Moisture Rides on Air Currents, and Warm Air Carries More Moisture Than Cool Air
To control air flows, make sure the air barrier is continuous
An air barrier helps control airflow both through and within the building enclosure. By controlling airflow, you also control moisture.
If moist indoor air contacts a cold surface — for example, exterior sheathing in cold weather — condensation can result. An air barrier prevents those cold surfaces from being connected with humid indoor air.
Air has a maximum storage capacity for water vapor which depends on temperature. Warm air can store lots of moisture, while cold air can store very little.
As the temperature falls from 90°F down to 20°F, the amount of moisture that can be stored in the air changes by a factor of ten.
Leaky homes didn’t have condensation problems
Older buildings rarely had condensation problems in cold weather because they were so well ventilated — meaning leaky. The relative humidity in an old home would rarely rise above 25%. As we have built tighter houses (and in some cases failed to provide mechanical ventilation), the indoor relative humidity has gone up.
In a heated, tight, unventilated house, the amount of moisture in the air and the amount of condensation that can occur are dramatically different than in an old leaky house. Condensation can occur wherever water vapor can find a cold spot — on roof or wall sheathing, on the inside faces of the windows, and inside the walls.
Let's say it’s 40°F outside and the outdoor relative humidity is 50%. If you allow that outdoor air to enter a building and heat it up to 70°F, the amount of moisture in the air stays exactly the same, but the “tank” gets bigger because the storage capacity of the air increases with the temperature. As a result, the relative humidity initially drops. Then, as moisture is added to the air, the relative humidity rises, and the absolute moisture content rises as well. How do you add moisture to the air? You breathe, sweat, boil water for spaghetti, take hot showers, grow houseplants — and all of those activities generate moisture.
When does indoor humidity become a problem?
Let’s say that air leaks out of a house through holes in the enclosure. As it reaches surfaces colder than 52°F or 53°F, the air will cool. Once it reaches its full capacity to store moisture, condensation occurs.
If the temperature of the outdoor air is around 30°F, the indoor air will drop all of the moisture that it gained on the way out, dumping it on the cold sheathing surface. That’s a typical example of the air leakage condensation cycle. Since condensation in walls can cause puddles — and in extreme cases, rot the framing — condensation is something you want to avoid. Installing an air barrier is one way to help prevent condensation.
Air conditioning can also create condensing surfaces
The same phenomenon can happen in reverse in the summertime. Let’s say the outdoor air is 85°F and the relative humidity is 75%. When outdoor air leaking inward contacts a surface below about 76°F, the moisture in the air will condense. So if you have an exhaust fan in your home, the air leaking in may cause condensation on the air-conditioned surfaces — for example, on the back side of vinyl wallpaper.
If moist air leaks into a house through gaps in the wall or roof, you can have problems. But in a tight house with a good air barrier and a supply-only ventilation system, most of the air that’s drawn inside is drawn in through the air conditioner, so the first cold surfaces it sees are the cooling coils.
If you have air leaks in your building envelope, you usually can’t see the condensation —Unless you have an Infrared Camera with a Visual Inspection. However, condensation is sometimes visible in the attic; all you have to do is look for frost or dampness forming on the underside of the OSB or plywood roof sheathing.