Posted by: SherlockHomesSY | 15 Oct 2010

Solutions Basics: Ventilation


Functions of ventilation systems:
– Bring in outside air
– Distribute outside air
– Exhaust strong pollutant point sources
– Filter the air
– Do it without noise, vibration, drafts, and odors

We encourage people to seal up their house tighter than a submarine and provide mechanical ventilation.  What does this mean exactly?  I’ll start with what it isn’t and then talk about what it is.

Infiltration & Exfiltration

A house drawing in outdoor air and letting it out in uncontrolled places is not ventilation.  To be sure, some amount of fresh air is coming in and stale air is leaving, but it’s doing it in an unpredictable way with equally unpredictable results.

Most of what we like to call fresh air is actually polluted with pollen, humidity, dust, vehicle exhaust, and whatever else happens to floating around in the air.  We’re still going to draw this air in, but we’re going to do it from controlled places and clean it before letting it get in the living space.

When a house lets out air, the great thing about it is that it lets out water vapor, volatile organic compounds (VOCs), soil gases, body odors and gases, allergens, and all kinds of similar stuff.  All of these things pollute our air and need to be let out early and often.  Unfortunately, with exfiltration, conditioned air we paid for is let out too.

Mechanical Ventilation Requires Pressure Differences

Controlled air intake and exhaust requires fans.  If the fans are going to do anything useful, they must be able to suck air out or push air in.  If the house has a lot of holes through the shell, the fans aren’t going to do much of anything useful because they can’t pressurize or depressurize the house.

A key step before providing ventilation is air sealing and duct sealing, so air can be forcefully moved through the house effectively.

Mechanical Ventilation Draws Air from Known Places

The problem with getting fresh air through holes in the house is that you don’t know where it’s coming from.  Air pulled in from the garage or crawlspace has all kinds of unhealthy stuff in it.

When mechanical ventilation systems are installed, the air intake is frequently through the wall—away from vehicle idling zones, crawlspaces, garages, chimneys and plumbing vents.

Types of Ventilation

There are two type of ventilation: spot and continuous.  Spot ventilation is designed to remove air from places like kitchens and bathrooms.  Continuous ventilation is supplied in one of three ways: exhaust only, supply only, and balanced.

Exhaust-only ventilation works like spot ventilation. These systems draw air through the accidental holes in the house to send stale air outdoors.

Supply-only ventilation works exactly opposite of exhaust-only ventilation.  They draw air in from known locations to push stale air through the accidental holes in the shell.  In some places, this is okay.  In colder climates, warm, moist air is forced into the walls, floors, and ceiling, where water vapor may condense and freeze.   The result can be pre-mature deterioration.

Balanced ventilation systems try to avoid letting conditioned air out or in.  Heat recovery ventilators (HRVs) use the heat in the exhausted air to heat the incoming cold air.  Enthalpy recovery ventilators (ERVs) try to leave humid air outdoors while letting fresh air in.  These systems have their own issues that must be addressed when installed.

Ventilation Characteristics

Ventilation needs to be done without noise, vibration, drafts, and odors.  The simple reason is this: If not, you’ll turn it off.  Then, you’ll be worse off than when you started.  To do this, you’ll need a better-to-best system.  This is where ENERGY STAR labeled systems can help.


Diagnosing and correcting ventilation issues can be found on my home performance website.

The New York State Energy Research and Development Authority published the pamphlet, Homeowners Guide to Ventilation.

The ventilation recommendations to the ENERGY STAR homes program is Recommended Ventilation Strategies for Energy Efficient Production Homes.

Bottom Line

Spot ventilation is always recommended.  Whether to add mechanical ventilation and what type depends on where the house is located and how air tight it is after all other work is done.

As always, if you have any questions or comments, share them with the rest of us by leaving your comments.  Otherwise, email me at

Next time: Heating and Cooling Equipment Sizing!

I hope you haven’t forgotten our warning . . .

WARNING! Do not implement any of these solutions without considering the impact on IAQ, moisture control, & heat transfer. A combination of solutions are usually needed to improve health, building durability, comfort, and energy efficiency. Ignoring this warning may lead to disease, deterioration, high energy bills or worse!



  1. […] The results for my house, of course, were different. My house is less than five years old. It turns out that our house is rated well into the extremely tight range! That sounds great – but that rating comes with its own issues. Mechanical ventilation is vital for an extremely tight house. We don’t have that. Yet. I’m still wrapping my head around why we really need mechanical ventilation. […]

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