Dear Jim: I need to determine the best indoor humidity level because my daughter has allergies. What is the most economical and efficient method to maintain the target humidity? — Ron H.
Dear Ron: It would be wonderful if there were just one ideal year-round indoor humidity level, but this is not the case. Typically, people are comfortable in the 30% to 50% relative humidity (RH) level and can tolerate 10% more to either extreme. The most comfortable humidity level for various seasons may not necessarily be the best to alleviate each of your daughter’s various allergies so expect to make some trade-offs.
Your first step should be to visit an allergist and determine to what allergens your daughter is most sensitive. People often think lower indoor humidify is always better, because mold and mildew are common allergens, but some others, such as irritating and dangerous microbes, can be worse at low humidity levels.
Maintaining a comfortable RH level indoors, as long as it does not exacerbate allergies, can reduce your utility bills. During winter, if the air gets too dry, there will be more evaporation from your skin making you feel chilly. To stay comfortable, you may set the thermostat higher, which uses more energy. The opposite is true when air-conditioning during summer.
It helps to understand what RH actually means. Warm air can hold more water vapor (moisture) than cold air. If the air at 75 degrees has a RH of 50%, the air is holding 50% of the maximum amount of water vapor it can hold at that temperature. At 100% RH, it would be foggy or drizzling indoors.
If the air drops to 50 degrees with the same amount of moisture in it, it may now have a RH of 70%. When air gets cool enough, next to window glass during winter or the refrigerator door during summer, it no longer can hold all the water vapor. This is called the dew point and condensation forms on the cool surface.
To maintain a comfortable level efficiently, you must control the sources of indoor moisture, and ventilate any excess away. Indoor sources are many. For example, each person gives off one-quarter cup of moisture per hour just breathing. Cooking for a family of four produces five cups of moisture per day. Using an exterior vented range hood, particularly during summer, can exhaust both this moisture and heat from the range.
Exterior sources are leaky roofs, plumbing, windows, doors, etc. Once you take care of these problem areas, check the slope of the ground around your home. The ground should slope downward slightly away from your house walls. If not, the soil adjacent to the wall will stay moist increasing the humidity of air that leaks indoors.
Installing replacement windows with efficient glass usually helps reduce window condensation problems during winter and heat gain during summer. On the other hand, if your family generates a lot of indoor moisture, installing new windows may cause excessive indoor RH because of less air infiltration through them.
Install a new bathroom vent fan with a humidity sensor. This comes on automatically and runs until the humidity level drops. With a manual switch, you have to either turn it off prematurely when you leave for work or let it run all day. Check the seal around the clothes dryer duct leading to the outdoor vent. During summer, hang your wet clothes outdoors to dry.
Install a new furnace or central air conditioner with a variable-speed blower and compatible thermostat to allow it to run in an efficient dehumidification cooling mode during summer or humidification during winter. Make sure the duct damper handle on the central humidifier is set for the proper season. Use electric countertop cookers and vegetable steamers in the garage instead of the kitchen during summer. I use an outdoor solar-powered oven on sunny days.
The best method to control indoor humidity is with an energy recovery ventilator system. This is a ducted system that bring fresh filtered outside air into your house. It exchanges heat and moisture between the incoming fresh air and stale exhausted air making it energy efficient. These systems can easily cost several thousand dollars to install.