Roof related issues, resolutions and news
Causes and solutions for excessive moisture in attics
The purpose of this page is to increase awareness among homeowners about the potential for moisture problems in attics and how to avoid or resolve them.
Some indications of excessive moisture problems include: rust, moisture or frost and ice buildup on roofing nails; dark stains, moisture or frost on the roofs sheathing; water or drip marks scattered on the attic floor; buckled or damaged roof sheathing; moist or wet insulation. These signs can vary contingent on the catalyst of problem and are generally worse on north facing or shady sides of roofs.
Moisture accumulation is most significant when outside temperatures are very cold because air can hold less moisture as the temperature decreases. If there is excessive moisture accumulation in your attic it will condense on cold areas mainly on the underside of your roofs deck.
During the winter months the recommended relative humidity inside of a house is anywhere between 30-50 percent. At higher levels relative humidity condensation can even occur inside of the house on windows, cold walls or ceiling surfaces. Depending on how tight a house is constructed and the outside temperature, even a relative humidity of 30 percent may be too high. Condensation may occur at low levels of humidity on single pane windows or windows that are not sealed well.
Sources for moisture in a home can vary. Some of the common sources are baths, showers, dish washers, washing machines, dryers, cooking, unvented gas or kerosene space heaters and over use of humidifiers. Even the average person produces about 3 pints of water a day while breathing. These sources are generally not a huge contributor to excessive moisture in the house or attic unless the amount of moisture is unusually high, the house is extremely tight, or proper venting is not used.
Another less noticeable common source of moisture accumulation is water moisture penetrating through foundation walls and floor slabs. Even when basements, floor slabs and crawl spaces do not appear excessively damp they can be a significant source for moisture. Because of the relatively moderate temperature in these areas moisture can go unnoticed and may not cause any problems in these areas. As the moisture migrates upwards to the colder attic areas during the winter months, the relative humidity is increased to the point that condensation can occur on cold surfaces. This can result in mold, rust and deterioration of wood and structural fasteners.
A simple qualitative test can be performed to see if moisture is passing through a foundation wall or floor slab. Take an 18 by 18 inch square plastic sheet and tape it tightly to the concrete and leave it in place for at least 16 hours. The presence of moisture under the plastic sheet is a positive indication that excessive water/moisture is likely present in the slab. However, a negative indication is not an assurance that the slab is acceptably dry below the surfaces. It is recommended to perform this test in several different locations.
How does moisture rise from the basement to the attic? In the winter your homes heating system warms air throughout the house. When water-moisture meets the warm air it evaporates. Moist air has a lower average molecular weight than dry air, so it is less dense. If the top floor of the house is not sealed tight, warm air will escape causing the pressure inside the house to decrease resulting in air infiltration from openings at the lower sections of the house. Infiltration can be caused by wind, negative pressurization of the building, or by air buoyancy forces also known as the stack effect. Any water vapor in the warm air rises until it reaches the cold roof deck area at which point it cools and condenses back to a liquid phase.
Most of the openings that allow air to escape into the attic can easily be overlooked. Some of the more common examples of openings found in a house include: Gaps at pull down attic stairways or other access openings; gaps at the tops of the exterior and interior walls; poorly sealed ductwork; opening in ceiling at wall cavities above stairways and closets. A considerable amount of air can leak through relatively small gaps and some openings can provide a direct path from the basement or crawl space directly to the attic. Air infiltration is also a common cause for heat loss and can significantly increase heating cost.
How do you resolve excessive moisture in your attic? Most will recommend increasing ventilation in your attic if ventilation is inadequate. However, this often does not solve the problem. Increased ventilation alone will increase exfiltration of air lowering the temperature of the attic. This may increase condensation and lower the effectiveness of the insulation. Also, because airflow is not uniform significant condensation may still occur. A moist attic means moist insulation, and moist insulation is less effective.
The best solution for controlling moisture is at its source. If the problem is the result of water-moisture penetrating the foundation or floor slab there are many remedial actions that may help. These can include: installation of seamless gutters and downspouts; if you already have gutters and downspouts make sure they are cleaned and working properly; re-grading the property near the perimeter of the foundation; sealing foundation walls or floor slabs; using a dehumidifier; installing a perimeter draining system. Because of the many conditions that can cause foundation water-moisture penetration and the many solutions, each house should be individually evaluated to determine the cause and the best possible solution for the problem.
After water-moisture penetration is controlled as best as possible, the next approach to reducing attic moisture is controlling air infiltration. Simply by sealing the various possible paths of air leakage already discussed, infiltration will be reduced therefore restricting the movement of air and eliminating the stack effect. Most handy homeowners should be able to handle this part on their own. If not, there are many weatherization contractors available to complete this type of project.
Keep in mind when air leaks are properly sealed reducing air infiltration into the house the relative humidity is higher. This moisture is contained to the living space where it is desirable during the winter months. Tighter homes have reduced heating and cooling costs and are generally more comfortable. If a home is too tight the need for mechanical ventilation might be necessary to assure that moisture or pollutants do not accumulate to unhealthy levels. This is not a concern for most homes even when properly sealed.
Now that the house is properly sealed it is crucial to verify the attics thermal insulation is adequate and properly installed. The insulation on the floor of your attic needs to have an approved vapor barrier installed on the warm-in-winter side of the insulation. In most cases rolled or batt insulation will have a vapor barrier already attached. On the vapor barrier the insulation should be labeled at least an R-30. If that number is lower than you should add more insulation. If the attic has blown in cellulose check to see if there is a vapor barrier and that the cellulose insulation is spread evenly throughout the attic floor. Attics with pull down attic stairs should have attic stair insulation covers installed.
The final step is the attics ventilation. Attics need to have a balanced ventilation system consisting of an intake and an exhaust. The soffits work as the intake and roof vents or ridge vents work as the exhaust. This installation method actually promotes the stack effect to continuously move the air contained within the attic. When a properly balanced roof ventilation system is installed the breeze that blows across the ridge of the house causes air to be drawn from the roof or ridge vents. The air is replenished through the soffit vents. When installing a balanced ventilation system it is important to block any older style gable vents located on the house. These vents will actually short-circuit the desired flow of air.
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