How To Prevent Attic Condensation

Attic condensation is one of the leading causes of house problems. If left unchecked, it damages insulation, framing, and roofing. Moisture invites mold and pests. Preventing condensation in attics increases longevity, reduces repair costs, and lowers energy use.

Attic Condensation

Causes Of Attic Condensation

Attic condensation results from warm moist air contacting cold surfaces or cold air. As warm air cools, it holds less moisture that turns into water vapor. It happens most often in winter and spring. Most warm air comes from the heated living spaces of the house.

In warmer climates, condensation is caused by moist exterior air entering the attic from outside. It meets cool dry attic air or cool ducting and piping in the attic.


Approximately 90% of US homes are under-insulated. Many lack a proper vapor barrier. Gaps and cracks are not sealed between living spaces and attics. Insulation can be installed improperly. Condensation causes insulation to become wet and lose its R-value.

Lack of insulation is also a problem in warmer areas. Attic air is often cooler than outside air because air-conditioned house air seeps into the attic.


Lack of proper ventilation allows warm moisture-laden air to build up in attics. Most building codes require a minimum of one square foot of venting for every 300 square feet of attic floor area. To be effective, vent openings should be split evenly between the soffit and roof peak. Incoming cool air pushes warm air out of the attic.

6 Ways To Prevent Attic Condensation

Add Insulation

Recommended R-values in warm climates are R-39. R-60 in colder areas. The proper amounts help keep warm air out of the attic and seal gaps and cracks. A vapor barrier like 6 mil poly prevents warm air movement better than insulation alone. Vapor barrier may be difficult or impossible to install in some situations.

Adding too much insulation creates more problems if it blocks air vents.

Add weatherstrip to attic access hatches or doors and ensure they seal properly.

Increase Ventilation

Add vent openings to wooden soffit to draw more outside air into the attic. (Cover with screen to prevent attic pests.) Install ridge vents, roof vents, or gable vents. The number of square inches of upper and lower venting should be about equal. Ventilation can be easily increased to 1.5 square feet per 300 square feet of attic floor area without adverse effects.

Mechanical Ventilation

Install a fan in the attic to increase air movement. They are available in hard-wired form, plug-in, or solar-powered. Mechanical ventilation is particularly valuable on hot days without wind.

Extend Vent Pipes

Kitchen and bath exhaust fans should vent outside. Many fans move 100 cubic feet per minute (CFM) of moisture-laden air. Ensure that they extend through the roof. Plumbing vent stacks also need to be extended outside to eliminate moisture and odor.


Any water leaking into the attic adds to the moisture content of the air. Find and repair all attic leaks. Standing water and wet insulation are a constant source of attic moisture that evaporates into the air.


Using a dehumidifier in the attic reduces moisture. Dehumidifiers work well but unless they can be drained automatically, they require constant attention. Many units have sensors that turn them on and off as required.

Why Prevent Attic Condensation

Over time, attic condensation can cause extensive time-consuming, and costly damage.

  • Mold. Humidity encourages mold growth.
  • Rot. Wet moldy framing members and roof deck eventually rot causing structural damage.
  • Pests. A source of moisture attracts pests like mice, squirrels, and termites.
  • Wet Insulation. Wet insulation loses its R-value and is costly to replace.
  • Ceiling Damage. Moisture soaking into drywall can cause mold and expensive damage.