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Hygrometers – Controlling Indoor Humidity

Condensation - Controlling Indoor Humidity

Several years ago, a friend of mine called me. She was frantic. It was bitterly cold outside. She opened an interior closet door to find one of the walls covered with frost! She thought something was seriously wrong and wanted my advice.

I went to her house and sure enough, the closet was frosty. I went into the attic and located the problem. Someone had cut a hole in the top wall plate of the closet wall. In addition, there was a gap in the insulation which allowed the cold, dense attic air to fall into the wall void. This cold air chilled the wall surface to such a degree that it was below 32 degrees F! The warm, moist inside air readily condensed on this cold surface. It didn't take long for it to then freeze.

Think Soda or Beer....

Condensation happens in warm weather all of the time. Unless you live in the desert or some other very dry location, surely you have seen the outside of a can of soda pop or beer get wet from condensation. The warm, moist outside air readily turns to liquid as it is cooled by the metal surface of the can. The temperature of the cold can is at or below the dew point of the humid air and condensation immediately begins to collect.

The easy way to stop this problem is to stop drinking beer....

Hidden Problems

If you have a condensation problem in your older home, it might not cause too much concern. Why? Old homes were naturally leaky. Many old house had no insulation or lack building materials which readily trapped moisture. As such, humid air could readily mix with colder air outside and dissipate.

However, modern building practices have created tighter and tighter houses. Foil faced sheathing products completely stop moisture transmission. Tight houses tend to have very high levels of indoor humidity as cold, dry air is not allowed to mix. The mixing of cold, dry air with indoor humid air lowers the overall humidity of indoor air. This is another reason you probably did not have a condensation problem in the house you lived in as a child.

Measuring Humidity Levels

Humidity is a relative thing. That is why your TV weather person says "..the relative humidity is now....." As you heat a constant volume of air it can hold more moisture. Eventually it becomes saturated. This is when the relative humidity of that volume at that temperature reaches 100 percent.

Dew point temperatures are also indicators of humidity. The dew point is the temperature when water will condense out of the air for a given humidity. This is why humidity is relative. Confused? It can be tough to understand, so don't worry.

Humidity is measured using devices called hygrometers or sling psychrometers. These devices are calibrated in such a fashion that they can tell you the relative humidity of air within a given temperature range.

The hygrometers can be electronic or ones with a standard dial face with a moving needle. The sling psychrometers are weird devices that incorporate two thermometers. One thermometer is dry while the other one has a wick attached to the bulb end. You get the wick wet and spin the two thermometers around in the air. Depending upon how humid the air is or is not, the water in the wick will evaporate at different rates. The faster the rate of evaporation (drier, less humid air) the lower the wick covered thermometer will read. Remember, evaporation is a cooling process (Why do you think Mother Nature created us so we sweat....?).

Using a chart, you correlate the temperatures of the dry and wet thermometers to determine relative humidity. They are pretty cool devices!

Checking Your House

If you have an indoor humidity or condensation problem, I recommend that you purchase an inexpensive hygrometer. Take readings in the same room(s) at the same spot each day at the same time. Start to chart your readings. Then begin to isolate sources of indoor humidity. Try to control them. See how it affects your readings each day. It may take several days to see a drop in humidity readings.

Take readings at different points within your house. Try to avoid the bathroom and kitchen. You can get widely spaced readings because of ever-present moisture sources.


Hygrometers - Manufacturers / Features

There are many manufacturers of hygrometers - instruments which measure relative humidity. I have listed just a few here. You can usually find these instruments advertised in the back of Popular Science or Earth magazines. These magazines always seem to have an abundance of ads about companies who sell weather related equipment. Your grade school or high school science teacher will also have science equipment catalogues you can browse through. Want REALLY sophisticated equipment? Call your local TV station's staff meteorologist - they absolutely know who makes top flight hygrometers.

