The first priority is to prevent or fix leaks of liquid water into the building or building envelope. Water entry through the above-grade roof and walls and water entry through surfaces in contact with the soil should be prevented. For above-grade leaks, normal maintenance measures, such as replacing or repairing the roof will often be all that is needed; however, professional assistance may be needed if there were errors in design or construction of the home. Below grade water entry is addressed by directing downspouts away from the foundation, grading of soil to slope away from the building (when possible), and having functional below-grade foundation water draining systems. In some situations a sump and sump pump may be required. Water leaks also occur from plumbing, both water supply and sewage plumbing, and these leaks need to be immediately fixed.
Cooking and bathing are key indoor sources of water vapor. Bathroom exhaust fans and range hoods that vent to outdoors should be used to vent much of the generated water vapor to outdoors. If exhaust fans are not available, window opening may help, except when it is hot and humid outdoors. Clothes dryers must also be vented to outdoors.
When it is hot and humid outdoors, the outdoor air that enters a home is an important source of water vapor or indoor humidity. Operation of air conditioning systems or dehumidifiers may be necessary to control indoor humidity. An air conditioning system condenses water vapor when it cools the air and the liquid condensate is drained to outdoors. High rates of outdoor air ventilation will make it more challenging to control indoor humidity when it is hot and humid outdoors.
When it is cold and dry outdoors, the outdoor air will have less water vapor than the indoor air. Indoor humidity can usually be maintained at an acceptable level by providing recommended amounts of outdoor air ventilation, preventing water leaks, and using bathroom fans and range hoods that vent to outdoors.
Even when the indoor air humidity is not elevated, when it is cold outdoors water may condense on the inside surfaces of windows, particularly energy inefficient windows, such as single pane windows. Water may also condense on walls or inside walls if there is insufficient thermal insulation or if the walls have been poorly designed or improperly constructed. The best solution is to fix the underlying building envelope deficiency, for example, by adding insulation or installing energy-efficient windows. Operation of bathroom and kitchen fans will help and dehumidifier use will help in some situations.
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