Condensation on the inside of your windows is usually not the windows.

Why do my windows sweat in winter?

Because the glass is the coldest surface in the room, and the air in your house is carrying more moisture than that surface can hold. The water comes out of the air onto the glass. It is the same thing a cold drink does in summer, running in the other direction.

The University of Minnesota lists condensation on windows in winter first among the symptoms of excess moisture in a home — alongside mildew in the bathroom, mould in closet corners, and peeling paint. It is grouped with those because it has the same cause.

Ordinary living produces the moisture: bathing, cooking, dishwashing and breathing. Add plants, a humidifier, firewood stored indoors, an unvented appliance or damp soil under a basement and it climbs further.

University of Minnesota Extension: “Symptoms of excess moisture in your home include: Condensation on windows during the winter. Excessive mildew in the bathroom. Mold growing on interior surfaces […] Indoor activities such as bathing, cooking, dishwashing, and just breathing all produce moisture.” Source: U of M Extension, Controlling moisture problems in your home

What humidity level should I actually be at?

The University of Minnesota puts a healthy and comfortable range at about 25% relative humidity in winter up to 50% in summer, and notes that above 50% moisture problems may occur.

The winter figure surprises people, because 25% feels dry. That is the trade: in a Minnesota January, humidity high enough to feel pleasant is high enough to condense on glass and, worse, inside wall cavities where you cannot see it. The colder it gets outside, the lower the indoor humidity has to go.

A cheap hygrometer settles the argument in a day. If it reads in the forties in February, the windows are reporting accurately.

University of Minnesota Extension: “When the relative humidity is more than 50%, moisture problems may occur. Generally, in cold winter conditions, the relative humidity levels need to be low to prevent condensation and potential damage to the house’s structure […] Keeping the relative humidity at 25% (winter) to 50% (summer) is generally considered healthy and comfortable for people, and better for the house.” Source: U of M Extension, Controlling moisture problems in your home

Will new windows fix it?

They will usually reduce it, because a warmer interior glass surface condenses less. But they do not remove the moisture from the house, and that is worth understanding before you spend the money.

There is a version of this that gets worse after a window replacement. Tighter windows reduce the air leakage that was quietly drying the house out. The moisture then finds the next coldest surface — an exterior wall corner, the back of a closet, the rim joist — where it grows mould instead of running down glass where you can see it.

So: if the windows are failed, draughty or rotten, replace them, and they will condense less afterwards. If the only complaint is condensation, deal with the humidity first. You may not need the windows at all, and we would rather tell you that than sell you a set that disappoints you.

What actually reduces it?

Source control first. Run the bathroom fan during and after a shower and the range hood when you cook, and make sure both vent outside rather than into the attic. Turn a whole-house humidifier down, or off, in the coldest weeks. Move firewood out. Cover bare soil in a crawl space.

Then ventilation. The University of Minnesota notes that tightly constructed houses require some form of controlled ventilation system — the house cannot dry itself out through leaks any more, so the air has to be changed deliberately.

Air movement helps at the glass itself. Open blinds and curtains during the day; heavy window treatments trap a pocket of cold, still air against the pane, which is why condensation is often worst behind them.

University of Minnesota Extension on reducing moisture: identify and remove the source first, and if that is not possible, use ventilation or a dehumidifier. Source: U of M Extension, Controlling moisture problems in your home

When is it actually the window's fault?

When the moisture is between the panes. Fog or haze sealed inside a double-glazed unit that you cannot wipe off means the seal has failed and the insulating gas is gone. No amount of humidity control touches that, and the unit or the window has to be replaced.

Also genuine: frost or ice on the frame rather than the glass, water pooling in the sill track and not draining, visible rot in the sash, or a draught you can feel with the window closed and locked. Those are failures.

The test is simple. Can you wipe it off? Then it is room-side condensation and it is a humidity question. Can you not? Then it is a sealed-unit failure and it is a window question.