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Mold, Relative Humidity, The Dew Point, and Your Home
Wed 14 Jan 2009 - 11:40
Mold, Relative Humidity, The Dew Point, and Your Home
By [http://ezinearticles.com/?expert=Steven_L._Smith]Steven L. Smith
Mold is an ongoing worry for homeowners. The problem has received so much publicity that many people wonder what conditions might lead to mold 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. Such conditions will increase relative humidity levels in the home.
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.
Another fact: The warmer the air, the more moisture it can accommodate without condensation forming. But, when the temperature goes down in a home (overnight hours) the cool air is less able to hold moisture without condensation. If we study a home that is 70 degrees F and the RH is 50%, what happens if the temperature goes down to 49 degrees F? Simple answer: The relative humidity shoots up to 100% and condensation forms.
Tests have shown that condensation, when it forms, tends to occur on walls/sheathing and not as often at the insulation or inside the wall cavities. However, moisture might build up through the insulation at walls if there is that opportunity as a result of poor building practices and if enough moisture is being transferred. Water vapor will move from an area of high vapor pressure to an area of low vapor pressure. Due to the warmth of the home inside, count on the moisture outside being drawn in. But, again, even though the inside of the walls might be the coolest part of the home, the result of the cold air at the exterior, that is not necessarily where condensation will form.
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. My experience is that those guidelines are reasonable in the summer, when we are not likely to be heating the house and when the home does not cool off so much overnight. But those same readings can be too high in the winter. Many physical factors come into play.
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 55 degrees F. That is 11 degrees higher. Condensation will form if the home drops to 55 degrees F.
In the final example, lets look at higher readings that are fully possible inside a home that is moist. With an RH of 77% and a temperature of 73 degrees F, the dew point is 65 degrees F. That is only 3 degrees under the 68 degrees that many people use as the ideal thermostat setting! That means that if the home drops to 65 degrees, there will be condensation problems. These are not RH or dew point readings that you want to see.
It is obvious that, especially in winter, homeowners want to keep the relative humidity low. That makes the dew point lower too. At least 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 certainly beyond what most people, including inspectors, will be getting involved in on an ongoing basis. Also, there are few absolutes. 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, you can 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 readings.
Steven L. Smith, owner of King of the House, Inc home inspection is a licensed structural pest inspector and a certified home inspector in Bellingham WA. Smith is the program coordinator for the college level home inspection training program at Bellingham Technical College. In 2008 Steve was appointed by the Governor to serve on the new Washington State Home Inspector Licensing Board. http://www.kingofthehouse.com
Article Source: http://EzineArticles.com/?expert=Steven_L._Smith http://EzineArticles.com/?Mold,-Relative-Humidity,-The-Dew-Point,-and-Your-Home&id=1869784
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