It can be very difficult, even for a pro, to track down the cause(s) of high relative humidity inside a home. A frequent cause is an improperly sized HVAC system. Your AC system needs to run long enough to pull moisture out of the air, not just cool it. So if your system is too big for your home, it will not run long enough and high relative humidity will result. Generally you need one ton of AC for every 600 square feet. Sizing a little small is actually better than sizing too big, and it will not be less energy efficient. It will have an easier time cooling and you will feel comfortable at higher temperatures, so you do not save money by increasing the AC tonnage.
Sometimes, after installing new hurricane impact windows, homeowners notice an increase in relative humidity. This occurs because the home is “too tight” and the AC is not running long enough. Your AC unit works by running so that the air passing by the cold coil condenses, thus dehumidifying the house. The AC needs to run at least 10 minutes to control humidity on a cold coil. If the house is very tightly built and insulated, and particularly if it has a lot of non-porous glass windows, it will not “breathe” and it will cool or heat faster than the AC system can pull moisture out of the air. This is compounded by moisture released from cooking and showering, as well as moisture trapped in carpet and wood paneling. In a very tight house, if it is 82 degrees outside, your AC won’t run very long to maintain a temperature of 76 degrees and you may have an issue with high relative humidity.
A leak in your ducts can also throw off your AC system. In fact, a dirty AC filter can throw off your relative humidity, so that is one more reason to routinely change your AC filter on the first of every month. An improperly working thermostat can also be a culprit. There are also more serious situations, such as humid crawlspaces that release moisture up through a wood floor. This does not occur in the typical crawlspace, but can be an issue if there is a plumbing leak or other problem that causes ponding water.
A whole house dehumidifier system is a last resort in severe cases where the humidity hovers around 65% or more. However, such a system will be necessary to prevent the growth of mold and wood rot. The cost to install a single whole house dehumidifier is around $1,700 in 2013, but will vary depending on the size of your home, the difficulty in removing debris, and the existing wiring in your home.