If you usually keep your house closed up in the summer and keep your air conditioner and ventilation system operating, check the indoor RH in various rooms throughout your house with a calibrated hygrometer.
If the relative humidity reads above 50 per cent, running a dehumidifier can take away the excess moisture. The highest RH will often be in the basement and, if that is the case, that is where you should put your dehumidifier.
If you open windows for ventilation in the spring, summer and fall, you will likely find the indoor RH on the upper floors of your house to be higher than 50 per cent. This will not cause problems such as dampness and comfort problems in the warmer parts of the year, but the house will accumulate moisture through the summer. That retained moisture can cause significant problems in the fall when you close the house up. You will have to reduce humidity aggressively during the few short weeks of dry fall weather through dehumidifier use and increased ventilation to avoid these problems.
For houses with no air conditioning, a basement dehumidifier can usually keep basement RH below 60 per cent preventing moisture and comfort problems.
Source: CMHC
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