Although mold is beyond the scope of my home inspection, during our home inspections mold questions sometimes come up.
This blog will focus on mold in attics. The hope is that this can provide some useful tools for explaining this complex subject.
If mold is found in a building, it needs to be addressed. There is an ongoing debate in the home inspection industry regarding mold testing and the effects of mold on our health. I wish to avoid that debate here, but simply state that 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 suggest that you direct all available resources to diagnosing and correcting the water issue first.
Why Is This More of an Issue in Prescott and not Phoenix?
Building standards and practices are regional and regional environmental factors 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 frequently associated with regions of the country that get cold in the winter.
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 winter or during summer monsoons, the roof sheathing can get wet from condensation. Roof sheathing is vulnerable to condensation because warm, interior air can migrate into the attic where it will drop its moisture on the first cold thing it hits, which is the roof sheathing. This can lead to frost on nail heads, one of the leading causes of a condensation problem. The nails are even colder than the roof sheathing, so often the first place to reach dew point.
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, plumbing and electric chases and attic access hatches.
Can Adding More Roof-Cavity Ventilation Prevent This Seasonal Condensation Problem?
Maybe. Adding more roof cavity ventilation may help but 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 your house is dry. If the main parachute fails and your house starts to get too wet, you sure do want to have a backup.
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 pull MORE interior air from the house up into the attic. In some cases, adding more roof cavity ventilation can worsen the situation, especially if the holes in the vapor diffusion retarder (the ceiling) have not been systematically sealed first.
Is Mold in the Attic Likely to Affect the Indoor Air Quality in the House?
Not necessarily. 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. There could be exceptions such as a leaky cold air return in an attic that pulls 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, mold can impact resale value. 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 homebuyers 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 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. 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… 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, or with high-efficiency construction, houses may be equipped with heat recovery ventilators; these facilitate 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 resistance heat such as baseboard heaters.
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 during 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 an attic may be getting wet and how to deal with a mold problem: stop the water!
