Frequently asked questions about a common problem in some climate zones
Mold is a 4-letter word in real estate and mold remediation is a multi-billion-dollar industry that thrives on the fear created by the lack of metrics available to quantify the risk of mold exposure in our buildings. Mold issues in buildings are complicated by television shows that use the fear of mold to dramatize their entertainment.
Unless home inspectors offer mold testing as an add-on service, mold is beyond the scope of a home inspection. Despite this limitation, we are constantly forced to discuss mold in the course of our home inspections and mold questions can quickly put a home inspector on the defensive; you can run but you can’t hide. This article will focus on mold in attics and is laid out as a series of questions and answers. The hope is that this format provides some useful tools for understanding this complex subject.
First, a Few General Mold Questions
Is mold in my house really a problem?
No. There is only one way to control mold in buildings and that is to keep them dry. Dry buildings do not have mold problems. Wet buildings have mold problems. If you have mold in a structure, I would direct all available resources to diagnosing and correcting the water problem as this will also solve the mold problem. This is an important distinction when seeking corrections to mold in buildings because a focus on the water problem allows a clear approach to remediation, where a focus on mold will lead to endless confusion due to the lack of reliable metrics.
Is mold harmful to my health?
Possibly. The common perception is that prolonged exposure to any species of mold can be harmful to human health and that humans can experience allergy, infection, irritation, and toxicity from mold exposure.  This perception is backed up by popular opinion, media, and even scientific papers, one of which is included here as a footnote. Whatever your opinions and thoughts are regarding mold as a health issue, it is worth considering the long scientific journey needed to travel from association to correlation and eventually to proof of causation; we seem to be a long way from proof of causation now. If you are certain that mold exposure causes human health hazards, it is worth reading:
The Health Effects of Mould by Caoimhín P. Connell. http://www.forensic-applications.com/moulds/sok.html. In this article, a convincing case is made that we have surprisingly little science to prove that mold exposure in buildings can affect human health and a great deal of evidence to suggest otherwise. In short, it can be difficult to distinguish the real dangers from mold exposure from the common misconceptions that are sold to us daily on television shows and by testing and remediation companies who profit from the fear.
 Is Indoor Mold Contamination a Threat to Health? Harriet M. Ammann PHD D.A.B.T. https://pdfs.semanticscholar.org/4843/2f929a65e606a003b69d85a21f183ba6e1fd.pdf
Should I pay for mold testing at my house?
Probably not. If mold is a symptom and not a problem, the best way to spend limited resources is to diagnose and repair the water problem that is causing the mold. If a home inspection reveals no signs that moisture problems are present, then testing is likely not needed as the risk of concealed mold is low. If a visual inspection reveals that a house is suffering from water problems, there is certainly a risk of concealed mold, but you are better off diagnosing and repairing the water problems which will also correct the mold problem.
The best case for mold testing is when mold is suspected due to odors or human health issues and no signs of a moisture or mold problem can be found after exhausting visual inspection techniques combined with tools such as moisture meters, infrared evaluation and even destructive testing. 
 RR-02-09: Mold Testing. Joseph Lstiburek, Terry Brennan, Nathan Yost. https://buildingscience.com/documents/reports/rr-0209-mold-testing/view
Now, let’s Talk Mold in Attics
Why do I get mold in my attic? The reason attics are prone to mold-growth in some parts of the country is that in the winter time, the roof sheathing can get wet from condensation. Roof sheathing is vulnerable to condensation because warm, interior air can migrate through the sheetrock ceiling and it will drop its moisture on the first cold thing it hits, which is the roof sheathing.
This is similar to the beads of condensation you see on your ice-cold beer glass on a hot summer day. So the answer is: temperature differentials are the reason for this problem. These temperature differentials create surfaces that can reach dew point and cause condensation; this creates water, which leads to mold.
Why is warm air migrating into my attic from my house? Houses are like chimneys. Cold air infiltrates down low and hot air escapes out the top, this is often referred to as the stack effect. A sheetrock ceiling with paint is called, in building science terms, a vapor diffusion retarder. This means your ceiling retards or slows the vapor moving through it. In most houses, the “vapor diffusion retarder” has lots of breaches and openings where interior air can easily pass into the attic – think can lights, fans and attic access hatches.
Why is this more of an issue in some climate zones? Building standards and practices are regional and regional environmental factors can have an enormous impact on building design and performance. Put simply, if you live in a hot arid climate, you are unlikely to find this problem. This is a problem most frequently associated with regions of the country that get cold in the winter.
