Indoor Relative Humidity

Relative humidity is the amount of moisture added to the air by one’s sweaty, talkative kinfolk. Just kidding. In fact, relative humidity is the amount of moisture that is actually in the air compared to the amount it could hold (when the temperature stays the same). For example, if 70 degree air is holding only half of the moisture that it could, the relative humidity is 50%. If that same 70 degree air is holding every bit of moisture possible, the relative humidity is 100% and it has reached its dewpoint.

Once relative humidity reaches dewpoint, water vapor condenses into visible droplets. This is what happens when your glass of iced tea “sweats” – even when the air feels fairly dry and comfortable to you, the surface of the glass is cold enough to lower the air around it to the dewpoint and cause condensation. If the temperature difference is large enough (75 degree room, 45 degree glass), it doesn’t take much relative humidity at all (35%) for the glass to reach dewpoint (check out the chart below for more dewpoint information).

[Air temperatures (in degrees F) are located along the left side of the chart. Relative humidity (in %) runs along the top. Where the two numbers meet is the dewpoint (in degrees F).]

Our bodies are comfortable with relative humidity levels between 30 to 60%. In comparison, our houses need a much narrower margin – 35% – 40% in winter and 40% – 55% in summer – to operate properly.

Even though our outdoor humidity levels remain fairly steady throughout the year, switching from air conditioning to heating makes a huge difference to interior humidity. Air conditioning involves removing water vapor (your outdoor mechanical equipment is called a “condenser” for a reason!) but heating the air simply adds warmth, so all of that moisture stays put. Raising the indoor temperature does drop the relative humidity somewhat (and creates that “dry” feeling), but our indoor habits like cooking, bathing, doing laundry and breathing add moisture right back into the air.

Here are some specific “culprits” that contribute to higher humidity:
Gas heating – Using propane or natural gas as a heat source actually adds water to the air. Approximately one ounce of water is released per 1,000 btus burned in one hour of runtime. A typical gas furnace for our climate uses around 75,000 btus, which produces a little over 2 ¼ quarts of water per hour of runtime.
Breathing (1/4 cup water/hour)
Cooking (cooking for a family of four releases 5 pints of water over 24 hours)
Showering (1/2 pint water)
Bathing (1/8 pint water)

Adding only 4-6 pints of water will raise the relative humidity in a 1,000 square foot house from 15% to 60% at a constant temperature. A typical family of four, from 6 p.m. to 10 p.m. (including heating) will add 16 pints of water to the air.

I highly recommend having an indoor moisture meter, or hygrometer (available at Radio Shack and pet supply stores in the reptile section), to monitor moisture levels. Given time, relative humidity balances itself throughout the house, however, certain rooms have temporarily higher humidity. Kitchens are definitely up on the list, as well as bathrooms. Also, rooms on the northern side of a house are slower to warm up and will maintain their higher relative humidity for longer, especially if air flow is restricted by closed doors. Knowing which parts of the house, if any, are more humid will help determine what course of action to take.

Next, see if changing some of your habits brings the humidity down:

Breathing – continue doing this
Plants – Reduce the number of plants per room, choose plants with low watering needs, water plants outdoors and let them drain thoroughly and use a dense mulch such as pea gravel on top of the potting soil.
Kitchen – Using lids on pots and pans, turning on the range hood fan and cooking with a microwave will reduce the moisture load. Don’t use the air dry setting on the dishwasher – it vents hot, humid air directly into the room. Instead, wait for the dishwasher cool down and then hand-dry wet dishes. Check for leaking pipes under the sink.
Bathroom – Make sure that exhaust fans have working dampers and truly vent to the outside of the house. Take shorter showers, if possible, and keep the bathroom door closed while the room is steamy. Open the door back up after the exhaust vent has removed most of the moisture.
Heating – If you use gas for heating, consider setting the thermostat lower.
Air circulation – Keep interior doors open to discourage moisture buildup in isolated areas.

Depending on how “leaky” your house is, even taking the steps above might not lower the humidity. In that case, I recommend using a dehumidifier to bring the levels down to an acceptable range.

This blog was written for Paul LaGrange’s BuildWrite website and was originally posted on November 26, 2008.


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