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Paul LaGrange

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Paul: Explaining indoor relative humidity and why it matters

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 and condensation can occur on cool surfaces. 
Once relative humidity reaches dewpoint, water vapor condenses into visible droplets of water.  This is what happens when your glass of sweet 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 temperature around it to the dewpoint and cause condensation.  If the temperature difference is large enough (70 degree room, 44 degree glass of tea), it doesn’t take much relative humidity at all (40%) to reach dewpoint (check out the chart below for more dewpoint information). 
Our bodies are comfortable with relative humidity levels between 40 to 60%.  In comparison, our houses need a much narrower margin – 35% - 40% in winter and 40% - 55% in summer - to operate properly and not be susceptible to moisture issues. 
Even though our outdoor humidity levels remain fairly steady throughout most of the year in the Gulf South, switching from air conditioning to heating makes a huge difference to interior humidity.  Air conditioning involves removing water vapor but heating the air simply adds warmth, so all of that moisture stays put.  Changing the indoor temperature does not drop the moisture content of the air.  However our indoor habits like cooking, bathing, doing laundry and breathing adds moisture to the indoor air.      
Here are some specific “culprits” that contribute to higher indoor 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. 
  • Humidifiers
  • Houseplants
  • 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)
  • Wood burning or Gas log fires
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. will add 16 pints of water to the air, presuming that the house’s envelope isn’t leaky. 



3 pints


½ pint/shower X 4 showers

2 pints


¼ cup/hour X 4 hours X 4 people

4 cups = 2 pints


¼ quart/hour of runtime X 2 hours

½  quarts = 9 pints

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. Knowing which parts of the house, if any, are more humid will help determine what course of action to take such as utilizing source control. 

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 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 properly vented 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. 
Bathroom – Make sure that exhaust fans have working dampers and truly vent to the outside of the house and attic space.  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 and supplementing with individual electric room heaters.
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 indoor humidity during our cooling season.  In that case, I recommend using a dehumidifier to bring the levels down to an acceptable range.  If your ac supply registers are reaching dew point ask your HVAC dealer that services your equipment to check for duct leakage in the air distribution system.
Try my suggestions to lower your indoor relative humidity and enjoy your summer! For more info on how to save energy and improve comfort levels in your home or office, visit LaGrange Consulting’s website at www.lagrangeconsulting.com  or call us at 985-845-2148.

07/12/2014 8:50AM
Paul: Explaining indoor relative humidity and Why it matters
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