[GSBN] Airtightness and ventilation - exhaust-only systems

[ejgeorge at riseup.net]

A response from the Natural Builders Northeast list with regard to the  
airtightness/how to ventilate thread on infiltration (or exfiltration  
as the case may be):


From: Robert Riversong <housewright at ponds-edge.net>

 From Tony Novelli: It
  also seems useful to show where infiltration occurs, and perhaps adapt
systems for the "point loads" of moisture to occur at those points?

In cold climates, where I suspect most of us work, envelope moisture  
problems occur at the points of exfiltration, not infiltration.  
Infiltration merely provides some of the air exchange that's necessary  
(and unavoidable) in any house, but does not contribute to moisture  
problems except in warm-humid climates. Under natural stack effect  
differentials, of course, infiltration is balanced by an equal rate of  
exfiltration.

What's often ignored is that a "balanced" ventilation system, such as  
an HRV, does not change the internal pressure dynamics of a building  
(stack effect pressure differentials), or - if it does - it is by  
increasing positive pressure and lowering the neutral pressure plane.  
This occurs with equal cfm intake and exhaust fans, since cold outside  
air will expand about 10% when brought up to room temperature,  
effectively introducing more air volume than is being exhausted. This  
can increase envelope exfiltration and is part of the reason that  
extraordinary levels of envelope air-tightness are necessary with such  
"balanced" mechanical systems.

An exhaust-only ventilation system does the opposite, increasing  
negative pressure and raising the neutral pressure plane (possibly to  
the upper ceiling), encouraging natural infiltration and discouraging  
envelope exfiltration, thereby reducing the probability of  
interstitial moisture accumulation. A hermetically-sealed envelope,  
then, is unnecessary and possibly counter-productive.

Depending on the operating efficiency of an HRV and the annual  
maintenance costs (including core cleaning and filter changes), an  
exhaust-only ventilation system can actually be less expensive to  
operate when heating impacts, operating energy and maintenance are all  
factored in. They are also less expensive to install, more effective  
in source removal (dedicated bath fans vs. HRV ports), potentially  
more effective in maintaining good IAQ, and make structural design  
simpler by avoiding the need for duct chases.

- Robert


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