If your windows are frosting on the inside, your humidifier is set too high

Almost every whole-home humidifier in this city is set to one number and left there from October to April. That is how they are usually handed over, and it is the reason so many Minneapolis houses have sills that have quietly rotted under the paint.

The correct indoor humidity is not one number. It moves with the temperature outside, and once you know that, the rest of this is easy.

Why our winter air is so dry in the first place

This is a physics problem, not a plumbing one. Cold air can hold very little water vapour. Outdoor air at 10 below, even at a respectable 70 percent relative humidity, contains almost no moisture in absolute terms.

That air is continuously leaking into your house, through every gap and every time a door opens. Once inside, it warms to 70 degrees. Warm air can hold far more moisture, and no moisture has been added, so its relative humidity collapses. The figure lands somewhere around 4 or 5 percent. That is drier than most deserts, and your house is doing it all winter, every winter.

Hence the static shocks, the split lips, the nosebleeds, the piano going out of tune, the gaps opening between the floorboards, and the houseplants that give up in February.

The setting, against the temperature outside

Here is the guidance we leave with every customer. It assumes a typical Twin Cities house with reasonable double glazing.

• Outside 20 degrees and above: aim for 40 percent inside
• Outside 10 to 20 degrees: 35 percent
• Outside 0 to 10 degrees: 30 percent
• Outside 10 below to 0: 25 percent
• Outside below 10 below: 20 percent or lower

If your windows are older, single glazed, or aluminium framed, shift the whole table down by five points. Aluminium frames in particular are effectively a cold bridge straight through the wall, and they will condense long before the glass does.

Your windows are a better instrument than any chart

Condensation and frost form when moist indoor air touches a surface below its dew point. In winter the coldest surface in any room is the glass. So interior window condensation is not a window problem, it is a message about the humidity level relative to how cold it is outside.

Turn the humidistat down five points and wait a day. Repeat until the frost stops forming. That is the correct setting for your house, right now, and it beats any table on any website including this one.

A little condensation at the very bottom edge of a north-facing window on the coldest night of the year is normal and not worth worrying about. Sheets of ice across the pane are not. That water runs down onto the sill, soaks into the wood, lifts the paint, and over three or four winters produces rot and mould behind the trim where you cannot see it until it is expensive.

The other direction: too little

We do not want to overcorrect. Running a house at 15 percent all winter has real costs beyond comfort. Wood floors and solid wood furniture shrink and check. Doors stop fitting their frames. Instruments will not hold tune. And there is decent evidence that very dry indoor air is harder on respiratory systems.

The table above is a balance between those costs and the condensation risk, which is exactly why it moves with the weather rather than sitting at one number.

If it cannot hold the setting

A humidifier that cannot reach its setpoint during a cold week is usually one of three things: an evaporator pad that has scaled up and stopped evaporating, a solenoid valve that is not opening, or a unit that is simply too small for the house.

The pad is an annual replacement and it genuinely is annual. A scaled pad looks fine and does almost nothing. If the pad is new and the water is flowing and it still cannot hold 35 percent in January, the unit is undersized, and the honest answer is usually to move up a type rather than to fit a bigger version of the same thing. There is more on the three types on thehumidification page.

One more thing about tight houses

If your house has been recently air sealed and insulated, which a lot of houses in this city have, you may find you need less humidification than you used to, because less bone-dry outdoor air is getting in. You may also find you need mechanical ventilation for the first time, because the house has stopped ventilating itself through all those leaks.

Those two changes arrive together and people often only notice one of them. If your windows started fogging the winter after an insulation project, that is what happened, andthe ventilation page covers the other half.