Is there such a thing as too much ventilation?

Yes, there can be too much ventilation in a bathroom, but it's rarely a practical concern for most homeowners. The main issues arise when ventilation is dramatically oversized or improperly controlled.

Excessive ventilation can create problems like pulling conditioned air out of your home too quickly, leading to higher heating and cooling costs. In winter, over-ventilation can draw cold air into the house through gaps and cracks, making your heating system work harder. Very powerful fans can also create negative pressure that interferes with other exhaust systems like your furnace or water heater venting.

In Ottawa's climate, the bigger concern is usually under-ventilation rather than over-ventilation. Most bathroom fans are sized at 1 CFM per square foot of floor space, so a 50 square foot bathroom needs about a 50 CFM fan. Going significantly higher - like installing a 150 CFM fan in a small powder room - would be overkill and waste energy.

The key is proper sizing and controls rather than just maximum power. A moderately sized fan (50-80 CFM for most bathrooms) with a timer or humidity sensor will remove moisture effectively without wasting energy. Quality fans with lower sone ratings (under 1.5 sones) can run longer without being disruptive, which is often better than a loud, powerful fan that people turn off quickly.

For your renovation, focus on proper installation with insulated ducting that vents directly outside, not into the attic. A well-installed, appropriately sized fan with good controls will handle moisture without the drawbacks of over-ventilation.

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