Apart from indoor air pollution (i.e. carbon monoxide from stoves, fumes from glue and paint), the big reason to make sure your home is adequately ventilated is to prevent excess moisture from accumulating. Breathing, cooking and showering create moisture, and this moisture needs a way to escape the house easily. Why such a big deal? Excess humidity opens the door to mold (which can lead to health problems) and can even compromise the cosmetic and structural integrity of your home. We've identified the problems of inadequate ventilation, and the unsightly pore known as "natural ventilation." The question becomes how to provide adequate ventilation properly, to make sure those .35 air changes per hour keep rolling through your house replacing contaminated, stale air.
The alternative to natural ventilation is a mechanical ventilation system, which is any ventilation system that relies on more than leaks and windows. Unlike natural ventilation, mechanical ventilation is reliable and measurable. There are two basic sub-categories of mechanical ventilation systems:
1. Exhaust-only ventilation:
This can refer either to a "spot" or a "multi-port" ventilation system. Spot ventilation removes air from a house in one location: a bathroom fan is a good example. It removes air and promotes circulation in a single area.
The shortcomings of spot ventilation are, first, it may not guarantee sufficient ventilation in a larger house, and second, it has no air intake. This means that all the new air coming into the house enters through existing cracks, leaks, chimneys, vents, or what have you, as a result of the negative pressure created inside the house by the fan's exhaust - basically, the air being discharged needs to be replaced, so you get air coming from wherever it can to replace it - what we call negative pressure ventilation. The problem here is that these existing leaks, cracks, or pipes may contain contaminants that, left unfiltered, will enter the house, and you'll wind up with a house full of dirty, polluted air. Still, spot ventilation is much better than nothing, and it's a good idea to have at least a bathroom fan and a range hood liner in your kitchen.
To minimize the downside of spot ventilation, you may want to install a multi-port ventilation system, which works the same way but has, (who knew?), multiple ports. The upside is that this covers more ground than basic spot-ventilation; the downside is that you're left with the same negative pressure equation (no air intake = negative pressure in the house = air entering wherever it can) that can bring unwanted moisture and contaminants into your house.
2. Balanced ventilation:
This is really the ideal solution for maximum ventilation and high indoor air quality. A balanced ventilation system has separate intake and exhaust ports, so the air being discharged is immediately replaced with clean, filtered air from the great outdoors, and you don't have to worry about chemicals and allergens. Even better is a heat-recovery ventilation (HRV) system, which is a balanced ventilation system using a heat exchanger, so that heat from outgoing air is transferred with up to 85% efficiency to the incoming air (or vice versa, during the cooling season); the cumulative effect being a well-ventilated house filled with clean, filtered air, topped off with a lower heating bill.
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