Controlling Condensation and Mold Growth in Winter

Mold likes humidity. While we don't usually think of winter as a time when there are humidity problems, the conditions for mold growth can be ideal. Knowing a few basic facts about humidity and mold can help you avoid situations that promote mold and moisture damage.

We've all heard the weatherman talk about "relative humidity", but what is it? Relative humidity, or RH, is a measure of how much moisture is in the air. 100% RH means that the air is holding all the moisture possible, any more and the water literally falls out of the air. But, did you know that warmer air can hold more moisture? Air that contains a set amount of water, might be 50% RH at normal room temperature but it could be 100% RH at 40 degrees. This is why windows get covered with condensation during the winter.

As noted, mold likes humidity. It is well known that mold growth is promoted by RH levels above 70%. So if you see mold, it means that the surface has a lot of moisture. Most likely, it is a cold surface like a window-frame or door threshold. But sometimes it can occur inside a wall or ceiling.

Usually, the outside air contains very little humidity during the winter, making it uncomfortably dry inside the house. To increase the comfort and avoid winter nosebleeds, most people use humidifiers. But we just saw that humid air leads to mold growth, so how do you make it comfortable without promoting mold growth?

The most important thing is to control the level of humidity. Studies show that most people are comfortable at humidity levels between 25% and 60%. So, if you are going to humidify your home, do so gently. Only raise the humidity the least amount necessary for comfort. Generally, it is best in our climate to keep the humidity between 25% and 35%. When it gets higher than this, you run the risk of condensation on cool surfaces in your home.

If you have a whole-home humidifier, do not trust the humidity dial to accurately control the indoor humidity. For $10-$20, you can purchase a small humidity monitor. Place one of these on each floor of your home. Humid air rises, so there's a good chance that the upstairs will be more humid than downstairs. Adjust the humidifier a little bit each day, keeping an eye on the monitor. This simple measure can help to greatly reduce the risk of mold and rot in your home.

Special note for people with forced air heating systems: furnaces and heat pumps are often called "dry" heating systems. People complain of nosebleeds or dry throats due to the dryness of the air with these systems. However, it turns out that furnaces don't really dry out the air. The culprit is usually leaky ducts. Many systems have ductwork routed through attics or have air registers mounted in the walls. When these leak, they suck cold, dry air into the house while forcing out the warm, moist indoor air. The result is a house that is very difficult to keep comfortably humid. If you think your house is like this, chances are you have leak, maybe big ones, in the return air part of your heating system. An energy auditor with a blower door and a thermal imaging camera should be able to track down these leaks.

Remember, mold grows wherever the humidity is high. This is often in bathrooms, bedrooms. If you see condensation on any surface, wipe it down immediately then find out why there's too much humidity in your house. Keep the humidity low, and mold can't grow!

This article was written by Ted Inoue of Energy Testing & Consulting LLC.

Ted Inoue
Energy Testing & Consulting llc.
New Hope, PA 18938
215-760-0211
www.etccreations.com