Wednesday, April 4, 2012

Making Our Home a Home Sweet Home: Part 5- Air Apparent

Last time I mentioned a bit about the progress of building shelters, first keeping larger critters out, then water—both ways of tightening up a home.  Air is next.  There are three reasons for this: moisture, temperature, and pollution.  Air brings all these things, most often in rather different quantities than indoors, which adds a burden to the house, likely to its occupants, and also to its heating and cooling equipment.  Conventional building wisdom has said, “a house needs to breathe.”  This is technically true.  But, then, ask: “How and when do you get fresh air?”  Like the body, a clear opening (mouth or nose or vent) and a controlled pressure regulator (diaphragm or fan) lets you know.  The air that comes into the body is tempered and filtered by the nasal passages so its contents are not so shocking to its more delicate and vulnerable interior: the lungs.  In a home, mechanical ventilation can do this work.  Like not breathing deprives lungs of fresh air, without this provision, air moves (or doesn’t!) unfiltered at the whims of wind and convection currents.  Convection currents are caused by warmer air rising or cold air sinking.  When air rises inside a home, it will find its way out the top, wherever it can, through the attic access, the light fixtures, or any other unsealed opening in the ceiling.  By the general nature of air pressure, when air leaves the top of my house, it will be replaced by other air. Where does that air come from?  Bad windows and poorly sealed doors, exterior outlets, and leaky floors allow outside and/or crawl space air to enter the house.  This is unwanted, unhealthy, unpredictable, and uncontrollable airflow—better put a lid on it.

To jump ahead a little bit, we decided to feed two birds with one scone and seal and insulate in one step with spray foam on the roof deck, and in our uninsulated back room (more on that, later).  The Building Performance Institute recommends adding mechanical ventilation when tightening more than 15 percent of the exterior of a building, so with our spray foam decision, mechanical ventilation would provide a choice to maintain a healthy interior.  The options were a bathroom exhaust fan on a timer (which requires repeated human input) paired with a passive and filtered vent opening through a wall to both exhaust and bring in an appropriate amount of air, or an Energy or Heat Recovery Ventilator (ERV, HRV), which can be set to run automatically.  More on that decision soon!

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