What is passive background ventilation?

Also known as Trickle Ventilation, it is a system whereby adjustable ventilation inlets and outlets are provided to every room of the house by way of the window system. This allows fresh air from the outside to replace stale and moisture-laden air inside the home through a natural process. It does not replace opening sashes but is in addition to them.

Why is passive ventilation needed?

Today’s homes are practically airtight with draught sealed windows and doors. Moisture and airborne pollutants, created by the residents and sometimes by the building process itself cannot escape. Excessive condensation is the most visible result, although the resulting poor air quality is also a serious health issue, particularly affecting asthma sufferers.

Doors and windows often cannot be left open for security reasons. Passive background ventilation overcomes this problem by providing continual, cost and maintenance-free, ventilation even when the home is locked up or left unattended.

Why should you install passive ventilation in the window system?

Nearly every room in the home has a window that allows direct access to the outside. This means that fresh air can be delivered to practically every room in the house.

By installing trickle ventilation in the windows, renovation work is minimised, the installation is very straightforward and no other contractor is required other than a qualified glazier.

How does passive ventilation work?

It relies on the principle of variations in air pressure, whereby fresh air enters the house on the windward side forcing stale and moisture laden air to exit on the sheltered side. Passive ventilation is very well suited to New Zealand conditions since it relies on natural air movement and we have a consistent level of wind in most areas.

How much passive ventilation is required?

Following extensive research in the UK a minimum requirement for background passive ventilation (also known as trickle ventilation) has been adopted. This minimum requirement is for 4000mm2 minimum air opening per average sized room that is used for normal living activities. This does not include bathrooms, laundries or kitchens. These may require more ventilation in the absence of other methods of moisture extraction.

This can be achieved by a minimum vent length of 600 mm. The above recommendation is the same as that recommended by BRANZ.

Are there any other factors to take into consideration?

The above is for an average style home in an average location. Variations in house design and location affect the amount of background ventilation required and can be predicted. The extremes at the top and bottom ends of the scale are shown below.

Multi level, sunny and exposed: A home which falls into this category will have few condensation problems. Natural convection will assist internal air movement while the sun will warm the air and wind pressure will allow fresh air to enter the home through any openings available. A few trickle vents distributed strategically around the home will suffice.

Single level, shady and sheltered: A home in this category will be prone to serious condensation problems, and will require the full recommended minimum number of passive vents. In addition mechanical extraction of moisture at source in kitchens, bathrooms and laundries would be recommended.

Will ventilation alone solve condensation problems?

Ventilation is one key factor in eliminating condensation. The other being heating.

Warm air has a greater capacity to hold moisture than cold air. So, as air in the home is heated, it expands and absorbs moisture from furniture, carpets, beddings and other surfaces in the home.

As air cools, the water vapour in the air condenses and the air releases it’s moisture content. This condensation appears first on cold surfaces such as windows.

Cold outside air is relatively dry, even on rainy days. This is because the colder the air is, the less moisture it is capable of holding. If you have ever spent any time in very cold environments such as ski resorts, you will have noticed how much drier the air feels than on a hot, humid summer’s day.

So, eliminating condensation requires a combination of heating and ventilation. It requires cooler, drier air from outside to be drawn into the living areas, warmed up to absorb the moisture from household surfaces and day-to-day activities, and then expelled from the home, taking the moisture and other pollutants with it. This needs to be a continual process, where the levels of heating and ventilation can be adjusted depending on the conditions. Ideally the internal temperature should to be 5 to 7 degrees warmer than the external temperature, or around 18 – 21 degrees for living areas.

If you heat the air in your home without also allowing it to be ventilated to the outside, as soon as the indoor temperature drops (for example when you turn your heaters off before going to bed), the moisture will be released back out of the air and into / onto furnishings and surfaces. In addition, you are actually creating a warm, damp environment, perfect for fungus, mould and bacteria to grow.

What about heat loss?

