What Humidity Should a House Be at in a Twin Ports Winter?

Why it gets so dry here

Cold air physically cannot hold much water vapour. At −15°F, outdoor air is nearly moisture-free even at 100% relative humidity. Bring that air inside, heat it to 70°F, and the relative humidity collapses — the absolute moisture content has not changed but the air's capacity has increased enormously.

That is why a Duluth or Hermantown house in January often measures 10–20% RH, which is drier than most desert climates. Every air leak in the house accelerates it, because every cubic foot of infiltrating air is bone dry.

The consequences are familiar: cracked skin and lips, nosebleeds, sore throats, constant static shocks, and more susceptibility to respiratory irritation. Expensively, hardwood floors shrink and gap, trim joints open, doors and drawers change fit, and wooden instruments and furniture crack. Dry air also feels colder at the same thermostat setting, so people run the heat higher than they need to.

Why more is not better

The instinct is to push humidity up to a comfortable 45–50%. In a −20°F climate that causes real damage.

Warm, moist indoor air migrates outward through the building envelope. When it reaches a surface below the dew point — the inside face of exterior sheathing, the underside of roof decking, the cold edge of a window frame — the moisture condenses. In an attic that means frost on the sheathing that melts in the spring thaw and stains ceilings, and over years it rots decking and grows mould. In walls it wets insulation and degrades framing, invisibly, for years.

Window condensation is the useful early warning. If your windows are streaming in cold weather, the indoor humidity is higher than the building can tolerate at that outdoor temperature. Some people also lower the target further during extreme cold snaps, because the colder it is outside, the lower the humidity a wall assembly can handle without condensing.

30–40% is the band that gives you the comfort and protects the building. A humidistat-controlled whole-home humidifier holds it. A portable unit you refill by hand cannot — it either does nothing in a house-sized volume or overshoots in one room.

The ventilation half of the problem

There is a second, opposite problem in newer or recently air-sealed houses: too much moisture and not enough fresh air. A tight house traps everything you generate indoors — shower and cooking moisture, CO2, VOCs, and radon.

That is what a heat-recovery ventilator is for. It exhausts stale indoor air and brings in fresh outdoor air through a core that recovers 60–85% of the heat, so ventilating no longer means throwing your heating dollars out the wall. In a tight Twin Ports house an HRV is often the right answer before a humidifier — you may find the humidity problem was actually a ventilation problem.

Which one your house needs depends on how tight it is, and that is measurable. We would rather test than guess, because installing a humidifier in a house that actually needed ventilation makes the moisture problem worse.