Investigating Attic Condensation

By: Russ Zeckner, P.E., CFEI, Forensic Engineer & Principal Consultant

Attic condensation and resultant mold growth are common problems for homeowners.  Attics generally have limited access and homeowners seldom, if ever, enter them unless they suspect a problem or if they store seasonal items there.  My personal experience suggests that these problems are most commonly detected in the months between November and January when homeowners either retrieve or store their Christmas decorations.  As a result, attic condensation or its results may not be detected until long after the conditions have changed, making it difficult to determine its cause.

Signs of Attic Condensation
The first sign of an attic condensation problem is most commonly mold because its dark colored patches stand out against the typically lighter colored wood of most roof sheathing.  The actual moisture that promotes the mold’s growth can go unnoticed because of its natural clarity and its appearance as a collection of very fine droplets.  Regardless of whether moisture or mold is detected first, condensation is the underlying cause.

Condensation most commonly forms in an attic on the underside of the roof’s sheathing.  Unlike roof leaks’ typically well-defined and isolated locations, condensation and its resulting mold will often cover the underside of an entire roof slope and be uniform in appearance.  If condensation amounts are significant, the moisture can drip from the sheathing and onto the top of the ceiling below and may give the impression that a roof leak has occurred.  These stains will likely be widespread and random rather than concentrated in a location as is typical for a roof leak.  Such symptoms will often occur in the absence of a rain event.

Condensation Causes
Like all condensation, attic condensation occurs when moisture in the attic’s air encounters a surface that cools the air to a temperature at or below its dew point.  Since the air’s temperature is susceptible to change, its ability to hold moisture can shrink or swell, holding more when it is warmer and less when cooled. 

Depending on the season, the humid air in the attic can originate within the house below, in its basement or crawlspace, or from the exterior (Figure 1).  Inside the house, the air will probably not feel humid or uncomfortable and if a humidistat is present, its readings can be well within the normal range of 40% to 60% relative humidity with a temperature of 68 to 72 degrees Fahrenheit.  However, during cooler months when this air migrates into the attic as part of a house’s normal air circulation, its temperature can be changed dramatically, particularly when it contacts materials with surfaces that are exposed to the exterior.  If those surfaces cool the air sufficiently, then condensation will form. 


Figure 1: Heated Air Pathway into Attic


The best chances for these surfaces to have cool enough temperatures for condensation to occur will occur during winter months, but such conditions can occur in summer months as well.  Evening temperatures can drop sufficiently to produce condensation and thunder and hailstorms can also create the necessary temperature differentials. 

Although normal humidity levels can produce condensation, elevated humidity levels make the problem more likely to occur.  Normal human activities such as breathing, bathing, cleaning and cooking all place moisture into a house’s air, but generally do not cause problems.  Excess humidity can occur because of damp or flooded crawlspaces or basements, maintenance problems such as improper clothes dryer, bathroom fan or kitchen hood exhausts, HVAC equipment failures, unvented combustion appliances, and even lifestyle issues. 

Lifestyle issues are the most difficult to diagnose because they might not be consistent or apparent during an investigation.  An informed interview of the homeowner may be the only way to obtain the necessary clues.  Lifestyle issues can include: changes in habitation, such as when a baby enters a family or when adult children and perhaps their families move back home; new hobbies such as candle making, brewing or pets are undertaken; or age- and health-related changes of the homeowner and the effects of medications they may consume.

Attic Ventilation
In attics, ventilation also plays an important role in the formation of condensation and mold.  Warm moisture-laden air enters the attic from the house by design.  Soffit and roof vents are intended to promote this ventilation in order to bring fresh air into the living space and expel stale and humid air to the exterior.  A typical house should have at least one complete exchange of its air every three hours, but this will differ depending on the age and construction of the dwelling.[2]  When designed and functioning properly, such ventilation should be able to remove excess humidity from an attic before it produces harm.

Problems with attic ventilation commonly occur because the soffit vents are blocked or simply aren’t provided.  Soffits can be blocked by improperly applied insulation in the eaves, or by paint filling the grills of the ventilator covers.  Without functional soffit vents the convection currents cannot form and little or no air will be exhausted from the roof vents.  When such conditions are present mold will often be present on the roof sheathing except within a few inches of the roof vents’ penetrations.

Short circuiting often occurs when a homeowner attempts to improve the attic’s ventilation, but inadvertently worsens it instead.  Powered ventilators contain an electric fan that is intended to suck hot air from within an attic and expel it to the exterior.  They are commonly sold as energy saving devices and are usually thermostatically controlled.  If, however, they are installed adjacent to other passive roof ventilators they can simply pull exterior air in through those nearest vents and then discharge it again with little benefit to the attic’s ambient conditions and in some climates even make the problem worse by introducing more humid air into the attic.  This problem is exacerbated if the soffit vents are missing or blocked.

Investigating Claims
When investigating an attic condensation claim consider the following:

  1. Is the problem really the result of condensation?
    1. Attic condensation will be widespread and typically will appear on the underside of the most northern facing slope’s sheathing.
    2. Condensation in an attic will form first on nails that penetrate the sheathing and thus check them for evidence of corrosion.
    3. Localized moisture problems, particularly around penetrations and at roof valleys are likely the result of leaks and not condensation.
    4. Check for drip marks on dust-covered surfaces on the attic floor.
  2. Is the attic ventilation adequate?
    1. Are roof and soffit vents present and unblocked?
    2. Do power vents produce short-circuiting of the ventilation?
  3. What is the source of the excess moisture?
    1. Wet crawlspace, basement or cellar?
    2. New hobbies including pets, canning, brewing, candle making, etc.
    3. Maintenance issues such as improperly connected clothes dryer vents, bathroom fans or kitchen exhaust hoods.
    4. Are unvented combustion appliances being used such as gas logs, stoves, or kerosene space heaters.
    5. Have changes in lifestyle occurred such as increased habitation, or a reduction in dehumidification due to desire for a warmer environment.


[1] Pierce, Mark, Causes and Cures of Attic Condensation and Roof Ice Damming Problems, Cornell University Cooperative Extension, February 2005.



About the Expert

Russell Zeckner joined Donan in 2000 and currently serves as forensic engineer and principal consultant, a position held by Donan professionals with the highest level of technical expertise and industry experience. Mr. Zeckner is a subject matter expert in mold, foundation failures, water intrusion, plumbing, spontaneous combustion, fire suppression systems, and other areas. His previous engineering experience includes work in the following industries: wastewater treatment, water supply, air pollution control, environmental compliance, safety engineering, hazardous waste, superfund cleanup, and above ground and underground storage tanks. Mr. Zeckner works on a variety of projects at Donan including water intrusion, structural damage impacts, construction defects, environmental issues, residential plumbing, and mold problems. He also provides fire investigator support related to appliance or similar fire causes. Mr. Zeckner is a licensed professional engineer in Indiana, Kentucky, Ohio, and Tennessee, and is a Certified Fire & Explosion Investigator. He earned his Master’s degree in Civil Engineering from the University of Louisville’s J.B. Speed School of Engineering (formerly Speed Scientific School).

View his full professional profile here.