Dew Point, RH and Mold (Bellingham Home Inspector) King of the House
The detailed information in this post goes beyond the standards of a normal home inspection. However,
the issues described are problems that a home inspector sees and, periodically, might report on. The topic
is high humidity levels inside the home. High humidity can lead to a myriad of problems.
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. When air is saturated, condensation forms on inside surfaces.
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.
The photo below is a thermal bridge. Thermal bridges occur at studs, top plates, sill plates
— locations where insulation is not continuous so the wood and drywall, both poor
insulating materials, conduct heat through the walls/ceilings. Bridges lead to localized cool
temperatures on interior surfaces, along with accompanying stains where dust collects at
the moist locations. If condensation is heavy, 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 think. The quick number, for my region of the country
and provided by the NW Clean Air Agency, is 30% to 50%. But, a reading as high as 60%
is not usually a cause for much alarm. My experience is that those numbers work well in
the summer, when we are not heating the house, and when the home does not cool off so
much overnight. Those same readings can be too high in the winter. Many factors come
into play.
Keep in mind that air is saturated and expelling moisture when relative humidity is 100%.
The dew point is the temperature at which water condenses. Some people think the dew
point is a low temperature and that such a temperature could never occur inside the home.
It is more complicated than that.
The RH gauge, below, has an RH of 40% and a temperature rounded to 69 degrees F. The
dew point is 44 degrees. The temperature would have to fall to 44 degrees before
condensation would form at inside surfaces. This example displays a pretty typical RH and
temperature based on my studies in this region of the country.
In the next example, RH is a bit higher, 56%, and the temperature is up less than 2 degrees.
That change in the equation leads to a dew point of 53 degrees F. When the mercury hits
53 degrees, with that much moisture in the air, condensation forms inside the house.
Now let’s take a big jump. These readings are possible inside a home that has excessive
moisture. With RH at 77% and a temperature of 73 degrees F, the dew point is 66 degrees
F — two degrees less than the 68 degrees that many people consider to be the ideal
thermostat setting. This is problematic because if the inside temperature drops to 66
degrees overnight, condensation will form on inside surfaces.
Especially so in winter, it is beneficial keep the relative humidity low which drives down
the dew point at a residence. Here in the Pacific Northwest, in fall and winter, it is more
practical to keep a house somewhere above 44 degrees overnight than it is to keep it at, or
above, 66 degrees.
It is an oddity but some homes that seem like they should have mold growth do not, and
the opposite is true as well. Being aware of the correlation between RH and dew point
helps a person better understand why excess moisture can lead to damaged sheet rock,
wood rot, mildew, mold, rust on metal, shrinking or expanding wood, reduced thermal
resistance of insulation and musty 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 interested in figuring out the RH and the dew point at your home, purchase a
relative humidity gauge (hygrometer) at an electronics shop or Online. Or, for under $100,
you can purchase a more accurate instrument such as the one pictured above. To compute
the data that you collect, search Online for a "psychrometric chart" or an interactive dew
point calculator.
Locally, if I have concerns about moisture inside a home, I often suggest that clients
should contact the Building Performance Center. It is a private, non-profit organization
that employs building scientists who will help homeowners and tenants troubleshoot
complicated humidity, air quality or mold issues.
Steven L. Smith
Bellingham WA Home Inspections
Thanks for stopping by,
Steven L. Smith
Rev: October 2, 2012