A Homeowner's Guide to Ventilation

The potential consequences of not thinking about your home's ventilation—or of just leaving it to chance—are either that you're over-ventilating, which is expensive and can be uncomfortable in the winter; or that you're under-ventilating, which can lead to a build-up of moisture (leading to mold and mildew), carbon dioxide (not particularly dangerous in and of itself, but a potential cause of grogginess and also a good proxy for other pollutants), volatile organic compounds such as formaldehyde, and general smells and other nastiness.

Our homes get most of their fresh-air ventilation not by design but by accident. Here's how the accidental ventilation works: Older homes (and many newer homes) have lots of holes in them — cracks and gaps around windows and doors, where the roof meets the walls, where the walls meet the foundation, and at any number of other places. Whenever there's an air pressure differential across one of those holes, air moves through the hole. If the part of the house where the hole is happens to be under lower pressure than the outdoors, air moves from outside to in. If that part of the house is under higher pressure than the outdoors, air moves from inside to out.

What causes air pressure differentials? The wind can, for one thing. A strong wind blowing against a corner of a house can cause some really interesting pressure differentials (remember studying Bernoulli's principle in grade-school science?). A poorly balanced forced-air heating or cooling system can, too: Imagine what happens when the air conditioning cranks on, blowing a bunch of air into a room. If the door is closed tight, and the return air grill is in the hallway and not in the room itself, that room will be pressurized, and air will leak from the room to the outside through any crack it can find.

One of the biggest causes of air pressure differentials, though, is temperature differences between indoors and out. The bigger the temperature difference, the bigger the air pressure differential. When do we have the biggest temperature differences between indoors and out? Winter, of course. Basically, our homes are leakiest when those air leaks cost us the most money: air that we have paid dearly to heat escapes through all sorts of nooks and crannies, and is replaced by cold air that we then have to pay to heat all over again. That's why utility-sponsored weatherization measures generally start with some air-sealing work.

A lot of people think leaky houses are going to be healthier than tight houses ("a house has to breathe!"). It's certainly true that some leaky houses have better air quality than some tight houses, but it's also true that some grilled cheese sandwiches look like the Virgin Mary. Best practice is to build a super-tight house, and then put in a simple, efficient ventilation system to assure a steady mix of fresh air all year round.

Here's a run-down on common ventilation strategies:

Leaky house, operable windows, no bath or range hood exhaust fans.
We see this in older homes and in rental apartments. In this case, we're relying on leaks in the house to provide fresh air and get rid of moist air. If it gets intolerably stinky or stale or damp inside, occupants are motivated to open windows, which can provide a lot of ventilation in a hurry. This sort of home is usually an older home with old, wood boards rather than plywood for structure, and plaster and lathe rather than sheetrock for wall finish, so the structure can deal with a fair amount of moisture before it starts to rot. If you don't care about potential mold or mildew, or stale air, or CO2 levels, or wasting energy in the winter, or what the building code or board of health requires, then this is a fine way to go.

Leaky house and operable windows still, but now we provide bath and range hood exhaust fans.
What the exhaust fans in the bathrooms and kitchens do (and only if you actually use them) is to pull air out of the house—and they pull it from the places where moisture and smells are most likely to build up (outside of the basement, that is). So this is good. What's not so good about this approach is that you're not sure where the fresh air is coming from, and you're not sure how much fresh air you're getting over the course of a day or a month or a season.

• Question #1: Where's the fresh air coming from? With exhaust-only systems (which is what you have if you just use bath and kitchen fans for your ventilation), the "fresh" air could be coming from almost anywhere (including, alarmingly, an attached garage). In an old bathroom, for instance, we've observed cases where the exterior wall in the bathroom was so old and leaky that almost all the make-up air was coming from that wall itself, and next to nothing was coming from adjacent rooms. Not a problem if all you want to ventilate is the bathroom; a big problem if you're trying to use the bath fan for general ventilation, including fresh air to an adjacent bedroom, for instance.
• Question #2: How much ventilation are you actually getting? Say you have a 2000 square foot, 3-bedroom house and run a bath fan with a capacity to exhaust 70 cfm (cubic feet a minute) for 30 minutes a day, and you run a range hood exhaust with a capacity to exhaust 400 cfm for 2 hours a week. That works out to about 63,000 cfm of ventilation a week. The code (broadly speaking) says that this house needs about 50,000 cfm a week. So you're over-ventilating—but not by that much, by the standards of these things. But let's say that the ductwork for these vent fans is badly installed (lots of elbows, lots of kinks) and the fans are actually only exhausting half their rated capacity. Let's say also that the bath fan is operated with a light switch, and the light is on for 15 minutes each shower rather than 30 minutes. All of a sudden you're down to about half the recommended ventilation, which could be problematic.

Tightened house, bath and range hood exhaust fans, balanced ventilation (with or without heat recovery).

• Pretty much all the drawbacks of the above two ventilation strategies are addressed by installing balanced ventilation system. In this set up, fresh air is brought in through a strategically located exterior inlet and distributed to living spaces such as bedrooms and the living room, and an equal amount (the "balanced" part of this strategy) of stale air is exhausted from the kitchen and bathrooms.
• The deluxe approach is to use a heat exchanger in this balanced system that in the winter transfers much of the heat from the warm outgoing air to the cold incoming air. This makes for a very efficient, comfortable system.

We at Byggmeister have learned a lot about ventilation over the years. We're monitoring several of our projects by means of data loggers that track temperature, relative humidity, and CO2 levels. The information we generate through those measures help us continue to learn and fine-tune our systems and strategies. If have any questions, thoughts, or concerns about your own home's ventilation, feel free to get in touch with us, and we'll do our best to steer you in the right direction.

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