House Ventilation

April 28, 2014 at 4:02 PM

As seen in the NZ Herald “Spaces” magazine published April 17th 2014



With winter approaching it may be time to look at your options for a ventilation system – to help replace cold damp air in your home, with warm dry air. By Steve Hart

Sure, you can open a window to let the steam out from your kitchen and bathroom, so long as you don’t mind the warmth going out too. It works – to a point – but it’s not a smart option.

Opening a window or door can only be a temporary solution, because once you start feeling cold, you close them up and the moisture gets another chance to build up again.

The best answer is an efficient and reliable ventilation system that will remove moisture in the air and reduce dampness in things such as fabrics and carpets, helping to prevent things such as mould developing.

Scratch the surface of the ventilation market and you’ll quickly find it boils down to two basic systems; positive pressure (roof cavity systems) and balanced pressure (a heat recovery system).

Positive pressure systems move warm air from your loft space into your living areas using one or more motorised fans. However you need a home that allows air to seep out for the system to work (sadly, that’s not a problem for most Kiwi homes).

A prerequisite for these systems working in your favour is that the air in the loft has to be warmer than the air in your rooms. It might be worth taking the temperature of your roof space one evening in mid-winter to see just how warm it is, and while you’re up there look for evidence of vermin and mould. Vermin like chewing through cables, and you don’t want mould ventilated into your living areas.

Also, if you have a particularly draughty home, the positive pressure system might not be as effective as you might like.

It also seems that suppliers have yet to prove that filters used in the positive pressure systems stop contaminants that may lurk in your roof from making their way into your living areas. The Energy Efficiency & Conservation Authority, a government funded organisation, recommends that home ventilation systems draw air from outside the home, not the roof cavity.

Glenn Murdoch is the chairperson of the Passive House Institute in Queenstown. The charitable trust works to, among other things, relieve fuel poverty through the ‘promotion of healthy and highly energy efficient homes and public buildings’.

“Positive ventilation systems are often marketed on television in New Zealand,” says Glenn. “They take air from the loft space and there are a variety of add-ons such as filters and heating elements to warm the air. They don’t meet the building code because in terms of providing ventilation, they do not bring fresh outdoor air into the home.

“Positive systems work by slightly over pressurising your home, pushing moisture out through the leaks in the house. The systems do remove condensation from inside your house.

“But where does that condensation, or water, go? During winter there is a really good chance the moisture is going to end up as condensation inside your (cavity) walls – where you can’t see it. As the moisture hits cold surfaces, it turns back to water.”

Glenn says owners of these systems will often say their home feels drier. But may be unaware the moisture is being moved from the rooms “to places they can’t see it”.

“They might also mention their house feels cold, because the air in the roof space is only hot on a sunny day,” says Glenn.

The New Zealand Building Code requires homes to have a means of ventilation using outdoor air to maintain air purity. Ventilation systems that draw air from the roof space do not comply with ventilation standard NZS4303:1990, says EECA.

It’s also worth remembering that come summer, loft spaces can become exceptionally hot. So a positive pressure system should have a way of dealing with this, a ‘summer bypass’ is one option.

The other ventilation system, and the one recommended by Glenn’s organisation, is the balanced system. These systems expel warm, moisture-laden air to outside – from places such as kitchens and bathrooms. The air passes through a heat exchanger. Fresh air is drawn in, warmed up by the heat exchanger, and fed into the living areas.

Glenn says: “This system gets rids of the moisture and any cooking smells, and at the same time it brings in fresh outdoor air for the bedrooms and living areas.

“As it does this, the two volumes of air pass each other through a heat exchanger, so the incoming air is warmed up. It recovers that potentially lost energy. There is no active heating going on, although that can be an option.”

Unlike the positive system, the home doesn’t need to be draughty for the balanced system to work. So they are ideal for those who have full double-glazing and insulation.

Glenn says there is a perception that balanced systems are more expensive than positive systems. But he says a balanced system installation for the average home can start from $6,000, and deliver around $12 of heat for every $1 spent on running the system.

“But the returns do depend on where you live, and how cold it is,” says Glenn.

He also says that even people who think they live in a dry home can feel the benefit of a good quality ventilation system.

“New Zealand homes, no matter how old or new they are, suffer from quite high levels of indoor contaminants, particularly carbon dioxide (Co2) – which is breathed out and trapped in the home when all the doors and windows are closed on winter nights,” says Glenn.

EECA recommends home owners always ask their ventilation system supplier for independent test performance reports for the system being offered. And says buyers should also get a ‘no questions asked’ guarantee of performance that includes removal of the system if it doesn’t work, and repair of all modifications of your home.

Steve Hart is a freelance journalist at



Posted by Jo Clare on
Interesting article. It makes perfect sense when you think about it. And it ain't rocket science, as they say!
Leave a Reply

(Email addresses will not be published.)

Please type the letters and numbers shown in the image.Captcha CodeClick the image to see another captcha.