Indoor humidity that's too high feeds mould and condensation; too low causes dry skin, static, and cracking woodwork. Keeping it in a comfortable middle range protects both your home and your comfort.
Too much humidity
Condensation on windows, musty smells, and mould signal excess humidity, common in basements and tight homes. Run bathroom and kitchen exhaust fans, use a dehumidifier in damp spaces, and ventilate. An HRV helps in very tight homes.
Too little humidity
In winter, heating dries indoor air, causing dry skin, static shocks, and gaps opening in hardwood and trim. A humidifier — portable or furnace-mounted — adds moisture back. Don't overdo it, or you'll create condensation on cold windows.
Find the balance
A cheap hygrometer lets you measure and target a comfortable indoor humidity. In winter you may need to lower humidity enough to avoid window condensation; in damp seasons, raise ventilation to keep it from climbing.