Building Science: Do houses need to breathe?

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When it comes to insulating homes, sealing them and providing proper ventilation, a number of myths and “old wives’ tales” persist. There are those who claim that houses need to allow in a certain amount of fresh air (even with the windows closed) in order to function properly. This low level of air leakage allows the home, they say, to be properly heated and cooled and to provide healthy indoor-air quality.

These claims are often made within the context of how best to insulate a home, make it tighter and make it more energy efficient. But, according to Gord Cooke, P.E., an associate with Building Knowledge Inc., a building science consultancy, there is only a small amount of truth to the statement that houses need to breath.

“It is one of those things that is kind of a half truth,” says Cooke. “We have always wanted houses to be ventilated, but we used windows as our ventilation system. We somehow lost that knowledge and thought that somehow we were getting our fresh air through the holes in attics and electrical outlets. And that is really the misnomer.”

According to Cooke, we have not always insulated walls. And when we, as a building community, started insulating walls, we began altering the temperature of those walls. We learned that in cold weather climates when insulated walls are not sealed and let warm air into the wall cavity, the warm air in walls condenses and forms moisture that must be allowed to escape.

“What we want and need is our attic and wall assemblies to be tight, and not let any warm moist air into the wall cavity,” says Cooke. “And if any moisture does get into the wall, we want the wall to be vapor permeable. We want water vapor to escape through the wall assembly to the outside.”

Cooke likens the desired effect for houses to that of what is required of good exercise clothing.

“You want clothes that are airtight, that won’t let the cold air in, but will let our body vapor out. So that is what we want in our walls: air tight, but vapor permeable,” says Cooke.

Bathroom Fans and Heat Exchangers

A remodeler needs to provide mechanical ventilation as part of any major renovation, says Cooke.

Setting aside energy efficiency goals and just examining what people require for fresh air, there is a need for about 15 cubic feet of air per minute (cfm) per person. Therefore, the calculation that a remodeler must make is to multiply 15 cfm by the number of people that live in a home. A three-bedroom house translates to four people or a need of 60 cfm in a ventilation system.

And there are two options for providing that ventilation. The first is to install a bathroom fan that is quiet and runs throughout the day at 60 cfm. Another is to install a heat exchanger, which can control the amount of air that goes out while bringing in the same amount of fresh air and minimizing the amount of heat lost.

“A bathroom fan should have a 60 to 80 cubic feet per minute capacity and it should run more or less continuously, whenever people are in the building,” says Cooke. “To my mind it is a remodeler’s job to ensure that any house they work in is left with a capacity for continuous mechanical ventilation. So in every remodeling job, you look and see if they already have bathroom fans that can run continuously. If they don’t and they are doing an addition that includes a bathroom, then they need to add one.”

In rare situations, where houses have been made very tight with insulation and air sealing, the step of adding bathroom fans may create negative pressure in a home. And if that house also has a chimney, air could be pulled down a chimney and create a dangerous situation. But this is very rare says Cooke, and there are simple depressurization tests that can be conducted to ensure that this unsafe situation is avoided.

“Some people might say continuous ventilation goes against the axiom of making the house tight,” says Cooke. “But you will almost never make a house tight enough that it is going to not be able to bring some air in some way.”