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It is pretty obvious if you can’t run your range hood and keep a fire going in the fireplace, that the house is too tight.  The reaction is to open a window.  It is not always obvious that carbon monoxide (CO) is flowing down the water heater chimney into the house when the bath fan or range hood is on.  Ventilation devices are systems.  They mechanically move air into or out of the house, pressurizing or depressurizing the space, and relying on building leakage to make-up the difference.  Some systems, like Heat or Energy Recovery ventilators (HRVs or ERVs) are balanced.  They don’t provide extra for other appliances like water heaters or clothes dryers.  Those systems need their own make-up air.

The tightness of the house can be measured using instrumentation such as a “blower door”.   Many green building and weatherization programs have criteria for how tight a home should be before mechanical ventilation must be added.  This may be a reference to a BTL or to a Minimum Ventilation Rate (MVR).  Indiana, for example, sets their MVR at 1200CFM@50 Pa.  (This is the “CFM₅₀” level.)  That means that a blower door must be used to exhaust air from the house until the house reaches a negative pressure relative to the outside (with respect to or WRT) of 50 Pascals.  If the amount of air flowing through the test fan to accomplish this exceeds 1200 cfm, then the house is leaky enough not to need additional mechanical ventilation (in their program).  Luckily homes are not being continuously exhausted to this artificially high level, which is used to exaggerate the conditions and to reach a standardized, comparative level.  The CFM₅₀ level can be converted to a more realistic CFM_Natural or CFM_Nat level by dividing it by a factor that takes into account the location or wind conditions and the height of the building known as the “N” or “n” factor.  Lawrence Berkeley Laboratory came up with variations on the “n” factor which used to be just averaged at 20.  They vary from a low of 9.8 for an exposed three-story house in a cold climate like North Dakota to 29.4 for a well shielded, one story house in a warm climate like Miami.  Dividing the CFM₅₀ number by the “n” factor provides CFM_Nat or approximately how the air will move through the home under average natural conditions.

For example a two story 1200 CFM₅₀ house located in Massachusetts that would be considered “normal” in terms of wind exposure would use an “n” factor of 14.8.  Its CFM_Nat would be 1200/14.8 or 81 meaning that under average conditions this house would be leaking 81 cubic feet of air per minute.  If the house were tightened up to 900 CFM₅₀, its CFM_Nat would be reduced to 900/14.8 or 60 cubic feet per minute.  This is under “average” conditions.  Much of the time it would be leaking less than this.

Because it takes such a small amount of pressure to potentially backdraft a combustion appliance, caution should be taken in aggressively tightening up the house and employing mechanical ventilation.  It is highly recommended that combustion safety testing be done before, during and after the air sealing/tightening process.  The calculated CFM_Nat number is an approximation under “average” conditions meaning that half the time the natural ventilation will be greater than this and half the time it will be less than this.  Using a continuous, primary mechanical ventilation system assures that some ventilation will be happening all the time and is an advisable approach unless the house is obviously too leaky to warrant it.