It’s that time of year again. The cold weather has begun here in the Northern Hemisphere, and people are talking about raising the humidity of the indoor air. I’m certainly no fan of bone-dry air, having experienced too much of it when I lived in an old house in Philadelphia. But I also know that keeping the humidity too high in winter can cause problems. Harvard University, in fact, once had to tear down a 17 year old building because of bad advice about indoor humidity.

The cause of bone-dry indoor air

Before I jump into that bad advice about indoor humidity, let’s first look at why the air in your house might be so dry to begin with. I’ve covered this before so I won’t go into all the details here. Here’s a quick rundown of the cause of dry indoor air:

Cold air is dry air. The lower the temperature, the less water vapor will be floating around in the air.
A leaky house allows a lot of cold, dry air to come in and mix with your indoor air.
Two of the driving forces behind air leakage are wind and the stack effect, both of which can be higher in winter.
When cold, dry air leaks into your house, it lowers the indoor relative humidity.
The leakier your house is, the drier the indoor air will be in winter.

Clearly, all this means that the first step to keeping your indoor air from being too dry in winter should be air sealing. It’s a pretty simple concept, right?