Checklist for Attic Moisture Problems

High moisture in the attic of a house can cause significant problems. Water vapor in the attic air can condense on wood surfaces, causing deterioration, mold, and rot on the framing and sheathing.

Figure 1. Frost forming on the underside of the roof sheathing

Figure 2. Roof sheathing damaged by condensation

When problems occur, the following are some common contributors to elevated moisture in an attic:

Roof leaks – Common causes of water intrusion through the roof include improperly installed flashings, deteriorated rubber boot flashings around roof penetrations, missing shingles or tabs, and ice damming.

Bathroom and kitchen ventilation fans – Bathroom ceiling fans and kitchen range fans ventilate warm, moist air from the bathroom or kitchen. Codes require that these fans be ducted and exhausted to the outside. However, many older houses and even some newer houses improperly exhaust bathroom and kitchen fans directly into the attic.

Heating ducts – Supply and return-air heating ducts are sometimes routed through the attic. If the joints in the ducts are not properly sealed, warm, moist air from the heating system can leak into the attic.

Damp basement or crawlspace – Humidity from the basement or crawlspace can permeate through the house and eventually to the attic. Actually, a wet crawlspace or basement can be the number-one contributor to high relative humidity in the attic. Water or moisture in a basement or crawlspace can have many sources: a crawlspace floor with no vapor barrier, poor outside surface water drainage, downspout discharges too close to the foundation wall, overflowing gutters, plumbing leaks, air conditioner condensate draining into the crawlspace, water softener recycle water discharging into the crawlspace, and clothes dryer vents exhausting into the crawlspace. Controlling moisture sources and providing adequate ventilation at the lower levels of the house will reduce relative humidity throughout the house, including the attic.

Central humidifier – Some houses have central humidifiers built into their heating system. The purpose of a humidifier is to reduce discomfort due to dry air in the winter. Some humidifier controls are linked to an outside temperature sensor that automatically adjusts the settings for the humidifier. If a humidifier is not automatic (most are not), the homeowner must manually adjust the humidifier controls as the outside temperature fluctuates. If this is not done and the outside temperature drops, then the humidifier can add too much moisture into the house. During an extended cold spell, this can cause condensation and even frost to form on cold surfaces in the house and attic.

Other indoor moisture sources – Day-to-day living activities in a house contribute to air moisture. Showers, tubs, clothes washers, dish washers, cooking, steam cleaners, steam irons, aquariums, portable humidifiers, vaporizers, firewood storage, and house plants all add moisture to the air. Excess moisture from any of these sources can raise humidity in the attic.

Attic ventilation – Keeping all moisture from reaching the attic is not possible. Therefore, properly ventilating an attic is essential. The recommended minimum amount of ventilation is 1 square foot of ventilation opening per 150 square feet of attic area. The ventilation area should be split between lower openings (soffit vents) and upper openings (box or ridge vents). This will create a natural flow or chimney action from the lower to upper vents.