Controlling Humidity in Florida Homes Year Round

When most people think of problems with high humidity in Florida homes, they think of summer weather, when outside humidity is extreme.  And such problems do develop, for many reasons.  However, humidity problems are also often common during spring, autumn and winter, especially when outside weather is cool and humid for extended periods. It’s important to be able to control humidity in Florida homes year round, not just during the summer. Under such conditions insufficient heat load is present to operate the AC system for long enough to keep up with internally-generated humidity (from cooking, showering, even breathing!)

Several factors often interact to make problems with humidity in Florida homes during cool weather worse:

  • Modern, highly energy-efficient homes are usually less well-ventilated to the outside. Internally-generated humidity is not diluted as much by infiltration of drier outside air.
  • Recent years appear to have more cool but humid weather in Florida as compared to past years when cool/cold weather was often drier. It is possible this is related to “climate change.”
  • Modern AC systems may in practice be less efficient at removing humidity, even when specifications indicate they should be more effective. In modern, tightly-built homes it is also easier to accidentally oversize an AC system, reducing its humidity removal ability significantly.
  • Exterior walls, windows, closets, etc. often form “micro-climates” with significantly cooler surface temperatures. The relative humidity on these surfaces is proportionately higher, creating the potential for mold growth and other problems.
Effects of Surface Relative Humidity in Florida Homes
humidity in Florida homes

In this thermal (infrared) image, the various colors show temperatures differences on an exterior wall. The temperature of the room air was 75°F, relative humidity (RH) was 50%, dew point temperature was 55°F. As shown by the vertical bar on the right, surface temperatures varied from 56°F to 72°F. Even at these really quite dry interior conditions, the numbers on the image show the RH on the surface at various points was greatly elevated. Any surface RH above 60% and especially 70% is quite capable of supporting mold growth.

Controlling humidity in Florida homes means reducing the dew point temperature, which is the temperature at which moisture condenses out of the air onto a surface.  Reducing or controlling the amount of moisture in the air, also called humidity, accomplishes dew point control.

If desired, actual dew point can be easily calculated and monitored. Purchase a thermometer/hygrometer, available at home centers, to measure temperature and relative humidity. With these numbers, use a psychrometric chart to determine the actual dew point.  For instance, here is how changes in relative humidity in Florida homes affect the dew point temperature at a temperature of 75°F.

(http://www.thermmax.com/psych-chart.jpg)

  • 75°F / 75% RH = 67°F dew point temperature.
  • 75°F / 65% RH = 62°F dew point temperature.
  • 75°F / 55% RH = 58°F dew point temperature.
  • 75°F / 45% RH = 52°F dew point temperature.
  • 75°F / 35% RH = 46°F dew point temperature.

An on-line psychrometric calculator that provides dew point temperature and other information when temperature and relative humidity are entered can be found at http://www.linric.com/webpsy.htm. Excellent free apps for these calculations can be downloaded from both the Google Play (https://play.google.com/store/apps/psycit) or Apple Store (https://itunes.apple.com/us/app/munters-psychroapp).

Significant condensation is much less likely to occur if the dew point temperature is 50 degrees or less than it is at higher dew points. Even when condensation does occur at lower dew points it is lower in volume and therefore causes fewer problems.

Dehumidification in Florida Homes

The most convenient way to control humidity in Florida homes is with dehumidification equipment. We like units that bring in outside air, deeply dehumidify it, then inject it into the home.  This creates positive pressure in the home, with multiple advantages for controlling moisture in the building envelope, ensuring good ventilation, etc.  IET has had great success specifying the highly efficient Ultra-Aire ventilating dehumidifiers.  (www.ultra-aire.com) These units control humidity effectively year-round.

However, less elaborate dehumidifiers can also be effective.  40 or 60 pint dehumidifiers from Walmart or Home Depot cost only a few hundred dollars.  One or more of these can easily remove excess humidity in Florida homes and prevent problems.  For convenience, we recommend the auto pumpout models or setting the unit up so it can drain rather than having to empty a bucket regularly.

Alternatives for Controlling Humidity in Florida Homes

It is also possible to maintain lower dew point temperatures and humidity in Florida homes in cool or cold weather without incurring additional costs for equipment or energy.  They can be used separately or often more effectively in combination:

  1. Assuming the bathroom exhaust fan has sufficient power to actually exhaust the room air when the door is closed, keep the door closed while showering and for perhaps half an hour afterwards, with the fan operating continuously.  Holding a sheet of toilet paper up to it while it is running can check exhaust fan effectiveness.  The fan should suck the paper up and hold it from falling.  If the fan does not have sufficient power to do this with the door closed, leave the door ajar a crack while showering and for some time thereafter.  Consider installing a grille or louver in the door, or trimming the lower portion of the door to allow air to enter the room with the door closed.
  2. When the weather is unusually cool, avoid extensive cooking, especially baking and extended boiling, or other types of cooking likely to add a lot of water vapor to the unit’s air. This increases dew point temperature and humidity in Florida homes.
  3. On unusually cool nights, consider leaving bathroom exhaust fans, and the kitchen range hood, if it vents to the outside, operating continuously.  This will put the unit under a slight negative pressure relative to the outside, pulling in a small amount of outside air.  As this cool, dry air enters and warms up, its relative humidity drops dramatically, and when mixed with the rest of the indoor air reduces the relative humidity of the air and its dew point temperature.
  4. Consider leaving one or more windows open a crack to allow outside air to enter.  This is another way of accomplishing the same goal as in number 3 above.  They are especially effective when used together. Make sure that natural airflow convection patterns past any partially-open window are not impeded by blinds or other furnishings, as the infiltration of air through the slightly open window will allow the air temperature between the blinds and the window to drop even lower and will cause additional condensation at this point, although condensation elsewhere would probably be eliminated.  Forced airflow past this point from a fan, even a very small fan, will also help prevent chilling of surfaces and resultant condensation in this area.
  5. Before going to bed, open the sliding glass door or windows fully for a few minutes and allow some cold outside air to enter.  This is a more direct approach to number 3 above, and tends to have the same effect, but much more quickly.  If the entry of exterior air causes the room temperature to drop even a couple of degrees while the door is open, dew point temperature of the air after it regains its normal temperature is likely to be at least 10°F lower and to remain significantly lower overnight.

Note: All these approaches should be carefully monitored using the hygrometer discussed above.

Note:  Controlling humidity in Florida homes year round in a hot/humid climate can be challenging.  Here is an excellent resource that spells out a number of options: http://www.buildingscience.com/documents/bareports/ba-0505-residential-dehumidification-systems-research-for-hot-humid-climates

Feel free to Contact IET to learn about our services or schedule an inspection.

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