Renovation & Design

Reducing humidity key to solving condensation issues

Question: I live in Louisiana and have been trying to solve a moisture issue. I was Googling and stumbled across one of your columns on your site. When the temperature drops outside, we have had moisture accumulate on the back side of a light switch cover and water drips down the wall. Sometimes it seems worse when we have the heater on in the house, but I am not sure if that is just because heating the house coincides with cooler weather. I suspect the issue could be a temperature differential causing moisture to condense inside of the wall. There is a hole in the slab underneath a bathtub, with exposed soil within the wall cavity behind this light switch. Is it possible that moisture is coming up through the soil and condensing on the cool wires and pipes within the wall during the winter? I realize you are in Canada, but you seem very informed on resolution of moisture issues. Can you make a recommendation on how best to solve this problem, to minimize moisture and prevent mould within the wall cavity?

Thank you, Corinne Meyer

Answer: Moisture dripping from light switches, electrical receptacles, or lights in cooler weather is most likely caused by condensation. Reducing the source of the moisture is the best remedy, but sealing around the electrical box containing the switch may be the easiest first response to stop the leaking.

When water is seen leaking or dripping near an electrical fixture, switch, or receptacle the first question I always ask is what was the weather like at the time? If the dripping happens when it is raining, or when the snow may be melting off a roof, then leakage from outside is the most apparent potential cause. Especially if the leakage occurs on or below the uppermost ceiling in the home, it is almost always getting in through a roof gap or penetration. Moisture stains, normally brown in colour, can often be seen around these areas before actual liquid water is noticed. Either way, an experienced roofer should be immediately called to go up on the roof and seal or repair anything that appears to be the source of the leakage. Also, looking up in the attic while the dripping is occurring, with a powerful flashlight, may further shed light on the point of entry.

When moisture is seen in colder weather, with the possible exception of a summer cold spell, condensation is undoubtedly to blame. Until recently, most electrical boxes were made of galvanized steel. As we know, metal is an excellent conductor of heat, so the box that houses the switch in your bathroom is subject to condensation, in certain conditions. When the weather suddenly changes and temperatures drop, the warm air surrounding the metal box in this area may reach its dew point. When that occurs, dissolved moisture in the air will condense, often on the coldest surface nearby. In your case, the metal box around the light switch may be the perfect target. Because the box is located inside a relatively small space in the wall cavity, there may not be enough fresh air circulation to easily allow that condensation to dry. Once enough moisture has built up, it can drip down the bottom of the box, the switch, or the cover plate, onto the surface of the wall below.

Diagnosing the cause of your leakage may be quite straight-forward, but locating the source of the high moisture may be trickier. You are correct that the exposed soil beneath the bathtub could be one possible source, but that may not be the most significant factor. More likely, the moisture from bathing, showering, and washing in the bathroom more directly raises the relative humidity (RH) in that room, making condensation much more prevalent. To combat that, ensure that you have a good quality exhaust fan that vents to the exterior, is clean, and in good working order. Even better, replace the fan switch with a timer-switch, so it can be left running well after steaming up the bathroom.

If you have access to the area under the tub, through a removable access cover on the tub or wall behind, filling in the concrete to close the hole where the soil is exposed may also lower the humidity. Otherwise, filling in or sealing any areas where air can pass between the soil and the offending wall cavity may prevent excess moisture migration. Regardless, sealing the open area of soil should be done to prevent pest and moisture intrusion and mould growth, especially in a location that has very limited accessibility.

Once the RH is reduced in the bathroom the excess condensation should abate, but further simple remediation may help, even before that is accomplished. Sealing the outside of the metal junction box, with expanding foam from a can, may help prevent it from cooling quickly and allowing an ideal location for condensation to form. This should be done after turning off the circuit breakers for the switch and temporarily removing it. The foam may be applied with the attached plastic straw between the box and the drywall, and/or by poking the straw through gaps or holes in the metal box, itself. Make sure as much of the outside of the box is covered as possible, and seal between the box and the drywall, to further prevent air and moisture leakage.

Dripping from the light switch area in your bathroom is most certainly related to condensation from the air in that high humidity location. Insulating and air sealing the electrical box should help prevent the leakage, but reducing the relative humidity in that room, by using a good exhaust fan and timer-switch, should be the ultimate solution to your problem.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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