One of the features in our new house that I’m most excited about barely raises an eyebrow with some of our visitors: the ventilation system. I believe it is the highest-efficiency heat-recovery ventilator (HRV) on the market -- or at least right up there near the top.

I’ll describe this Zehnder HRV and its amazing specifications and features -- but not until next week. This week I’ll provide a little background on ventilation.

Why ventilate?

For centuries homes weren’t ventilated, and they did all right, didn’t they? Why do we need to go to all this effort (and often considerable expense) to ventilate houses today?

There are several reasons why ventilation -- usually mechanical ventilation -- is more important today than was the case long ago. Most importantly, houses 100 years ago were really leaky. Usually they didn’t have insulation in the walls, so fresh air could pretty easily enter through all the gaps, cracks, and holes in the building envelope.

Also, the building materials used 100 years ago were mostly natural products that didn’t result in significant offgassing of volatile organic compounds (VOCs), formaldehyde, flame retardants, and other chemicals that are so prevalent in today’s building materials, furnishings, and belongings.

Ventilation options

Ventilation systems can be categorized very broadly into about a half-dozen generic types.

-- No ventilation. This is almost certainly the most common option in American homes. There is no mechanical system to remove stale indoor air or bring in fresh outside air. In the distant past, when buildings weren’t insulated, this strategy worked reasonably well -- relying on the natural leakiness of the house. It’s worth noting, though, that even a leaky house doesn’t ensure good ventilation. For this strategy to work there has to be either a breeze outside or a significant difference in temperature between outdoor and indoors. Either of these conditions creates a pressure difference between indoors and out, driving that ventilation. On calm days in the spring and summer, there might be very little air exchange even in a really leaky house.

-- Natural ventilation. In this rather uncommon strategy, specific design features are included in a house to bring in fresh air and get rid of stale air. One approach is to create a "solar chimney" in which air is heated by the sun, becomes more buoyant, and rises up and out through vents near the top of the house; this lowers the pressure in the house, which draws fresh air in through specially placed inlet ports. The rest of this column will focus on mechanical ventilation.

-- Exhaust-only mechanical ventilation. This is a relatively common strategy in which small exhaust fans, usually in bathrooms, operate either continuously or intermittently to exhaust stale air and moisture generated in those rooms. This strategy creates a modest negative pressure in the house, and that pulls in fresh air either through cracks and other air-leakage sites or through strategically placed intentional make-up air inlets.

-- Supply-only ventilation. As the name implies, a fan brings in fresh air, and stale air escapes through cracks and air-leakage sites in the house. The air supply may be delivered to one location, distributed more broadly through ducts, or supplied to the ducted distribution system of a forced-air heating system, through which distribution is achieved. A supply-only ventilation system pressurizes a house, which can be a good thing in keeping radon and other contaminants from entering the house, but it risks forcing moisture-laden air into wall and ceiling cavities where condensation and moisture problems can occur.

-- Balanced ventilation. Optimal ventilation is provided through a balanced system in which separate fans drive both inlet and exhaust airflow. This allows us to control where the fresh air comes from, where that fresh air is delivered, and from where exhaust air is drawn. It makes sense, for example, to deliver fresh air to spaces that are most lived in (living room, bedrooms, etc.) and exhaust indoor air from places where moisture or pollutants are generated (bathrooms, kitchen, hobby room).

-- Balanced ventilation with heat recovery. If there are separate fans to introduce fresh air and exhaust indoor air, it makes a lot of sense to locate these fans together and include an air-to-air heat exchanger so that much of the heat in the outgoing airstream can be transferred to the incoming airstream. This air-to-air heat exchanger -- more commonly referred to today as a heat-recovery ventilator or HRV -- is the way to go in colder climates. A slightly different version, known as an energy-recovery ventilator or ERV is similar but doesn’t transfer moisture.

I’ll focus more on HRVs next week, especially our new top-efficiency Zehnder system. Following that I’ll address why "commissioning" an HRV is so important and how that’s done -- or at least how it was done with our system.

I’m a firm believer that all homes should have mechanical ventilation. With better-insulated, tighter homes that ventilation is more important. But even in a very leaky house, one can’t count on bringing in much fresh air or calm days in the spring and fall when there isn’t a pressure differential across the building envelope.

If budgets allow, going with a balanced ventilation is a very good idea, and if you’re doing that in a relatively cold climate, like ours, then providing heat recovery is a no-brainer. Mechanical ventilation always takes energy; with heat recovery the energy penalty of fresh air is minimized.

Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to alex@buildinggreen.com.