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 we have to quantify the risk of mold in our buildings. Mold issues in buildings are complicated by television shows that use the fear of mold to dramatize their entertainment. (Help, I’ve got mold in my attic!) This article will focus on mold in attics and will approach this topic in the form of a series of questions and answers in the hopes of shedding some light on a murky subject.

First, a Few General Mold Questions

Is mold in my house really a problem?

In my opinion, mold in a house is never a problem, it is a symptom of a problem. There is only 1 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. So mold is not the problem, the mold is informing you of a water problem.

Is mold harmful to my health?

To my understanding, there has never been a proven case of a toxic reaction to mold in human beings. I am open to being proven wrong here, but this is the best available information I have found. It is absolutely true that prolonged exposure to any species of mold can be harmful to human health, but this is because we develop allergies to mold. Mold allergies are a serious human health hazard, so nothing to toy with, but the idea that we will experience a toxic reaction to mold has no grounding in science that I can find.

Should I pay for mold testing at my house? 

This is a subject that raises a lot of angst. The simple answer is no. If mold is a symptom and not a problem, the best way to spend limited dollars is to diagnose and repair the water problem that is causing the mold. If a home inspection reveals no red flags that moisture problems are present, then testing is likely not needed as the risk of concealed mold is low. If a visual inspection reveals many red flags that a house is suffering from water problems, there is certainly a risk of concealed mold, but you are better off spending limited money fixing the water problems which will also fix the mold problem. Also, if prolonged exposure to any species of mold can be bad for human health … does it really matter what species of mold are present? I am sure one can devise scenarios where mold testing saved the day and helped find a latent or concealed mold problem, but I would suspect these scenarios are the exception rather than the rule.

Now, let’s Talk Mold in Attics

Frost on nail heads, the leading edge of a condensation problem. The nails are even colder than the roof sheathing, so often the first place to reach dew point.

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.

This photo shows the stack effect as balloons are pulled toward a fireplace that had the flue damper open overnight
This photo shows where interior air can leak up into an attic around a can light.










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.

photo shows indoor humidity
Condensation on a single pane window indictes high relative humidity inside the house.

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. I once inspected a house that had serious problems with indoor relative humidity. There was mold around the windows and mold around the toilet tank in the bathrooms, but the attic roof decking looked great. This was because the house had almost no insulation, so the roof decking was nearly the same temperature as inside. No condensation problems despite the real problems with high relative humidity inside the house. 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.

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 a significant risk to indoor air quality. But most of the air in your attic is going up and out the upper roof cavity venting and not into your house.

another example of indoor humidity
Mold on an insulated window frame – often a sign of high indoor relative humidity.

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 one of the best reasons to have attic mold problems treated professionally. Because condensation is a mysterious and seasonal problem related to dew point it is difficult for an inspector to determine if an attic condensation problem is active or not. Mold remediation companies are essentially being paid to put their name on it, 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 between the house and the attic and air-seal the ceiling. They will also evaluate fan terminations and roof cavity ventilation in the attic to try and prevent further condensation. Finally, they will often remove or encapsulate any existing mold on the framing. Where I live this is often done by using an industrial paint/sealer on the attic framing in question.

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 below 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 have a 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 helps you understand more about mold problems in residential construction as well as how and why we get mold problems in our attics.