When is a House Tight Enough to Need Mechanical Ventilation?
Building Scientist John Straube explains the basics of systems that bring fresh air into a house, including when you need one, how much air it should move, and what types of ventilation systems can get the job done.
Q:
I have gradually been plugging air leaks and improving the insulation in my leaky 1925 house, and I’ll be the first to admit that it’s still far from being tight. With lots of remodeling contractors and homeowners like me tightening up existing houses, when do we have to worry about crossing that threshold where the house is tight enough to require planned ventilation? At that point, is a simple solution like a trickle vent a good retrofit option?
Justin Fink, Plainville, CT
A:
John Straube, Ph.D., P.Eng., a principal of Building Science Corp., replies: If you tighten up a very leaky house, you may reduce the amount of airflow under many natural conditions to such a low level that not enough outdoor air is supplied to dilute the level of indoor pollutants. Unfortunately, there are no field studies to address the question of when a house is tight enough to require supplemental ventilation. No one has a precise answer, although some people say they do. (In other words, they make guesses.)
For small houses with a wet basement or a relatively high number of occupants, a blower-door-test value of 2.5 or 3 air changes per hour (ACH) at a pressure difference of 50 pascals (Pa) might be too tight to leave unventilated. For large houses with few occupants, even 1.5 ACH@50 Pa may work fine without any mechanical ventilation. My reasonably conservative recommendation is that if the blower-door-test value falls much below 5, add mechanical ventilation. Of course, you also could watch for signs of condensation on windows during cold weather to decide if you need additional ventilation.
If additional ventilation is needed, a further question arises: How much? A good start is to follow the ASHRAE 62.2 standard, which roughly says to ventilate continuously at 7.5 cfm per person (if you don’t know, assume the number of persons is the number of bedrooms plus one) plus 10 cfm per 1000 sq. ft.
Now for the matter of how to provide ventilation if you determine that a house requires it. Trickle vents don’t work reliably. You would still need an exhaust fan operating all the time to drag fresh air into the trickle vents, and such a fan also would drag air through cracks and openings that remain in the building enclosure, potentially compromising the indoor-air quality.
Heat-recovery ventilators (HRVs) are a balanced ventilation system: They provide a reliable flow of air into the house and then exhaust the same amount, and they use a heat exchanger to minimize the energy cost of ventilation. They are not cheap, however—or rather, their dedicated duct system is not cheap. It’s also not easy to retrofit.
A lower-cost solution for houses that have existing ductwork is to use the Aprilaire 8126 Ventilation Control System (www.aprilaire.com). This device is used to draw in fresh outdoor air and to mix it with air going through the furnace or the air handler.
Editor’s note: Another affordable option, and one that doesn’t require ductwork to install it, is the Lunos E2 system of paired, through-wall HRVs. Watch this Job-Site Diaries video to learn more about this simple ventilation system: Simple Mechanical Ventilation System for a Small Cabin
