Definition — the 30-50% RH target
Health Canada and most building-science practitioners recommend residential indoor relative humidity between 30% and 50% year-round. Below 30% RH, dry-sinus and static symptoms increase; above 50% RH, condensation on cold surfaces and mould-friendly conditions increase.
In a GTA winter (–5 °C outdoors at 70% outdoor RH), incoming cold air holds very little moisture and the air dries out fast once it warms up inside — pushing homes to 15-25% RH without active humidification. In summer the reverse: high outdoor humidity infiltrates a home and can push indoor RH above 60% unless the AC or a dehumidifier removes it.
When to add humidification, dehumidification, or ventilation
Winter symptoms — dry skin, static shocks, cracking floors, sinus irritation, shrinking wood trim: add a whole-home humidifier plumbed into the furnace (bypass, flow-through, or steam).
Summer symptoms — musty basement, condensation on cold-water pipes, feel-clammy AC operation: check that your AC isn't over-sized (over-sized ACs cool without dehumidifying), then consider a stand-alone dehumidifier for the basement.
Tight new-build with stuffy air, no humidity extremes but stale feel: add whole-home mechanical ventilation — an HRV (Heat Recovery Ventilator) or ERV (Energy Recovery Ventilator). Modern Ontario building code effectively requires one on tight new homes.
- Winter dry → whole-home humidifier
- Summer damp → right-sized AC + dehumidifier
- Stuffy tight home → HRV or ERV
Failure modes — the common humidity mistakes
Winter over-humidification: running a humidifier at 45% RH when outdoor is –15 °C drives condensation onto every cold window and (worse) into wall cavities where it can rot sheathing. Rule of thumb: lower your winter humidifier target as outdoor temperature drops — 45% at 0 °C, 30% at –15 °C, 25% or off below –20 °C.
Summer under-cooling: an over-sized AC cools the thermostat quickly and shuts off, never running long enough to condense moisture out of the air. The house feels cool but clammy. The fix is proper sizing, not a bigger AC.
HRVs and ERVs installed but left unbalanced: an out-of-balance unit either pressurizes or depressurizes the home, defeating the point. Balancing is done at commissioning with an anemometer, not by eye.
Proof — how to measure what's actually happening
A $20 hygrometer in the main living area tells you your actual RH. A smart thermostat with humidity display gives you both temperature and RH at a glance. Log a week of readings before spending on equipment.
The room-to-room RH variation matters too. A basement bedroom at 65% RH while the main floor sits at 40% is telling you the basement needs a stand-alone dehumidifier — not more central humidification.
Action — how Metroheat approaches humidity
Every furnace or heat-pump quote we deliver includes a humidity assessment — current indoor humidity swing, existing humidifier condition (or absence), ventilation status, and a recommended target range for your home's envelope tightness.
For homeowners retrofitting older Toronto or Scarborough homes, we typically pair a right-sized central system with a bypass or steam humidifier and (if the home tests tight) an HRV. Book a consultation at /contact.