by Arnie Katz


Q: I suffer from allergies and sinus problems. My doctor recently recommended buying a humidifier for my house to help relieve my congestion. Last summer, another doctor recommended a de-humidifier. Which one is right?

A: While every person–and every house–is different, it’s likely that both doctors were “right” in the sense of recognizing that the amount of moisture in the air in your house can have a real impact on your health. Whether the best solution is to install these kinds of machines is a bit more complicated.

Most “experts” in the field recommend that we maintain the relative humidity (RH) inside our homes at between 40% and 60%. In this range, most people are comfortable; building materials, such as wood moldings, don’t shrink or swell excessively; and there’s not enough moisture to promote the growth of molds, mildew, and other little nasties that can look awful and make you feel even worse.

There are some people, such as John Bower of the Healthy House Institute, who recommend keeping the RH as low as 25% in the winter. The reasoning is that most houses have cold spots–leaky windows, poorly insulated corners, certain parts of the floor–which can cause moisture in the air to condense into water and create ideal growing conditions for micro-critters. The lower the RH in the house, the less chance of this happening.

While 25% RH may be fine for many people, it may make you uncomfortable. If your throat feels like sandpaper, it’s likely that it is too dry in your house–or maybe just in your bedroom. The first thing to do is to find out what the RH is in various parts of your house at different times of the day and at different times of the year. This is something that will change constantly, depending on things like the weather and how many teenage boys are wrestling in the back bedroom.

Fortunately, there are now digital meters available for about $25 that accurately measure both temperature and relative humidity. Get one for your wife for Valentine’s Day.

The next step is to figure out why the house is too dry. Usually the answer is in an unexpected place–excessive infiltration of cold air. How does this work? Relative humidity is the amount of moisture in the air relative to how much moisture the air can hold at a given temperature.

Let’s say your house is at 70o and 50% RH and you bring in outside air that’s at 40o and 50% RH, a fairly common condition in North Carolina in the winter. Since warmer air can hold a lot more moisture that colder air, mixing the cold air with the warm air will reduce the relative humidity inside your house. If you bring in enough cold air, it won’t take long to turn your nasal membranes into something resembling a dried prune, not to mention the money you have to spend to heat all that cold air.

So, the most likely cause of your house being too “dry” in the winter is that the house itself, or the duct system, or both, are too leaky. The way to find out is to have your house tested. Several of the electric utilities in North Carolina have programs to do this testing, and others can refer you to local contractors.

Based on our research at Advanced Energy, you shouldn’t assume your house is tight just because it’s new, or expensive, or beautiful. Sadly, most houses we test, even new houses, are leaky. And we almost never hear complaints of “dryness” from people in tight houses.

Managing moisture inside your house is one of the most important things to pay attention to, for the long-term durability of your home as well as for your health and comfort. There are a lot more problems caused by too much moisture than by too little. The trick is to have enough, but not too much. A couple of weeks of high moisture in the house can cause bacteria and other organisms to get established and colonize your rug for a year.

One problem with using a humidifier to try to solve the “dryness” problem is that they are difficult to control. I’ve been in homes with portable humidifiers running and water running down the windows. Since many allergy and respiratory problems are made worse by molds and bacteria that thrive in moist conditions, these devices may not be the most sensible strategy in the long run.

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