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07 Oct 2019

The problem with airtight homes

‎A recent article on the New Zealand Institute of Building Surveyors website says that New Zealand homes have gone from being extremely well-ventilated to potentially too airtight.

Our older weatherboard houses were famously cold and draughty. The average house leaked the equivalent of a one metre square hole. That’s a lot of ventilation! Unsurprisingly, these homes were very hard to heat.

The other extreme arrived in the 1990s, when monolithic cladding came along to create airtight boxes that retain moisture like crazy. The result was modern homes with internal dampness and mould, which are both associated with serious health issues.

Between these two extremes there’s a happy medium: a home that has the ability to breathe and continually change its internal air, allowing moisture to escape, but is still insulated enough to be easy to heat. To achieve this nirvana, many homes need some help in the form of a positive pressure ventilation (PPV) system, like Xgen by Unovent.

Installed in your roof cavity, a PPV system brings fresh, filtered air into the rooms below. The pressure of air coming into the rooms pushes stale, moist air out of your house through the tiny gaps under doors and around windows. If your house is one of those ‘airtight boxes’, leaving a few windows open a little (with security stays) will allow the old air to escape. Alternatively, your installer may come up with a vent solution to assist the through-flow of air from inside to outside.

Within a few days of installation, you’ll notice that your home is drier. Very likely it will smell better as well. Even during the coldest days of winter, there will be very little mist or condensation on windows. And if your home has been prone to mould around windows and mildew on curtains, those days are over. A PPV system is the ultimate damp closet solution too, especially if you get into the habit of leaving your wardrobe doors slightly open. You’ll also find that a drier home is easier to heat in winter, because dry air heats faster than moist air. The ultimate home ventilation and heating/cooling solution is PPV + heat pumps – they assist each other to give you fresh air at just the right temperature.


  • Humans are 60% water. The essential act of breathing puts moisture into the air. We also exude water as perspiration.
  • Kitchens put water into the air via steam. Every time you boil the kettle or cook on your stove top, water enters the internal atmosphere.
  • Bathrooms are all about hot water. Every hand wash, shower or bath puts moisture into the air.
  • Drying and airing clothes inside adds to the water load. When you come in from the rain and hang your coat on a hook, all that water’s going to evaporate into the air.
  • A passion for indoor plants can put extra moisture into your home (and if you believe plants are cleaning the air for you, that’s an urban legend).

If you’ve been thinking about getting a proper ventilation system installed in your home, it costs nothing to talk to the experts at Unovent. For a no-obligation assessment and quotation over the phone, call 0800 UNOVENT (0800 866 836) or email and we'll be in touch.