High humidity in a house can be caused by many things, but the biggest reason is poor ventilation—in other words, air isn't being circulated properly, which causes the air in your house to become stagnant and humid. The other major reason? Too much air exchange with the outside.
Experts generally agree that the ideal indoor humidity levels for comfort and for avoiding health effects are between 35 and 60 percent. When you're spending time in a home or workplace with humidity levels in excess of 60 percent, it's increasingly likely that you will experience certain health issues.
Research from the Building Science Corporation found that humidity of 70% or higher adjacent to a surface can cause serious damage to the property. The Health and Safety Executive recommends that relative humidity indoors should be maintained at 40-70%, while other experts recommend that the range should be 30-60%.
Water can enter your home by leaking or by seeping through basement floors. Showers or even cooking can add moisture to the air in your home. The amount of moisture that the air in your home can hold depends on the temperature of the air. As the temperature goes down, the air is able to hold less moisture.
Not only is high humidity in a house uncomfortable, but it can also contribute to mold growth, poor indoor air quality, and wood rot (as well as health concerns like skin issues and respiratory problems).
During the winter, if you open the window when the outside air is colder than the air indoors, the cold air will come into contact with the warmer air and the level of humidity will fall automatically.
If the relative humidity is 100 percent (i.e., dewpoint temperature and actual air temperature are the same), this does NOT necessarily mean that precipitation will occur. It simply means that the maximum amount of moisture is in the air at the particular temperature the air is at.
Because the air holds different amounts of humidity at different temperatures, it is measured on a “relative” basis. For instance, 90 percent relative humidity means that the air is holding 90 percent of the amount of moisture that air is capable of holding at that temperature.
High humidity can increase the amount of mucus in your nose and throat, causing congestion, sneezing, runny nose, and post-nasal drip.
While there is no set humidity upper threshold, the National Oceanic and Atmospheric Administration (NOAA) typically considers relative humidity levels of 50% or more and dew points above 65% to be uncomfortably high.
If there are no cold-condensing surfaces and the relative humidity (RH) is maintained below 60 percent indoors, there will not be enough water in those materials for mold to grow. However, if the RH stays above 70 percent indoors for extended periods of time, mold will almost certainly grow.
Uncomfortable Humidity
There's no set humidity threshold above which discomfort starts to increase. However, the National Oceanic and Atmospheric Administration considers relative humidity levels of 50 percent or more and dewpoints (a direct measure of relative humidity) above 65 F to be high and uncomfortably.
Most of the time when it is raining the relative humidity is 90-99 percent, but not often is it 100 percent. Clouds form when invisible water vapor gas condenses into visible water droplets, and rain occurs when droplets grow too large to be supported in the air and they fall.
Research shows that the ideal relative humidity for your home is between 40%-60%. However, to discourage the growth and spread of mold, levels should be kept below 50%.
It's a condition known as supersaturation. When air is saturated with water vapor, its relative humidity is 100%, but in carefully controlled laboratory experiments the humidity has been raised above 200%.
"It is empirically, scientifically, physically, mathematically impossible to give 110. The fact is, 100 percent is the maximum. That's everything.
So, when you hear a weather forecaster say, "It's 85 degrees outside, but with the humidity, it feels like 92 degrees," that 92 is combination of the temperature and the dew point, also known as the heat index.
Your house is usually more humid because it's all sealed up in the summer, and moisture produced in cooking or running the shower stays locked up inside. Your humidity level in summer could actually reach about 40 percent.
So if you've noticed that your AC isn't dehumidifying your home, you most likely have one of the following problems: Your thermostat fan is set to ON. Your evaporator coils are dirty. You have a refrigerant leak.
Cold air can't retain as much moisture, so high humidity is less common in chilly weather.
Will the humidity rise if the heat is turned up? Contrary to popular belief, heating the air actually lowers the relative humidity. This is because, as the temperature of the air increases, the air can hold more molecules, and its relative humidity goes down.
High humidity can have a number of adverse effects on the human body. It can contribute to feelings of low energy and lethargy. In addition, high humidity can cause hyperthermia — over-heating as a result of your body's inability to effectively let out heat.