With any ventilation system there is a degree of heat loss. This is unavoidable. However, rather than thinking of inside air as warm and outside air as cold, it’s more important to focus on the fact that inside air quickly becomes stale and damp, while outside air is fresh and dry. It’s also far easier and cheaper to heat dry air than it is to re-heat damp air. For these reasons, you need to replace the inside air with fresh dry outside air at a rate of about one air change every three hours.

The key is to provide sufficient air flow to achieve this without over-ventilating. The Easy Air vents have been designed to achieve the recommended air flow with one vent installed in each room. They are also completely controllable, so they can be partially or fully closed during windier periods.

At these ventilation rates, heat loss is minimal and the net effect on your heating bill is usually negligable.

What sort of heater should be used?

It is important to warm the air to allow moisture from internal wall coverings and furnishings to be absorbed. The best type of heater to achieve this will depend on your specific situation, however an electric convection heater, fan assisted and thermostatically controlled is a good cost effective option with a small investment. Heat pumps are very energy efficient, but they are expensive to install. Fuel can be wood fired or electric. The exception is gas or LPG heaters which create water vapour and exacerbate the problem when the room cools down. Radiant heaters which heat people and not the air are ineffective.

How much does Easy Air passive ventilation cost?

The average cost is between $110 to $120 per vent supplied and installed into existing windows inclusive of GST. For an average house with eight rooms, the total cost would be between $880 and $960. This cost is based on a home on reasonably level ground. There would be a small additional installation cost for homes requiring ladders and/or scaffolding.

If you intend ordering the vents to go into new windows, ask your window manufacturer for a quotation, which will be considerably less than the prices shown above, since they can be installed when the windows are made.

Can I include passive vents with retro-fitted double glazing?

Yes you can. In fact this is a great way to improve your insulation and ventilation all at the same time, and for very little extra cost.

While it may seem counter-intuitive to install double glazing and then include air gaps along the top, remember that a certain level of natural ventilation is always required, whether you have single or double glazing. And installing our double glazed vent at the same time is by far the most cost-effective way to achieve this.

Another mis-conception is that double glazing makes your home drier. Double glazing doesn’t remove moisture at all, so it can’t dry your home. Double glazing is a form of insulation. It insulates the inside pane of glass from the cold outside air making it easier and cheaper to keep your home warm. As the inside glass pane is warmer, the moisture in the air is less likely to condense on it. Hence, Double Glazing reduces window condensation.

Adding Easy Air vents at the top of double glazed windows is a great way to reduce the moisture level of the inside air without affecting the insulation of the glass. So they complement each other very well.

If you’re interested in getting our vents installed along with your retro-fitted double glazing, please get in touch with us. We can also recommend The Double Glazing Company who are familiar with our product.

Who are some of your clients?

Easy Air trickle vents are currently being installed in Housing New Zealand homes as part of their Healthy housing programme to improve indoor air quality and reduce condensation.

Both the Manukau City Council and the Wellington City Council specify Easy Air vents in their upgrades to council housing.

We have also installed vents in the race control tower at the Royal Yacht Squadron at Westhaven marina in Auckland.

Easy Air vents have been installed in a number of school and tertiary institution buildings.

Force 3 transportable homes install our vents in their homes.

Easy Air vents are also regularly specified in new home builds and renovations of existing homes where condensation and air quality are a concern.

What about forced air (positive pressure) home ventilation systems?

Home ventilation systems such as DVS, HRV and MoistureMaster start from around double the price for a basic system, require regular maintenance and use electricity. They are no more effective than passive ventilation under normal situations but may be useful in the single level sheltered and shady scenario.

However, forced air systems – or domestic ventilation systems – still require the circulated air to be expelled from the home to be effective, and they generally rely on natural, uncontrolled gaps in the building envelope for this. As modern homes are built very airtight, these natural gaps will not be sufficient for the moist, stale air to be expelled. It will simply be circulated through the home.

