Mold is an ongoing worry for homeowners. Due to the complexity of the biological process, mold is excluded in home inspection contracts. The problem has received so much publicity that many people wonder what conditions might lead to fungal growth in the home. First, more often than not, the problem will be brought about because of a water problem on the exterior such as a leaking roof or uncontrolled water that has allowed standing water in the crawl space or the basement.
In analyzing homes, professionals are usually concerned with relative humidity (RH) and that is a percentage of the moisture in the air. Air is saturated at 100%. It cannot hold any more moisture so, when air is saturated, condensation forms on surfaces. We start seeing obvious problems when that occurs.
The warmer the air, the more moisture it can hold. But, when the temperature goes down overnight, the cool air is less able to retain moisture. If a home is 70 degrees F and the RH is 50%, what happens if the temperature goes down 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 excess moisture forming, the condensation, is on windows but moisture could be occurring at or in walls as well. Research and experience indicate that condensation usually occurs on walls/sheathing and not as frequently inside walls or on insulation inside walls.
People often ask this question: What is the optimum relative humidity for my home? That is 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% not usually being a cause for much alarm. Keeping readings lower is best in the winter.
The problem is the dew point. Remember that air is saturated when relative humidity is 100%. Well, the dew point is the temperature at which water condenses inside the home. Some people think the dew point is a low temperature, around freezing, and that such a temperature could never occur inside the home. It is more complicated than that.
Some practical examples are in order. In a home, if the RH is 40% and the temperature is 69 degrees F the dew point in the home is 44 degrees — the temperature would have to fall 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 home drops to 53 degrees F.
In the final example, lets look at higher readings that might exist inside a home that is moist. With an RH of 77% and a temperature of 73 degrees F, the dew point is 66 degrees F — only 2 degrees under the 68 degrees that many people use as the ideal thermostat setting! If the home drops to 66 degrees, condensation will form on the inside.
It is obvious that, especially in winter, homeowners need to keep the relative humidity low. Where I live, in colder weather, it is more practical to keep a house somewhere above 44 degrees overnight than it is to keep it above 65 degrees.
This detailed information is beyond what most people, including inspectors, will be getting involved in on an ongoing basis. A word of warning, while you can obtain useful information from a relative humidity reading, it takes more than a single reading to gain enough information to make much of a determination as to whether or not a problem exists. It takes some study and analysis. And, some homes that seem like they should have mold growth do not, and the opposite can be true as well. Being alerted to this information helps one better understand why some houses have damp areas or stains. Excess moisture caused by high relative humidity can lead to damaged sheet rock, wood rot, mildew, mold, rust on metal, shrinking or expanding wood, reduced thermal resistance of insulation, odors. Frequently people ask what causes high relative humidity. It is not always easy to say but some of the usual suspects are showers, baths; cooking; washing clothes, dishes, floors and walls; breathing, perspiring; pets; uncontrolled surface water, wet crawl spaces and basements.
If you are, on a personal basis, interested in taking a look at the RH inside your home, purchase a relative humidity gauge (hygrometer) at an electronics store or online. You can easily find, online, dew point calculators or “psych charts” which allow you to determine the dew point from your relative humidity rand temperature readings.