Have modern energy codes exacerbated this problem? Yes. The more insulation you add to the floor of an attic, the more heat loss you prevent from inside the house and the colder the roof decking becomes in the winter. The colder the roof decking gets, the easier it is to reach dew point and have condensation problems when warm interior air hits the decking. In addition, homes constructed to modern energy codes are more air-tight than older homes, making it easier to develop high indoor relative humidity in the winter months because there are fewer natural air changes.
Can adding more roof cavity ventilation prevent this seasonal condensation problem? Maybe. It could also make the problem WORSE. The best way I have heard roof cavity ventilation described is that it is like your backup parachute. You should not really need it if everything is going well, meaning your house is generally dry. If the main parachute fails and your house starts to get too wet, you sure do want to have a back-up. So if your attic is nearing dew point, the flow of exterior air can help keep the wood sheathing dry. However, if you add too much ventilation, you can create a negative air pressure in the attic and exacerbate the stack effect and actually pull MORE interior air from the house up into the attic. In some cases, adding more roof cavity ventilation can worsen the situation.
Is mold in the attic likely to affect the indoor air quality in the house? No. The difficult thing here is finding undebatable metrics to prove this. But logically, mold in the attic is not likely an indoor air quality issue due to the stack effect. Mold in a crawl space below your house could contribute mold spores to interior air, which is no guarantee of a health hazard, but simply a statement of fact: we breathe the air that is below our house. But most of the air in your attic is going up and out the upper roof cavity venting and not into your house. Exceptions can be found, notably, a leaky cold air return duct in the attic could deliver signifigant amounts of attic air into the house.
Is mold in my attic likely to affect my enjoyment of my house? No. Unless you have a significant problem, the mold and seasonal condensation in your attic could go undetected for years. The one area of concern would be the indoor relative humidity inside the house. Because this seasonal condensation problem in your attic can be related to high relative humidity inside the house, there is a chance you have a more problematic moisture problem inside that you would need to diagnose and repair. A simple example would be water accumulation in the crawl space below your house. You might start to see signs of this inside like condensation on window frames and toilet tanks.
Could mold in my attic impact the resale value of my house? Yes. This is the best reason to have attic mold problems treated professionally. Because condensation is a seasonal problem related to dew point, relative humidity and occupant behavior, it is difficult for an inspector to determine if an attic condensation problem is active or not. In addition, when home buyers see mold on roof decking, they can’t un-see it. If they are unfortunate enough to watch any form of reality TV, they probably think it will kill them and, in my experience, there is not much I can do to convince them otherwise.
As I see it, mold remediation companies create value by putting their name on a seasonal and unfounded problem when it arises during a real estate transaction, so you want to choose a reputable company that has been in the business for a while and will stand behind their work.
A good company will diagnose the water problem first and foremost. They will look inside the home for issues that could cause high relative humidity and then shift their focus to the air barriers that separate the house from the attic. They will also evaluate fan terminations and roof cavity ventilation in the attic to try and prevent further condensation. Finally, they will remove or encapsulate any existing mold on the framing. Where I work, this is often done by using an industrial paint/sealer on the attic framing in question. Once the framing is cleaned or painted, it provides a fresh surface for monitoring to see if the condition returns.
Can people in the house contribute to attic mold problems? Yes. We call this occupant behavior. Remember that people are constantly dumping moisture into a house by breathing, cooking, bathing, washing and even with hobbies such as aquariums and indoor plants. The objective in winter months is to keep indoor relative humidity around 50-55%. You can often accomplish this by using bath and kitchen fans to exhaust moist air to the exterior. In especially cold climates, houses may be equipped with heat recovery ventilators; these help you get air changes in the house without losing too much heat in the process. Small, modern houses with lots of people living in them, are prone to high relative humidity and mold problems. This is especially true of houses that have electric heaters and do not have forced air heat as the leakage out the ductwork in a forced air system can dry the interior.
How do I know if I have an active condensation problem in my attic? Attic condensation problems are seasonal and can vary depending on occupant behavior, so they can be difficult to understand, especially in the midst of a real estate transaction. The best time to check your attic for condensation is in the winter, first thing in the morning, when you see frost on the grass outside. These are the mornings I frequently find condensation in the attic.
How can I fix this problem? By now you should understand both why your attic may be getting wet and how you should deal with a mold problem: stop the water!
I hope this article provides some useful tools for understanding and explaining mold problems in residential construction as well as how and why we get mold problems in our attics. The concepts laid out here have been simplified slightly to provide a reasonably short article and are from climate zone 4. Your experience with this condition could vary by climate zone.