When it’s Raining Inside: Clues to High Indoor Humidity in a Dry-Heat Climate

Recently, a client from Mesa AZ filled out our free Air Quality Consultation Survey to get help with their indoor humidity mystery.  We promote air sealing a lot, and often in very humid climates, the little cracks and crevices in our building “envelopes” will allow that humid outdoor air in, where it makes indoor air feels even more humid as it’s cooled down (cooler air can carry less water vapor).  However, this particular customer lives in an area of the country known for its dry climate, which is currently only 10-15% relative humidity outdoors!  This leads us to believe that air leaks from outside were not responsible for the “home feeling damp or humid”.  

Given the current excessive heat in Mesa, AZ, we can infer that AC units are running 24/7.   Most central AC units are set up to do one thing well: cool the home down quickly.  Unfortunately, they do not dehumidify well, especially if the unit is oversized.  We explain more about this problem in another post, but in a home such as this one in a dry climate, where is the excessive humidity coming from in the first place, if not from the outdoors?

You may realize that human activity such as cooking and bathing generate significant water vapor, and that’s one reason exhaust fans are installed in kitchens and bathrooms.  Using them during and after (for a minimum of 10 minutes after) cooking and bathing is essential to evacuating that moist air.   These are the top 2 reasons for high humidity, but let’s explore some more you may not have thought of. 

• Drying clothes: Ever wonder where all the water from your wet clothes goes when you put it in the dryer?  Ideally, it goes out the vent with the hot air generated.  If the vent is even partially clogged with lint, however, the clothes dryer will struggle to dry the clothes and the laundry room (and adjacent rooms) will fill up with hot, moist air.   Vents also often fall off and expel the humid, dusty air into the laundry room.  Take the time to clean the dryer vent at least once a year to help save energy costs, lower humidity, and prevent a fire!  

• Check the drain pan of the AC air handler.  If there's water collecting in the drain pan, this means that the drain line is clogged, and that standing water will evaporate back into the air and make it more humid.  If you need help unclogging the drain line, this video can help.

• If the AC is oversized, it can run very quickly to cool down the house, but not long enough to remove humidity from the air.  If the unit is 10 years old or more and you can consider replacing it in a smaller size as measured by a reputable AC tech, we recommend a high-SEER unit with inverter motors, and if possible, get it configured to provide "reheat dehumidification".  We have an article on this subject.

• Along with an inefficiently dehumidifying air conditioner, you may be keeping the home at a temperature that causes air to come out of vents below the dew point.  When an air conditioner runs a cooling cycle for a thermostat set on 72 degF, the air that comes out of the vents is not 72, it’s 14-20 degrees cooler than that (52-58 degrees).  The dew point of air in a 72 degF, 65% relative humidity space is 57.7 degrees (see a dew point calculator here).  Just by raising the thermostat setting to 75 degrees, the dew point will go up to 62.4 degrees, assuming the same relative humidity.  This minor temperature adjustment may mean the difference between condensation dripping off your registers, and dry registers!  For more information on dew point, check out our article here.

• While Mesa, AZ doesn’t get a lot of rain, the rain that does come could be penetrating the exterior walls and seeping into the home.   The same goes for  incorrectly adjusted sprinkler systems.  If your sprinkler system sprays directly onto the side of the home (and even the foundation), it may be pumping moisture into your home!   We wrote about how brick and stone are not waterproof and must have proper water-shedding structure behind them (like an air gap) in order to keep moisture from coming in.

• High ceilings: Cathedral ceilings are beautiful and make even a small room feel very open.  However, they are notoriously difficult to seal and insulate properly.  As a result, humid air rising from all the human activities inside (that didn’t get exhausted with the vent fans) hangs up at the peaked ceiling.  When the outside air cools at night, rafters in the ceiling will conduct the heat inside, to the outside, and condensation may form on the ceiling and in the roof cavity if it’s not properly sealed.  It’s a recipe for mold! (Why Is My House So Humid?)

• Cracks and leaks in the AC ductwork:  Even though the air outside is super-dry, the air under a home may not be.  Even vented crawlspaces typically have very little airflow, and moisture rising from the ground has nowhere to go.  If your AC ductwork runs through the crawlspace and there are openings in the ductwork (either by ducts falling off or animals trying to repurpose the insulation for their homes), then this humid, dirty air can be pulled directly into your home.  Likewise, the same thing can happen in your vented attic if the ductwork is routed through there.

• Improperly routed exhaust fans: it doesn’t do your house any favors to use a  bathroom vent which vents into the attic!  Sometimes it’s shoddy workmanship, sometimes ignorance, but kitchen and bathroom vents must extend all the way to an external register, so that the humid air doesn’t get trapped in the attic.  

It seems like a conundrum that while living in a super-dry climate, your house feels like a wet sauna.  The moisture definitely has a source and it can cause all kinds of problems: mold, dust mite growth, bacterial overgrowth, etc.  If you’re experiencing this problem and can’t figure out the cause, contact us with your questions and we’ll do our best to help you get your home more dry, comfortable and healthy!

Photo by Anne Nygård on Unsplash