Mold, Relative Humidity, Dew Point, and Your Home
Mold is a constant concern for homeowners. Due to the complexity of the biological process, mold is excluded from home inspection contracts. The problem has received so much publicity that many people wonder what conditions could lead to mold growth in the home. First, most of the time, the problem is due to an outside water problem, such as a leak in the roof or uncontrolled water that has allowed standing water to form in the crawl space or basement.
When analyzing homes, professionals are often concerned with relative humidity (RH), which is a percentage of the humidity in the air. The air is 100% saturated. It can’t hold more moisture, so when the air is saturated, condensation forms on surfaces. We start to see obvious problems when that happens.
The warmer the air, the more moisture it can hold. But, when the temperature drops overnight, the cold air is less able to retain moisture. If a house is 70 degrees F and the RH is 50%, what happens if the temperature drops to 50 degrees F? Simple answer: the relative humidity shoots up to 100% and condensation becomes apparent.
Probably the first place a person will notice the buildup of excess moisture, condensation, is on the windows, but moisture could also be occurring on the walls. Research and experience indicates that condensation generally occurs on the walls/siding and not as frequently within the walls or on the insulation within the walls.
People often ask this question: What is the optimal relative humidity for my house? That’s harder to answer than one might, at first, think. The quick number, at least for my region of the country and provided by the Northwest Clean Air Agency, is 30% to 50% with a reading as high as 60% generally not cause for much alarm. Keeping the reading lower is better in the winter.
The problem is the dew point. Remember that air is saturated when the relative humidity is 100%. Well, the dew point is the temperature at which water condenses inside the house. Some people think that the dew point is a low temperature, around the freezing point, and that such a temperature could never occur inside the house. It’s more complicated than that.
Some practical examples are in order. In a house, if the RH is 40% and the temperature is 69 degrees F, the dew point in the house is 44 degrees; the temperature would have to drop to 44 degrees before condensation would form. This is a typical RH and temperature based on my studies in this northwest region of the country.
In another example, if the RH is 56% and the temperature is 70 degrees F, that change in the equation leads to a dew point of 53 degrees F. Condensation will form if the house drops below 53 degrees F.
In the final example, let’s look at the highest readings that can exist inside a damp house. With a RH of 77% and a temperature of 73 degrees F, the dew point is 66 degrees F, just 2 degrees below the 68 degrees many people use as the ideal thermostat setting! If the house goes down to 66 degrees, condensation will form inside.
It is obvious that, especially in winter, homeowners need to keep the relative humidity low. Where I live, in colder climates, it is more practical to keep a house somewhere above 44 degrees overnight than to keep it above 65 degrees.
This detailed information is beyond what most people, including inspectors, will engage with on an ongoing basis. A word of warning, while you can get useful information from a relative humidity reading, it takes more than a single reading to get enough information to determine whether or not a problem exists. It takes a bit of study and analysis. And, some homes that appear to have mold growth don’t, and the reverse may also be true. Being alerted to this information helps to better understand why some houses have wet areas or stains. Excess moisture caused by high relative humidity can lead to damage to plasterboard, wood rot, mold, rust on metal, shrinkage or expansion of wood, reduction in insulation thermal resistance, odors. People often ask what causes high relative humidity. It’s not always easy to tell, but some of the usual suspects are showers, toilets; Cooking; wash clothes, dishes, floors and walls; breathing, sweating; farts; uncontrolled surface water, basements and wet crawl spaces.
If you’re personally interested in looking at the RH inside your home, purchase a relative humidity meter (hygrometer) at an electronics store or online. You can easily find online dew point calculators or “psych charts” that allow you to determine the dew point from your temperature and relative humidity readings.
