Wet Windows or Dry Air: Humidity in Winter

Get a meter and read it for a week

Stand a humidity meter in the main living space, away from the kitchen and away from any vent, and write the number down each morning along with roughly how cold it was outside.

Below thirty per cent. That is the dry end, and it is where static shocks, sore throats and shrinking wood come from.

Above the line for the weather. That is the wet end, and the windows will tell you before the meter does.

The important part is that there is no fixed target. At forty degrees outside a house can comfortably sit at forty per cent. At ten below, the same house needs to be down near twenty or the windows will run.

It is the same measurement as the summer problem, but everything about the answer is different: summer humidity and what the air conditioner is doing.

Which side of the glass, then where the water comes from

First, which side. On the inside is indoor moisture and this page. On the outside is condensation on cold glass in mild damp weather, which is normal and a sign the window is insulating well. Between the panes means the seal has failed, and that is a window job rather than a heating one.

Then find the moisture. Showers, cooking without a lid or a hood, laundry dried indoors, a lot of people in a small house, an unvented crawlspace, and a humidifier turned up too far.

The extractor fans. Run them for twenty minutes after a shower rather than two, and check they actually vent outside rather than into the loft. Hold a sheet of paper against one to see if it is moving air at all.

Open the curtains in the daytime. Heavy curtains and closed blinds cut the glass off from the warm air in the room, which makes it colder and the condensation worse. It is free and it surprises people.

In a tight modern house, ventilate rather than heat. A bathroom fan on a timer, trickle vents, or on a very airtight house a heat recovery ventilator, which brings fresh air in without throwing the heat away with it.

Seal it first, then add moisture

Cold air holds almost no water. Bring it into the house and heat it to seventy and its relative humidity collapses, which is why the driest houses are the leakiest ones in the coldest week of the year.

So seal the draughts first. That is the actual cause, and it cuts the heating bill at the same time. Adding moisture to a leaky house is pouring water into a bucket with a hole in it.

Then add moisture. A portable humidifier for a bedroom, or a whole house one plumbed into the furnace ductwork.

If you already have a whole house humidifier and it does nothing, the usual suspects are all small. The water panel scaled solid and never replaced, which is a yearly part. The solenoid valve stuck shut. The humidistat set for colder weather than you are having. The little saddle valve on the water pipe blocked or never opened.

And the one nobody checks. A bypass humidifier has a damper with a summer and a winter position. Left on summer, the humidifier runs and the air never goes through it.

Keep it clean. A humidifier that is not serviced puts whatever is growing inside it into every room in the house, through the same ductwork that carries everything else: what else your system is spreading around.