Ventilation & air exchangers

Are houses too airtight? No they aren't. Fresh air is essential to prevent mold and mildew in homes, but in a modern energy-efficient home it is provided by heat recovery ventilation.

Condensation on windows leads to mold and mildew in homes
Condensation on windows leads to mold and mildew in homes © Creative Commons

Homes built in the last 40 years in Canada are relatively airtight. Before that we generally relied on leaky uninsulated walls to provide fresh air and prevent mold and mildew, and they did that very well.

Cost and comfort had us adding insulation, but not necessarily sealing our walls. The folly of this was quickly realized, and vapour barriers became a part of the building envelope shortly thereafter. 

Vapour barriers stopped the flow of moist air through walls, this of course led to a buildup of moisture in homes, and condensation on windows was common place. This led to a build up of mold and mildew in homes. Modern day airtight homes need mechanical assistance to stop moisture damage and protect indoor air quality.

There are still those that profess that walls need to breath and that 'houses are too airtight', but this myth is completely false and very damaging to your home. Walls need to be able to dry, ideally in both directions.

Check out our new video building guide, this episode covers ventilation systems:



Natural air flow

If you hold a door open a crack in winter, natural convection will draw air in at the bottom, and force it out at the top. Your home will behave in a similar way, this is called the stack effect.

Warm air rises, forcing air out the top portion of your home and drawing in cold air from the bottom to replace it. How much air will be changed depends on how well-sealed your home is.

While natural convection will offer a certain amount of new air, with most new homes it just isn't sufficient. Properly sealed homes require mechanical ventilation systems to remove moisture and provide sufficient fresh air for occupants.

Ventilation systems- What is an HRV?

Mechanical ventilation systems are known as heat exchangers, HVACs (Heating, Ventilation and Air Conditioning) or HRVs (Heat Recovery Ventilators). The point of these systems is to remove moisture and provide fresh air to your home that is pre-heated by the outgoing air.

Air exchanger
© Venmar

The core of an HRV has small separated channels that air passes through, allowing incoming air to be preheated by exhaust air. There are no heating coils, you are simply operating fans, so they are relatively cheap to run. And you will certainly save money overall, as heating moist air eats up a lot of energy.

Depending on the quality of the machine you buy, you can expect to recoup anywhere from 50% of the heat in the air, to as much as 95%. Plan on spending around $2,000 installed, that's  for a reasonably efficient one. Double that for the high end models with aluminium cores that conduct heat better than plastic.

Indoor air quality is important for several reasons:

  • Preventing moisture issues such as rot and mold

  • Preventing damage to windows from condensation

  • Preventing respiratory illnesses caused by interior contaminants

  • Reducing heating costs by not heating excess water vapour which will leak from your home.

Ideal humidity levels

Healthy moisture range
© Health Canada

Along with removing contaminants from the air, there are health consequences from having either too much or too little moisture in our homes. There are bacteria, viruses, molds and mites that will show up on either end of the spectrum if your air is either too humid or too dry.

Somewhere in the range of 35 to 50% relative humidity is generally assumed to be the best for avoiding most health risks and irritants. It’s high enough that you won’t have cracked furniture, chapped lips or constant nose bleeds, and it’s not too damp for comfort, condensation or heat consumption.

If you are in an older home, don’t panic. What we write in these pages is meant to inspire ideas and solutions, not fear and anxiety. If you feel good, your air smells good and your windows aren’t dripping, relax.

For peace of mind, consider buying a hydrometer to measure your indoor relative humidity, which will cost you maybe $20 to $30 at most hardware stores. If you have an issue, crack your window a bit for now until you sort it out. Humidifiers, dehumidifiers and air purifiers are available to deal with some of these issues.

A dehumidifier will cost between $200 and $300 to purchase, and maybe $10 to $15 a month to operate. This added cost will likely be negated by heat savings, as heating moist air requires a lot more energy than heating dry air.

The inside of an HRV
The inside of an HRV © Ecohome

If you plan to undertake an HRV installation project yourself, do your research first to determine proper vent placement. An intake in the bathroom rather than a simple exhaust fan for example, will mean heated incoming air instead of just creating negative pressure and letting cold air find its own way in everytime someone turns on the fan.

You won't need to install a bathroom fan of course if you have an intake in there, just be sure to put a timer switch so you and your guests can turn it on. Having an intake in or near the kitchen helps pick up general moisture and contaminants, but don’t hook it up to your stove hood. It's not a good idea to send cooking grease through an expensive heat exchanger.

And as for installing duct work, the flexible tubes are cheaper and easier to work with, but can be quite noisy and the ribs slow air movement forcing your air exchanger to work harder.

Since your fresh air vents are best located in living areas and bedrooms, you might find that it’s well worth the added cost of solid ducts simply for noise reduction.