Positive pressure ventilation systems

We’re bombarded with advertising these days telling us that we need ventilation systems to have dry, healthy houses.  Slick images compare these ‘happy’ houses and their ‘happy’ residents with those living in misery without ventilation systems.

Those of us with a background in home performance advice take these adverts with a giant grain of salt.  Why? Firstly, the research tells us a more mixed picture of the benefits of positive pressure ventilation systems.  And secondly, there are far more fundamental – and cheaper – steps that can address condensation before you need to consider an expensive piece of kit.

The two types of whole house ventilation systems
  1. Positive pressure ventilation systems

These systems bring filtered air from the roof space into the house through a single, or multiple, ceiling vents. The pressure forces the stale air to leak out through gaps, windows and doors. These are the most common type available in New Zealand.

  1. Balanced pressure or heat recovery ventilation systems

These systems can pre-warm the incoming air from the heat energy in the outgoing exhaust air. 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.  These are much less common in New Zealand.

Balanced ventilation systems with heat recovery make more sense for newer airtight houses but make sure you have oriented your home so that you make the most of the sun for warming your house, upspec’ed the insulation above Building Code minimums, added double glazing, specified rangehoods and extractor fans in wet areas, and specified thick, well fitted curtains and heating for both living areas and bathrooms BEFORE you add the ventilation system.  There’s more but that’s a good start! 

What does the research say about positive pressure systems?

Firstly, it says that positive pressure ventilation systems are not heaters – they do not heat your home.  An Otago University study in 2011 found that positive pressure ventilation systems did not draw enough heat from the roof cavity to warm a typical weatherboard house (pitched iron roof and ceiling insulation) in Dunedin.

Smarter Homes notes that some systems use an electric in-line duct heater to warm the air going into the house but that these don’t have sufficient capacity and are a relatively expensive and inefficient way to heat a home.

Secondly, it says that systems that blow air from the roof space can be bringing in more moisture or more cold air.  A Beacon Pathway study in 2010 found that that during the day in winter there can often be more moisture in the roof space than in the house. Operating a positive pressure ventilation system during the day actually brings more moisture into the house. At night, the air in the roof space may be drier than the house but it is also much colder. Operating a ventilation system at night replaces warm air with cold.

Check out former Eco Design Advisor Ian Mayes talking about positive pressure ventilation systems:

So what are our reservations?

We think that to get your house to be warm and dry, you need to look at the whole house and how it’s working together. That’s what Home Performance Advisors and Eco Design Advisors do when they assess a home or house plans.

There are a lot of factors that contribute to making a house damp, mouldy or dripping with condensation (remembering that condensation is moisture in the air that condenses on cold surfaces).  We think there are basic steps you should take FIRST.  These are

  1. stop moisture coming in from outside,
  2. get rid of internal sources of moisture
  3. extract moisture from wet areas, and
  4. ventilate by opening windows and doors for 20 minutes once a day. 

The good news is these are often relatively easy to do:

Some steps you can take are totally free – for example, drying your washing outside means that is 5 litres of moisture NOT released into your home.

Some steps are simple and cheap – for example, putting a ground moisture barrier on the ground underneath your house.  BRANZ research says an average of 0.4 litres of water can evaporate from 1m2 of uncovered ground in 24 hours. In a 150m2 house that’s up to 60 litres/day on average, some of which can be drawn up into the house.  If you have enough access under the floor, you can install a ground moisture barrier yourself at the cost of polythene and some tape.

Some may require a tradesperson but are relatively low cost – for example, installing extractor fans in your kitchen and bathroom. A ten minute shower can put as much as half a litre of water into the air inside your house – a decent extractor fan can send that outside.

Some steps are about maintenance – for example, it’s always worth checking for leaks in pipes, roofs or guttering which might be letting moisture into your house.  Draught stopping is another easy win, as is improving drainage around, and under, the house.

Some steps are more expensive but bring improvements in warmth and health as well – for example, higher than Code ceiling and floor insulation, good well fitted curtains, and heating devices that properly heat both living areas and bedrooms.  Heating is important because the warmer your interior surfaces are, the less likely that you will get condensation. AND it’s important because cold temperatures are linked to poor health.  If you’re going to invest in a whole house system, here’s a good place to invest. So we see a lot of smaller steps most homeowners of existing homes can take BEFORE they think about spending upwards of $5000 on a positive pressure ventilation system.