  • NovaLynx Corporation ... 800-321-3577
    Humidity Dial Model # 220-730 - Features a brass case and sufficient openings to allow good air flow over the inner humidity element. Certified to +/- 3 percent accuracy. $120.00

  • Radio Shack ... 800-THE SHACK
    Wireless Weather Forecaster Model #63-1090 - Handy electronic gizmo that forecasts using temperature, barometric pressure and humidity. Shows date and Atomic time. Requires 2 AAA batteries for sensor; 3 AA for meter. $39.99

    Indoor / Outdoor Thermometer with Hygrometer Model #63-1032 - Displays interior temperature, exterior temperature and relative humidity and humidity comfort readings. Electronic. Requires 2 AA batteries. $19.99

  • Robert E. White Instruments 800-992-3045
    Call for current pricing!
    Model BA777 Compact and accurate. 2 1/2 inch diameter Wall mount $45.00
    Model BA410 4 inch dial, graduated every 1 percent. Accurate to +/- 4 percent, $66.00
    Model CONH6 Outdoor dial hygrometer, 4 1/2 inch diameter, swivel mounting bracket. $39.00
    Model 730 Most accurate (+3 percent), 6 inch diameter, brass case. Wall mount $120.00

Tips on Controlling Indoor Humidity / Window Condensation

People who live in colder climates can become victims of condensation. Water can collect on both visible and hidden surfaces. My eyeglasses always fog up when I enter my favorite chili parlor on a cold winter night. The thin glasses get cold walking from the car to the restaurant. When I hit that humid environment, BINGO! my glasses steam up.

If you see visible condensation on windows, walls, etc., it is a sure sign that your indoor humidity might be too high. Water can collect in wall cavities and attic spaces if you do not take aggressive action to lower the humidity and/or ventilate the hidden spaces.

Identify the Source of Humidity

All types of things add humidity to indoor air. Aquariums, cooking, washing clothes, bathing activities, house plants, human and animal breathing, slabs, crawl spaces, basement floors & walls lacking vapor barriers, etc.

It can be tough to identify any single source of excess humidity. All the above activities or conditions can collectively add to the problem. What's worse is that as the outside temperature drops, it takes less humidity in the indoor air to cause a problem! That is why indoor humidifiers are supplied with controls that tell you what the setting should be in relation to the outdoor temperature.

Controlling Indoor Humidity

Bathing activities (hot showers and baths) probably cause more indoor humidity problems aside from humidifiers that are set too high. Try to see if you can't take shorter showers and/or ones that use less hot water.

Install and use an excellent bathroom exhaust fan. Turn it on during your shower or bath. Leave it on after your bath for 10 minutes or so. Be sure the exhaust air is piped to the outside of your house. DO NOT dump this air into an attic space. It will rain in your attic!

Do you have a crawl space? Does it lack a high quality vapor barrier? If so, the soil in the crawl space is liberating vast quantities of water vapor into your house. Cover the soil with a vapor barrier and you will drastically lower your indoor humidity. I have written a past column and bulletin on this very topic.

Older homes that have basements are just as bad as houses with crawl spaces. However, it is tough to solve this problem. I am currently doing research on water based sealers that can be sprayed on indoor masonry surfaces which will stop moisture transmission. Stay tuned for that upcoming column!

Cooking liberates lots of water. Boiling water for noodles, vegetables, etc. puts lots of moisture in the air. Try to minimize these activities or install a ducted kitchen exhaust fan.

Window Condensation

I get mail every winter from people who install new windows. They think the windows are bad because water condenses on them. Well, the source of the problem usually lies in the fact that the windows are doing too good of a job! The old windows leaked vast amounts of cold air into the house. This very dry air mixed with the indoor humid air and lowered the overall humidity in the house. The new windows stop the dry air resulting in a higher indoor humidity! This humidity then fogs the cooler glass surface.

To minimize new windows from fogging or condensation, be sure to order ones with soft coat Low-E glass. This type of coating produces a warmer interior glass surface. The warmer the glass surface, the less chance of condensation. Read my past column and Builder Bulletin on Low-E glass.

Look for windows that offer warm edge technology. This refers to special spacers that are used to separate the pieces of insulated glass in your new windows. The spacer strips are made so that the cold outer pane of glass conducts a minimum amount of cold to the inner pane of glass. Ask tough questions to get the right answers when you window shop!

Column B157

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2 Responses to Hygrometers – Controlling Indoor Humidity

  1. Thanks for this helpful article. I'm wondering if you can answer a question. My basement is humid, so I run a dehumidifier. My hygrometer (Caliber III) reads 46-50% if it is placed at least one foot above the floor. If, however, I place the hygrometer just 1-2 inches above the floor, it reads 55-57%. I know that I need to keep humidity below 50% in order to prevent mold growth. Is it necessary for me to buy a second humidifier in order to reduce the humidity near the floor?

    Kirk Winters
    Arlington, VA

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