Condensation is caused when warm, moist air moves into a cooler air space, or comes in contact with a cooler object. The warmer the air is, the more ability it has to hold water. The cooler the air is, the less ability it has to hold water (i.e., when warm, interior air comes in contact with a cold surface such as a metal window frame or a cold water pipe, the vapors in the air turn to liquid on the colder surface).

Moisture in the Attic

In an attic during the winter, moisture or ice may form on the roofing nails and the roof sheathing for the same reason.

Moisture in the Bathroom

Mould/fungus develops in bathrooms when vapors permeate through the surface of drywall or plaster, and turn to moisture before it exits through the other side of the wall material. This is due to the change in temperature between the living space (warm side of the wall/ceiling), and the exterior (cold side of the wall/ceiling).

On milder days (or during milder climates) condensation is less of a concern because the temperature differential between the inside and the outside are not far apart. The solution is ventilation, additional insulation, or stopping the permeation of vapors through the wall/ceiling material. In existing situations, access is typically difficult for proper ventilation or insulation, so the simplest approach would be to paint the wall and ceiling surfaces with a material which would significantly reduce or stop the permeability of vapors. Most enamel or hard finish paints will do a good job controlling or reducing the amount of vapor which may pass through the wall/ ceiling surface.


The air in the attic is warmer than the outside air in the summer or winter. It is obvious how warm attics are in the summer, but it might not be so obvious in the winter. Assuming the outside temperature is 35 degrees, the thermal loss from the house will raise the attic temperature 5 to 10 degrees, depending on the insulation, wind, etc.

Based on this information, the air in the attic will always be lighter than the air outside because it is warmer.

The best way to ventilate an attic would be high-low ventilation because the only condition we can be sure of is that warmer air will be lighter, and have a tendency to rise if the opportunity presents itself.

With high-low ventilation, warm air rises out of the high vents (preferably ridge vents), and this air is replaced by air from low vents (typically soffit vents).

The amount of air, and the speed at which it moves, is dictated by the temperature difference between the attic space and the exterior. This is called Thermal Convection. When the ventilation is correct, mother nature will control the air changes, and mechanical help is unnecessary!

General Criteria for Improving Attic Ventilation

  1. For every 300 square feet (SF) of attic floor space, you need 1 SF of clear If the clear air is not distributed 50% high and 50% low, you will need additional ventilation.
  2. Louvers are typically only 50-60% of their total measurement in clear Be sure to calculate this when you buy louvers or vents.

NOTE: If you cannot develop high-low ventilation, you will have to increase the horizontal or high ventilation by 50-100%, or approximately 2 SF for every 300 SF of attic floor space.

Without a proper vapor barrier, it may be necessary to have considerably more ventilation.

attic condensation
thermal convection