How can you tell when your wintertime indoor relative humidity (RH) is too low? When you get zapped every time you touch a doorknob and your nose is dried out and sore.
How can you tell when your wintertime indoor RH is too high? When there is water or ice on the inside of your windows. Yep, it’s just as simple as that.
Why does indoor RH get so low in the winter? Because, although the outdoor air may have a high RH (as when it’s snowing), there really isn’t much moisture in cold air. And when you bring that air inside and warm it, the warmer air can hold much more moisture, so the RH is much lower.
Why does RH get too high in the winter? Well, the answer to that is far more complicated.
The natural situation is for your house or business to have low RH. So, whenever the indoor RH is too high, the humidity is obviously coming from an indoor source where there is water, such as a bathroom or kitchen. The problem is created by a lack of proper venting - either someone not turning on the vent fan when showering or cooking, or with a vent fan that is too small for the job.
There are also more ominous causes for high indoor humidity that can be overlooked, such as gas furnaces or water heaters not venting properly. Understand that gas combustion actually creates moisture. Every molecule of natural gas (CH4), when combined with oxygen, creates one molecule of carbon dioxide and two molecules of water vapor. So, if a gas appliance isn’t venting properly, not only is there too much carbon dioxide in the air, there is also a lot of humidity.
Too much humidity is bad because it causes indoor-air quality problems. Bacteria and mold thrive in damp locations, so it is best to find and fix any sources of high indoor-air humidity.
And solving the problems at the source is always the least expensive option. Installing properly sized outdoor-vented fans and fans that go on automatically and stay on for a reasonable period (after a bathroom shower, for example) is the best choice, if showering or cooking is where the excess moisture is coming from. However, installing a whole-house ventilation system is an even better option when it comes to solving indoor-air quality problems.
Additionally, high RH can damage the walls, ceilings, roofs, and even the exterior paint of a structure, because it seeps through to the outside. And somewhere in between it freezes, causing paint to peel and creating other damage.
In homes, there is usually enough moisture during the winter because of the showering and cooking that goes on there. However, because of their lifestyles, some families just don’t generate a lot of moisture. And in cases such as this, installing a humidifier is the best option.
Understand that raising the indoor RH also tends to make a building feel warmer, since the moisture on our skin doesn’t evaporate as quickly. However, don’t get the idea that adding a humidifier will reduce utility bills (since you can set the thermostat lower), because that just isn’t true. For instance, it takes about 1,000 Btus of heat from your furnace to turn one pound of water into vapor.
Also realize that as the outdoor temperature drops, you must reduce the indoor RH proportionately to keep it from condensing.
Here is a suggested list of settings:
Outside Inside
Temperature Humidity
20º to 40ºF Not over 40%
10º to 20ºF Not over 35%
0º to 10ºF Not over 30%
-10º to 0ºF Not over 25%
-20º to –10ºF Not over 20%
-20ºF or below Not over 15%
