I’ve been meaning to post something about the absurdity of the almost religious devotion some people have to “natural” ventilation in a building. How is a double-hung window “natural”, by the way? That’s why I prefer the term manual ventilation.
Fortunately, the Passive House Academy of New Zealand beat me to it with a great Google+ post.
Would you throw your laundry into a river and hope to pick the clothes up cleaned somewhere downstream? Are you doing all your laundry by hand? Do you get fresh air in your car only by opening the windows? Probably not.
Yet, there is this absurd insistence on natural ventilation when it comes to houses – despite research demonstrating poorer outcomes for indoor air quality, comfort and energy efficiency. In many aspects of our daily routines, we accept that a machine can do a better job. What’s different with ventilation of houses? In a mechanically ventilated home, the windows can be opened and closed at will. Freedom of choice! In a manually ventilated home however, the windows cannot be closed without risking the build-up of air contaminants and moisture indoors. The effectiveness of this form of ventilation furthermore depends mainly on the direction and force of wind. In other words: it is relying on the weather. With mechanical ventilation, there is no reliance on power – as in a power outage, occupants can still fall back to manual ventilation. For most of the heating season however, they enjoy great indoor air quality, while saving massively on heating costs, and not having to worry about opening and closing of windows.
Note: ventilation in a Passive House is – deviating from the experience that most of us have with cars or offices – silent, non-draughty, the air is typically not conditioned at all, and sourced filtered directly from the outdoors. Fresh as!
In the past ten years, the efficiency of mechanical ventilation with heat recovery has improved so much that manual ventilation is rarely the most appropriate strategy.
UPDATE: An analysis out of Ireland showed a savings of 643 euros per year for a Passive House with heat recovery ventilation versus natural ventilation.