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Humidity is a strange beast: allow humidity to get too high, and all manner of issues crop up, but let it drop too low, and unique new problems rear their ugly heads. For this reason, homes should be designed with a specific range of humidity in mind. Engineers, architects, and other home designers must be careful to prevent humidity from dropping too low or from going too high, all while keeping in mind regional variations that may make high humidity a problem in one area and low humidity a problem elsewhere. Luckily, the best indoors humidity range is fairly wide. Engineers also have several tools in their kits to manage humidity rates, making humidity a fairly simple problem to solve.
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Best Indoor Humidity Range
The ideal humidity range is one that takes into account all of a home's inhabitants, both living and inanimate. Humans, books, and electronics all have their own particular ideal humidity range; fortunately, these ranges overlap. The best humidity range is the area where all three overlap. (The best ways to control humidity are discussed towards the end of this article.)
Before discussing the ideal humidity range, though, it's important to define units. When we talk about humidity, we usually mean relative humidity: the ratio of water vapor currently in the air to the maximum water vapor at that temperature, multiplied by 100. Air at higher temperatures can hold more water vapor, so the air with equal amounts of water vapor at 32o F and 90o F will have drastically different relative humidity. This becomes an issue in the winter, when heating systems warm up air but don't add any water vapor. This makes the relative humidity very low. On the other hand, cooking, bathing, and other living habits all may substantially increase humidity. Air conditioners are typically designed to lower relative humidity to between 30-60%, due to increased evaporative cooling at those ratios.
According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the ideal humidity range for humans is between 30 to 60 percent relative humidity. The ideal is somewhere around 45-55%. Very high levels of humidity contribute to the growth of mold, funguses, dust mites, and other pests. Mold contributes to a number of diseases and thrives in humid climates, generally above 60 percent humidity. For people suffering from asthma and other respiratory disorders, humidity should not exceed 50 percent, as high humidity can aggravate symptoms. Dry or itchy skin conditions are aggravated by low humidity, which tends to dry out the skin.
Though books aren't alive, they're made out of a formerly living material (wood), and so are subject to degradation. High levels of humidity can allow mildew and destructive pests to damage books, while low levels of humidity can make the paper pages brittle. Brittleness can occur at humidity levels as low as 40 percent, while mildew grows at levels over 50 percent RH. The range recommended by experts is between 30 and 50 percent RH, and temperatures no higher than 70 degrees Fahrenheit.
Image: Flickr - Andrew Ratto - Mildew on Walls
Similarly to books, electronics are sensitive to extremes of humidity. High humidity can alter the conductivity in the devices, leading to damage and malfunction, and possibly corrosion. Condensation becomes a very real problem at high humidity. Very low humidity, on the other hand, can cause the components of a device to become brittle. Electronics are usually designed for operation in normally-experienced humidity ranges, and so will operate correctly in the healthy human range of 30 to 50 percent.
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Best Ways to Control Humidity
There are several effective ways to control indoor humidity. Humidifiers and dehumidifiers both perform valuable functions, increasing and decreasing humidity, respectively. Humidifiers are particularly useful in the winter, when air is typically very dry. Again, this dryness is exacerbated by increasing the temperature of low-moisture content air, and can cause dryness of skin and lips, illness, and other ailments.
Humidifiers can be small, designed for use in individual rooms, or larger for humidifying whole houses. Whole-house humidifiers are attached to the furnace to alleviate overly dry air. This type is recommended for homes with forced air furnaces. There are several types of whole house humidifiers: drum, disc wheel, bypass flow-through, and spray mist styles. These can be very effective ways of increasing the humidity of a house. Though drum and disc wheel models are cheapest, they may have issues with fungal and bacterial growth on the drum pads.
Image: Flickr - Eli Duke - Steam Humidifier
Similarly, there are one-room and whole-house dehumidifiers. Though whole-house humidifiers and dehumidifiers are relatively expensive, they are generally more efficient and effective, particularly when compared to using several smaller machines. This .pdf provides technical details of a number of energy-efficient dehumidifiers.
Additionally, home builders can reduce humidity in several ways. Adding fresh air ventilation and venting laundry appliances outdoors will both work.