Stop Growing Mold In Your Home- (re-Blog)

The information in this post goes beyond the standards of a normal home inspection. It is rather detailed. However, the issues described are problems that a home inspector sees and, periodically, might have to report on. The topic is high humidity levels inside the home. High humidity can lead to a myriad of problems in the home.

First, in analyzing homes, we 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 we get condensation.

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 opporunity 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.

The photo below is probably a sign of a thermal bridge. Thermal bridges occur at studs, top plates, sill plates. These are locations where insulation is not continuous so the wood and drywall, which are poor insulating materials, conduct heat through the walls/ceilings. These bridges will cause heat loss but they often also lead to localized cool temperatures on interior surfaces. Cool interior surfaces are likely to show surface condensation. This can simply cause marks where dust collects at moist areas. Or if condensation is heavier, the moisture can cause various problems that are associated with damp buildings -- such as microbial growths/mold.

 

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 NW 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 levels work well 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.

Why is that? The answer is the dew point. Remember that air is saturated when relative humidity is 100%. Well, the dew point in this case 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. But wait. It is all more complicated than that.

Some practical examples are in order. This RH gauge below shows an RH of 40% and a temperature rounded to 69 degrees F. The dew point in this home is 44 degrees. The temperature would have to fall to 44 degrees before condensation would form. This, by the way, is a pretty typical RH and temperature based on my studies in this region of the country. 

 

In the next example, we go just a bit higher on the RH, 56%, and the temperature is up less than 2 degrees. Yet, that change in the equation leads to a dew point of 55 degrees F. That is 11 degrees higher.

 

Now lets take a big jump. These readings 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, overnight, drops to 65 degrees, we will have some condensation problems. This is not a reading you want to see in the home.

It is obvious that, especially in winter, we 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. 

Since I periodically try to figure out the dew point, largely for my own information, I have scouted out a handy online calculator. They make graphs and charts to unravel all this information but, with the calculator, if you know the temperature and the RH, then you can figure out the dew point in a flash. You can find the calculator below.

Dew point calculator

This detailed information is certainly beyond what most people, including inspectors, will be getting involved in on an ongoing basis. Also, there are no absolutes. Some homes that seem like they should have mold growth do not, and the opposite can be true as well. But knowing this information is interesting and 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 figuring out the dew point or the RH inside your home, you can purchase a relative humidity gauge (hygrometer) at Radio Shack. Or, for under $100, you can purchase a more accurate instrument such as the one pictured above. 

In this complex area of indoor air and moisture, when I find problems that concern me, I suggest to clients that they talk to the Building Performance Center. It is a local government agency in this county that has building scientists who will help homeowners and tenants troubleshoot their way through air quality or mold issues.

Steven L. Smith

Bellingham WA Home Inspections