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.
Leaky house and operable windows still, but now we provide bath and range hood exhaust fans.
Tightened house, bath and range hood exhaust fans, balanced ventilation (with or without heat recovery).
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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