your existing home

Ventilation can be improved in an existing home without making significant alterations. Moving a door or window, or removing an internal wall might make a significant difference.

For ventilation to work as effectively as it should, your home should be well insulated. Then you can control your ventilation, rather than being draughty and cold.

Older homes tend to be less airtight than more modern homes. This can allow for some natural ventilation - but can also mean they're draughty and harder to heat. As a general rule of thumb, houses built before the 1960s will be very draughty, and houses built between the 1960s and 1980s will be quite draughty.  Modern construction, however, is much more airtight, meaning that problems with inadequate ventilation become more frequent.

Passive ventilation

How does it work?

Passive ventilation uses doors, windows, vents, louvres and other openings to bring fresh air into your home and let stale air out. The size and placement of these openings can be used to guide air into and through your home.

Where cooling is required, windows or other openings on upper levels can be opened to let warm air escape. In winter, well-designed passive ventilation refreshes the air in your home without creating draughts or letting out too much heat.

Passive ventilation can only work if air has clear, uninterrupted pathways through your home. You can maximise air flow by designing open plan areas or having high vents or other openings between rooms. In general, windows should be larger on one side of the home than the other in order to encourage air flow.

If your home is designed for passive ventilation, all you'll need to do is open and close windows, doors or other vents as needed to reduce the temperature and improve the quality of the air you're breathing.

To encourage cool air flow, you'll need larger windows opening to the breeze and smaller, higher windows on the walls on the opposite side of the house

Options

The appropriate ventilation options for your home will depend on the climate and microclimate of the area you live in, and what prevailing breezes there are. As a rule of thumb, the area of windows, doors and other vents that can be opened up to the outside should be at least 5% of the floor area for each living space - and more for high-use areas.

Some points to consider:

• Windows or other openings on opposite sides of your home will help draw air through.
• Opening windows on the south and east side are best for allowing cool breeze into your home from early in the day. Openings on the north and west sides, higher up, will keep the air moving.
• Vents or other openings in the roof or on upper floors will allow air to escape as heat rises.
• Built-in vents, louvres, slots and gaps in door or window framing can provide low-level ventilation over long periods without creating draughts or security risks.
• Different types of window can be used to guide air into your home - for example, side opening windows are better at catching breezes and pulling them into the house, than awning opening windows
• If your home is on more than one level, make sure there are opening windows and doors on each level. As hot air rises, high windows which can be left open on upper floors can be a good way of ventilating your house during summer.
• Fly screens and security stays installed on windows mean they can be left open at night, or when you’re out during the day, to help the house keep cool in summer
• Don’t forget to ensure cross-ventilation under your floor to get rid of dampness (see Moisture for more information)

See Glazing for more about window design.

Background air leakage

Some features will provide low-level background movement of air between your home's interior and exterior. This is often called air infiltration, and can cause draughts and heat loss in winter. For example:

• timber joinery around windows and doors
• flues and chimneys
• recessed ceiling and light fittings
• extractor fan grills.

It is better to plan good ventilation together with a well-insulated house, than rely on leaks and draughts which you cannot control when you need to and won’t necessarily ventilate the right places.

Active ventilation

Active ventilation is ventilation provided mechanically - for example, by extractor fans, range hoods and whole house ventilation systems. These systems run on electricity - the bigger the system and the more components, the more power it will use.

A well-insulated, well-designed home may only need to use active ventilation for rooms where moisture is generated (bathroom, laundry and kitchen), while passive ventilation will be sufficient for maintaining air quality through the rest of your home.

Active ventilation may also be needed to get warm air into cooler, damper areas such as south-facing rooms - for example by heat transfer systems (see Tips for efficient home heating for more information).

Extractor fans/range hoods

Extractor fans quickly remove moist air from bathrooms, toilets and laundries. Range hoods do the same job for kitchens.

It's important to choose the right-sized fan for the job. A fan that's too small won't remove enough moist air to keep your home dry. A fan that's too large can create draughts. For a typical bathroom or toilet a ventilation rate of 25 litres per second should suffice. For more information see Table B1 in AS 166 part 2.

Extractor fans should be placed as close to the moisture source as possible. They must be vented to the outside or the moist air will end up in your roof space, damaging your insulation and roof supports (see Moisture for more).

Because extractor fans remove moist air but don't bring in fresh air to replace it, you'll need some other way of getting fresh air into the room. By placing air vents on the opposite side of the room from the extractor fan, or slightly opening doors or windows, you can encourage air flow.

Solar or electric-powered roof ventilation

These simple fan, duct and vent systems take hot air from the top of your home or roof space out through the roof.  Solar-powered systems are available that cost nothing to run.

Whole house ventilation systems

Whole house ventilation systems can be useful to bring in fresh air and combat condensation in modern airtight houses. There are two main types of whole house ventilation systems:

• Positive pressure / Roof cavity ventilation systems
• Balanced pressure / Heat recovery ventilation systems.

Positive pressure / Roof cavity ventilation systems

Positive pressure or roof cavity ventilation systems are the most common type available in New Zealand. They bring filtered air from the roof space into the house through a single, or multiple, ceiling vents. This forces the stale air to leak out through gaps, windows and doors. The performance of these systems depends on the sizing of the fans, the distribution of the ceiling vents throughout the house and how airtight your home is.

In an airtight house, pushing the filtered air into the house creates a positive pressure inside the house which causes inside air to move out.  However, in draughty houses, there are too many gaps and leakage points - the ventilation system will not be able to force the air into each room of the house.

The ventilation system will also not work properly if the roof space is not properly sealed from the inside of the house (for example, if you have downlights).  The stale indoor air will leak back into the roof and be pumped back into the house again.
Ventilation systems should bring fresh air into the house, but your roof space may be polluted by dust, mould and vermin. Most systems are fitted with filters - the quality of the air entering the house depends on the filter type and whether you regularly change or clean filters.

The Energy Efficiency and Conservation Authority (EECA) recommends that the home ventilation systems source their ‘fresh’ air from the outside, not from the roof space.

Balanced pressure / Heat recovery systems

Balanced pressure / Heat recovery ventilation systems are particularly suitable for homes in colder areas of the country, if they are already well heated and if they are reasonably airtight.

These systems have two fans: an intake fan which supplies fresh outdoor air into the house through several ceiling vents; and an exhaust fan which takes stale air from inside the house and discharges it to the outside. An air-to-air heat exchanger (usually in the roof space) transfers heat from the inside air to the incoming fresh air from outside. In this way, most of the heat is recovered.

Some products include additional features to utilise heat in the roof space when it is available on sunny winter days, or to avoid warming incoming fresh air in summer when it is hot.

To ventilate effectively, these systems need gaps or vents in internal doors so that air can flow through all areas of the house between the intake and exhaust.

In winter, the heat exchanger transfers a portion of the heat in the warm exhaust air to the colder outdoor air, thus reducing the heat loss associated with the ventilation. To be effective, the house should be airtight so that almost all ventilation air passes through the heat exchanger, rather than being leaked out through draughts.
Heat recovery systems provide good fresh air ventilation but they are not a heating system.  However, they can recover between 67–95% of the heat from the inside air which means that the fresh air coming in will be warmer. This means you will need less heating to warm your home.

http://www.smarterhomes.org.nz/design/ventilation/