A Passive Houses Breathes

A Passive House experiences 7 complete air exchanges in every 24 hour period! That’s a lot of clean fresh air

When I speak with builders about the Passive House Standard they stop me in my tracks as soon as I mentioned that these buildings need to be virtually air-tight.  “But doesn’t a building NEED to breathe?  Don’t you know about the dangers of building too tight!?!”  

You can open the windows in your Passive House whenever you want, just as in a regular house.  But let’s consider the hot humid summers and cold winters when the windows are typically closed….how does a typical house “breathe” during these months as compared to an airtight construction?

People need fresh air inside their buildings, but let’s think about how we bring that  air indoors.  When the wind blows around a typical house negative pressure  causes the outside air to infiltrate through “cracks” in the building envelope, typically around window and door frames or up from the crawl space.  There is a scientific approach to measuring just how much air infiltrates through these cracks – it is called a blower door test.  Houses built in the 1950’s and 1960’s might measure 10.0 air changes per hour (ACH) on such a test and are considered very leaky (it is very difficult for the house to hold not the heat you generate).  New houses built to current energy conservation codes aim for air tightness levels around 3.0 ACH.  Why then are builders alarmed when I say that a Passive House aims for 0.6 ACH?  Because at 3.0 ACH there are still enough cracks in the construction for outside air to infiltrate when the wind blows. However, by relying on outside air infiltration you also invite air pollutants, allergens, humidity, and of course the temperature extremes.  

To address concerns about indoor air quality and thermal comfort, many new houses include Energy Recovery Ventilators (ERV) as part of the mechanical design.  The system provides continuous fresh air ventilation without  sending out significant heat or coolant with the exhaust air.  The fresh air intake can include MERV 13 or HEPA filtration and humidity control.  

With a Passive House you depend on mechanical ventilation, but have the freedom to eliminate the traditionally large furnace and air conditioner.  Building to the Passive House standard is an amazing opportunity to reduce the building’s carbon footprint while dramatically improving indoor air quality.