Q: Two years ago, I purchased and renovated a three-bedroom, ranch-style home. In the winter, condensation accumulates daily in a swath 2 or 3 inches wide in the guest bedroom where the ceiling meets the north-facing wall. I have to wipe off the water at least twice a day. I renovated the house soon after I bought it, so I do not know if this problem existed previously. The only structural changes I made affecting that room were a new roof and new vinyl siding on part of the exterior. I have ruled out roof leaks, and the contractor came back and upgraded the attic insulation. That helped a little, but not entirely. I have had the gas hot-air furnace and humidifier checked (they are operating properly) and I have turned down the humidifier to 22 percent, lower than recommended for winter. I keep my thermostat at 69 degrees during the day and 63 overnight in the winter. The guest bedroom door stays open most of the time. What can be done to stop the condensation?

Kensington, Md.

A: Undoubtedly, the condensation is happening because warm, relatively moist air is touching a surface cold enough to force some of the moisture to become liquid water, rather than water vapor. The temperature at which this occurs, called the dew point, is elastic. It varies depending on both the temperature of the air and the relative humidity. Online calculators, such as on calculator.net/dew-point-calculator.html, show that if the air in your guest room is 69 degrees and the relative humidity is 22 percent, the dew point would be 29 degrees — cold enough to form ice, not just water droplets.

For a ceiling in a heated room with a door open to reach that dew point, something is obviously amiss. The actual temperature and relative humidity are probably not what the settings on your equipment indicate you want, and there could be a combination of other issues, such as air leaks, surfaces that aren’t insulated properly and humidity from something other than your humidifier (which you might not need).

Mohammed El-Ghoul, owner of Home Energy Saving Solutions (301-842-8818; marylandenergyaudit.net) in Rockville, Md., suggested you start by hiring a company to do an energy audit that includes use of a blower door and an infrared camera. His company typically charges $400 for this test, El-Ghoul said, but in Maryland, the state subsidizes much of the cost, leaving you to pay only $100.

A blower door is a fan secured in an exterior doorway. By pulling air out of the house, it lowers the air pressure inside so the auditors can use a smoke pencil or other equipment to trace places where air is sneaking in through gaps. The Energy Department has a primer that explains the process and what needs to be done to ensure accuracy; read it by going to energy.gov and searching for “blower door tests.”

An infrared camera sees light that is in the infrared spectrum, which is invisible to the human eye. By scanning inside or outside surfaces of a house, the camera detects temperature variations, which help pinpoint where insulation is missing or inadequate. When the camera is used while a blower door is running, air leaks are especially noticeable. To read the Energy Department’s advice about how to prepare for this type of test, go to energy.gov and search for “thermographic inspections.”

El-Ghoul said the problems in your case could include heavy air leakage in your guest bedroom wall or the ceiling above. Or the insulation that the contractor added might not be in direct contact with the ceiling. That would keep the ceiling from staying at relatively the same temperature as the room, creating the cold band where the droplets are forming. Insulation where roof rafters meet the north wall could be too compressed or thin because space there is tight, or there could be a problem with the baffles that are supposed to direct outside air from eave vents up and over the attic insulation near the wall. It’s possible, too, that a bathroom fan could be part of the problem. Some don’t actually move much air, allowing relative humidity in the room or rooms where air flows to be much more moist than you intend.

It might seem that installing a dehumidifier in the guest room and keeping that door closed would solve the condensation problem, but El-Ghoul recommended against that. “A dehumidifier would be a Band-Aid,” he said; it wouldn’t address the underlying cause or causes. But if you start with an energy audit, “I’m pretty sure by the end I’d know what the problem was,” he said.

Scott Eslick, a comfort adviser at James A. Wheat and Sons (301-670-1944; wheatandsons.com), a plumbing, heating and air-conditioning company in Gaithersburg, Md., said he received a call from a customer with the same issue you have. He sent out a crew to check out that house. Their conclusion after going out and looking around? “My guys recommended an energy audit for the customer this morning.”

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