Going Negative

Amy Musser

In honor of the election year, I want to address a contentious issue.  Some people have dug in their heels and won’t even consider questioning conventional wisdom, so there’s really only one thing left to do:  go negative.

No, I’m not talking about the upcoming election, I’m talking about whole house ventilation.  Something I hear over and over in the Southeast is that exhaust-only ventilation systems can’t be done here, that negative pressure is the worst thing you can do to a house, and you will cause all sorts of moisture problems if you try it.  It’s got all the elements of the perfect story:  it sounds right (sometimes negative pressure is bad), it elicits fear (moisture problems!), and it’s got a well-known spokesperson (a paper Joe Lstibrek wrote in the early 90s is often cited).  The only problem is that I’m not so sure that it’s true.

Let’s start with the fear part.  The story goes that an exhaust only system creates negative pressure in the house, pulling in moist outside air through the building envelope, where it will hit its dew point somewhere and condense, creating mold.  It’s true that these systems do create a small negative pressure – typically less than 2 Pascals (which is really small).  A lot of other things can create this much negative pressure in any house:  using a bath or kitchen fan, a windy day, and the temperature differences between inside and outside (called stack effect).  If your building envelope can’t handle a small amount of negative pressure, you’ve got bigger problems than your whole-house ventilation system.  This is why we don’t put plastic on the inside of walls or use vinyl wallpaper here anymore.  If the purpose of this blog post was to talk about how to prevent moisture problems in walls, my focus would be on wall section details, not whole-house ventilation systems.

Next, let’s address the facts.  Negative pressure can be bad – particularly if it gets too high.  It can cause gas appliances to backdraft and poison the occupants with carbon monoxide.  It can pull in so much humid outdoor air that the air conditioner can’t keep up with it.  In really tight houses, the front door might whistle.  But these things usually require a MUCH bigger fan – like a gigantic kitchen range hood.  In fact, as homes get tighter and range hoods get more “gourmet”, gigantic is the new normal.  Big kitchen range hoods are a real, widespread problem.  And in a small percentage of super-tight homes, a balanced ventilation system is a good idea because pressure control can get tricky.

Another fact:  a whole house ventilation system isn’t a dehumidifier.  Some people seem to think it is.  Actually, you can integrate your ventilation system with an actual dehumidifier – which I think is a fabulous option for homes in humid climates.  But the reason you have whole-house ventilation is to dilute contaminants that you don’t know are there or can’t get rid of any other way.  If your house isn’t properly dehumidified, most whole-house ventilation systems aren’t going to help.  At most, your choice of ventilation system makes a minor contribution toward adding or removing humidity.  The effect is complicated and deserves a blog post of its own.  In the case of winter humidity problems, this type of system could actually help.

Finally, what about the well-known spokesperson?  He can speak for himself (and always does), but a newer 2006 report on his company’s website may be of interest.  Check out slide 53.

What are the positives?  Exhaust-only systems are inexpensive, effective, and they’re often installed in a bathroom, where using a fan is always a good idea.  There’s no heat recovery, but the fan motors are so efficient that their overall energy performance is not bad compared to other systems.  They’re not dehumidifiers, but there’s no evidence that they create moisture problems in homes otherwise constructed for moisture durability.

Copyright 2012 Amy Musser

A non-scary ventilating bath fan hard at work.