Recently I was contacted by a homeowner who had been part of Beacon’s HomeSmart Renovation project.  She had received a renovation plan to make her home warmer, healthier and more energy/water efficient, and she had been working through this, replacing windows and adding insulation while recladding.

Her question to me was: What kind of ventilation system do we need?

This is a question that always gets me a little hot under the collar!  That’s because there is increasing evidence that these systems often don’t solve dampness problems, and in most instances don’t even improve heating.

If you have a tile roof, for example (a lot of 1950s houses do) then most types of ventilation systems won’t even work – because the tiles let in so much airflow.  Similarly  if you have a skillion roof (with no attic space) then most systems won’t work either – for systems taking air from the roofspace, there isn’t a space there!  Yet I’ve still seen them installed in those circumstances.

Most older homes, in fact are classed as draughty.  That means that lack of airflow is not a problem – if there’s dampness in the house you need to look for the causes.

Positive pressure/forced air roof cavity ventilation systems (the type that has mostly been installed in New Zealand) act by pushing air from the roofspace through the house and out through the gaps.  Because the air in roof spaces is often quite dusty they rely on having very good filters to clean the air – changing the filters frequently is essential otherwise the air being blown into the house can be quite dirty.

In some houses (but not all) they might reduce the amount of condensation you see on places like windows – because the air is moving so fast the moisture doesn’t have time to settle, but in doing so they will often make the house colder – especially in winter.

Both University of Otago and Beacon research show that positive pressure or roof cavity systems bring cold air into the house in winter, and bring in warm damp air during the day and in summer.  Neither EECA nor Consumer NZ recommend these systems for heating.

Heat recovery systems are a new product to New Zealand but are widely used in the colder parts of Europe and North America.  They use a heat exchanger, bring fresh air in from the outside and are usually more effective at heating.  However, they don’t work in older houses because they require very airtight homes (more airtight than most new construction in New Zealand) combined with a severe climate (think Central Otago) before you save more energy than you spend in the running costs.

Whole house ventilation systems are an expensive investment.  Before you invest, you should be sure that you actually need one. If you have moisture and dampness problems or if your house is cold then you should sort out the cause not the symptoms.

For this homeowner I made the following suggestions:

To address dampness, first get rid of any moisture inside:

1. Make sure you have a range hood in the kitchen, an extract fans in the bathroom, and for the dryer.  Most importantly, these should all be extracted to the outside NOT the roof space.
2. Put a polythene vapour barrier down over the ground under your house if there’s room to get under there.  A tremendous amount of moisture (about 25kg) comes off even quite a dry underfloor each day – and that just travels up into the house.
3. Make sure you can leave windows open safely and air the house regularly.
4. Get rid of any unflued gas heaters – they pump out a litre of water every hour!
5. Fix any drainage problems or leaks in or under the house.
6.  Don’t dry clothes inside – all the moisture from the clothes goes into the house.

Next keep warm inside

Warm air holds more moisture and keeping the air inside your home warm will also keeps surfaces warm so moisture won’t form on them when it gets cold outside. An insulated room will have fewer problems with condensation on walls and windows. Insulating, double glazing and using passive heating or an efficient heating system to keep your home warm will also help to reduce moisture.

1. Insulate, insulate, insulate!  Insulation is also a key way of reducing dampness – and actually for older homes it’s critical that you don’t just insulate ceilings and underfloor, but the walls as well.  If you are planning to reclad or reline then installing insulation at the same type (with building wrap around the outside) is a must.  In fact I would prioritise it ahead of any other intervention, just because you so rarely get the opportunity to install wall insulation easily.   I’d stick as thick a batt as will fit – and make sure it’s a rigid product that won’t slump in the walls.  Stiff fibreglass (eg R2.6 or R2.8), thick polystyrene or stiff polyester are the best options.   And try to get the highest R value possible. Of course if you haven’t already you should take advantage of the Warm Up New Zealand subsidies (while they are still there) and get your ceiling and underfloor insulated. With all insulation products good installation is critical.  Ideally get someone who has been trained by the Insulation Association of New Zealand (IAONZ) because they will do a good job.
2. Depending on how extensive your renovations are, consider replacing single glazing with double glazing – there are a lot of options here so I might save these for a separate blog.
3. Heat your home efficiently (don’t forget the bedrooms) and use heat transfer systems to move the heat around your home.

So my advice is: Before you think investing in a ventilation system, address moisture sources and insulation levels in the home first – it’s simple, relatively low cost, and you get the added benefit of a warmer home.

If you really want to install a heat recovery ventilation system get your house tested for draughts first (often called a blower door test) and I recommend you only consider one of these systems if you live in a cold climate and your house is very airtight.