So in these cases, installing a few strategically positioned trickle vents around the house will provide the necessary paths for the stale air to escape and will increase the effectiveness of any existing forced air home ventilation system.

Forced air home ventilation systems work by pushing air from the roof cavity down into the home. They often claim that the air in the roof cavity is fresher, drier and warmer than the outside air. The air in the roof cavity may be warmer than outside during part of the day, but at night, when the heat is needed, roof space air is almost always colder than inside, and is usually no warmer than the ambient outside air. Therefore, the forced air ventilation system is either pumping cold air through your home at night, or it shuts down leading to insufficient ventilation. The opposite is the case during the summer.

This is supported by findings of a study conducted by Otago University recently and reported by Consumer.org.nz

The ability of the roof space to retain heat depends on a number of factors including the roofing materials used, time of year and daily conditions. During the winter, even with favourable weather, the roof space will generally not reach a suitable temperature until late morning or early afternoon. By early evening, the temperature in the roof has generally dropped below indoor temperatures. Also, under common winter conditions, the roof space can fall below the temperature of outside air. You can read more information on this topic in a report to EECA titled “Home Ventilation Systems Assessment”

The claim that roof space air is clean and fresh is also questionable. The roof space in a home is often rife with insects, dust and potentially rodent droppings and urine. Roof spaces are rarely cleaned and the filters supplied with these ventilation systems will not prevent all of these pollutants from making their way into your living areas. You only need to ask yourself why these systems require expensive filters in the first place.

Furthermore, the 1992 Building Code specifies that ventilation systems must comply with the New Zealand Standard 4303: Ventilation for Acceptable Indoor Air Quality. This specifies the minimum residential ventilation rate with outdoor air at approximately one complete air change every three hours to be distributed throughout the home.

Outdoor air is defined as air taken from the external atmosphere. Roof space air does not qualify as outside air.

Finally, it’s worth noting that the UK building regulations do not endorse the use of positive pressure ventilation systems (positive input ventilation). They recommend trickle (passive) vents with a variety of extraction methods, or balanced pressure systems with heat exchangers.

What about dehumidifiers?

These cannot control the entire house. They are noisy, need to have the water container emptied and consume large amounts of power. They can be useful to solve an acute condensation problem in a particularly damp area in the house, but you would need a number of dehumidifiers runnings throughout the house to control the moisture.

Approximately 15 – 20 litres of moisture is generated in a typical home every day, so your dehumidifier(s) would need to be able to meet this capacity. In comparison, passive ventilation can quite easily cope with this level of water vapour with no power requirements, and no need to continually empty water collectors.

Another important disadvantage of dehumidifiers is that they only tackle the water vapour in the air. They still don’t replace the stale air with fresh outside air. Only ventilation can do this.

What about simply opening the windows?

Opening windows are certainly effective in achieving passive ventilation through your home. However, there is the issue of security, over-ventilation, weather and insects.

Windows can’t be left open while the home is unattended.

Even leaving your windows open slightly leads to over-ventilation, which causes excessive heat-loss. Excessive heat-loss increases your heating bill and it lowers the indoor temperature closer to, or below the dew point. This means that even with the dryer fresh air, you will still get condensation due to the low temperature. Put simply, it is possible to have too much fresh air.

Unless a full insect screen is fitted to the window, it will allow insects and other pests inside.

Open windows will also allow rain in during extreme weather conditions.

Fitting security stays to your opening windows will go some way towards addressing the security issue, but they do not cover the issue of over-ventilation, insects and rain. Security stays are also not as secure as a glazed-in vent and would not be recommended if the home was being left unattended for some time.

Further reading

You can learn more on the topic of home ventilation from our Further reading web page where we have provided a collection of independent web sites and studies.

Contact us now

Contact us for further information, or to arrange a quote to install passive vents in your existing home.

For new home builds, get in touch with your chosen window manufacturer and ask about including EasyAir passive vents in your windows. We can also recommend a window manufacturer in your area.