How to control humidity in your home

In order to control humidity, you must look at the problem in a completely different way.  Humidity is simply the amount of water vapor in the air divided by the amount of water vapor that the air can hold.  For example at 90 degrees and  50% humidity the amount of moisture in the air is one half of the maximum (at 90 degrees).  The warmer the air the more moisture that it can hold, conversely, as air cools it can hold less and less water vapor.  As our 90 degree air at 50% humidity cools, the humidity increases by 70 degrees, the humidity is 100%.  If you cool the air further, the water vapor will turn back into a liquid and form dew (hence the name dewpoint).  The warmer the dewpoint the more moisture is in the air.

Lets take an example of a 3 level house.  Upstairs, the temperature is 82 degrees and the humidity is 55%.  Downstairs, the temperature is 76 and the humidity is 60%.  In the basement, the temperature is 70 and the humidity is 70%.  Which one has more moisture?  If you look at the dewpoint you realize that they all have the same amount of moisture.  All three temperatures/humidity’s have a dewpoint of 62 degrees.  If you want to control humidity you have to control moisture.  The standard way to measure moisture is the dewpoint. 

On a typical summer day in NC, you might have a dewpoint of 70 degrees.  The dewpoint will remain relatively steady all day.  In the morning, it might be 70 degrees with 100% humidity (dew) and then by afternoon, it may be 90 degrees with 50% humidity.  After sunset the temperature will fall rapidly to 70 degrees and then much more slowly.  This is because once you cool past the dewpoint you are no longer just cooling air, you are condensing water.  There is a large amount of heat released when the water vapor condenses.  This extra heat prevents the temperature from going more than a few degrees below the dewpoint.  (High humidity is one of the reasons that rainforests are almost constant temperature, while dry deserts have such large temperature swings). 

If you want to keep humidity down, you have to keep the moisture out.  Moisture normally gets in to your house in the form of water vapor.  (If water is leaking directly into your house, you will have major problems.  Correct the problem IMMEDIATELY).  Just as water runs downhill, water vapor diffuses from high dewpoint to low dewpoint. 

Many people like to “air out” their house at night when the temperature falls.  If your bedroom is 80 degrees, 70 degree air feels cool.  The problem is that the air is 70 degrees and 85% humidity.  Anytime the dewpoint outside is higher than the dewpoint inside, opening a window will let in lots of moisture.  Worse yet, because the air is cooler, it will prevent your A/C from running to remove any of that moisture.

Even if you keep you windows shut, anytime the dewpoint is higher outside than inside, moisture will diffuse into your house.  The more the difference in dewpoints, the more moisture will get in.   I own a modern townhouse.  In the summer, the AC can keep the dewpoint inside about 10 degrees lower than the dewpoint outside.  I try to keep the inside dewpoint less than 60 degrees.  If the dewpoint outside gets above 70 degrees, I usually have to turn on a separate dehumidifier. 

Once your house is built there is not much that you can do to prevent moisture from entering your home.  You can however remove it from your house.  This is usually done with the Air Conditioning system.  Air conditioning (AC) works by compressing and moving a gas (R-22) to carry heat from inside to outside.  (if you have a heat pump it can also move heat from outside to inside).  Inside your house is a cold coil where the R-22 boils, adsorbing heat (the evaporator coil) and outside is the compressor and a large coil where the hot gas condenses into a liquid (the condenser coil). 

The air in your house is forced past the cold evaporator coil and sent through the ducts in your house.  If the air in your house is 78 degrees and 55% humidity (dewpoint of 60 degrees) and the evaporator coil in the AC is 60 degrees, how much water do you remove?  None!  The air will cool to 60 degrees and 100% humidity, but none of the moisture (in the air) will condense out on the evaporator coil.  AC can only remove humidity when the air is cooled below the dewpoint.  Obviously, you want the evaporator coil to be as cold as possible.  (If you have it less than freezing, you will have to have a regular defrost cycle to remove the ice from the coil) 

Heat pumps have to run in both directions and must compromise on their design.  For a given size compressor, a small coil will get cooler than a large coil.  In general, the condenser coil is much larger than the evaporator coil.  Because the inside coil must act as both an evaporator coil in summer and a condenser coil in winter, the coil must be fairly large.  This large coil tends not to get as cold as a smaller coil would.  (more expensive heat pumps have a 2 stage coil and don’t have this problem)  If the coil is not getting cold enough, you can have humidity problems.

Even if you have a nice cold coil, it can take 8-10 minutes from the time the AC starts until the moisture condenses on the coil, builds up and starts dripping, fills the pan, and runs out of your house.  Ideally you would want the AC to run for 45 minutes or more.  In my apartment, the AC tended to run for 15-20 minutes.  I also would leave the fan on, in order to keep the upstairs more even temperature with the downstairs.  This meant that any moisture that condensed onto the coils would evaporate back into the apartment once the AC shut off.  With the AC spending the first 10 minutes of the run cooling the coils, only 5 minutes of a 15 minute run were actually removing moisture.  This meant that my heat pump (which was not ideal for removing moisture to begin with) was now operating with only 33% efficiency for removing humidity.  No wonder the humidity was always high in my apartment.  If you have an electronic thermostat, you can lower the sensitivity to allow the temperature to swing more between cycles.  Try to get each cycle to run at least 45 minutes.  Also sometimes you can help matters by shutting off the AC duct closest to the sensor.

If you live in an apartment, there is not much you can do about the AC.  You can buy a separate dehumidifier from your local hardware store for less than $200.  For best effect, try to place the dehumidifier next to an air return so that the dry air is spread around the apartment.  If you live in a house you might want to make some simple modifications to your AC system to make it more efficient at removing humidity.  In my townhouse, I reduced the humidity by almost 10% just by slowing down the fan.  This modification consisted of changing a couple of jumpers.  If you want more information about tweaking your AC for better humidity control I recommend:

http://www.centralcityair.com/University.htm.

Sometimes even the best AC systems are not enough to keep the humidity low.  Recently in NC there was a stretch of 5 days where the temperature hovered around 70-80 degrees with intermittent rain and almost 90% humidity.  Under these circumstances, the AC hardly runs and there is as much moisture in the air as a muggy summer day.  In these cases the only solution is a separate dehumidifier.  Not only does the dehumidifier remove some of the moisture that enters your house, it also generates heat when doing so.  This additional heat forces the AC to run and remove even more humidity.  I use a small 25pint model that in addition to the AC is enough to keep my townhouse below 60% humidity at all times.  I tried this in my apartment, but the standalone dehumidifier was not enough to make up for the humidity entering from outside and in the end was a futile gesture as it was only able to lower the humidity down a few percent at most.

If you don’t know what your humidity is, then you wont have anywhere to start from.  I personally have a radio shack wireless temperature/humidity sensor.  Although this is overkill for most people, I like it because it is wireless so I don’t have to run any wires.  I paid about $80 for the sensor and 2 remotes.  You can get a perfectly serviceable humidity gauge for about $20. 

If your humidity is below 60% at all times you can be relatively certain that you are not growing mold.  If you can keep the humidity below 50% you can prevent all sorts of indoor allergens, such as dust mites.  If you have a humidity problem and your AC is up to par then you might have a water leak somewhere.  This needs to be taken care of immediately.  Most of the nightmare mold stories that I read about are caused by water leaks into wall cavities.  Mold is something to be taken seriously, but there is no need to panic.  If you can control the moisture levels in your house, you can prevent the mold from growing in the first place.