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Why Attics Get Moldy

Source: Habitat Quarterly

By Jason Earle Google+

As the holidays approached, the ornaments and decorations started coming out of the attic.  In the process, you may have discovered that you’ve got a mold problem. Or maybe you’re selling your house and the buyer’s home inspector found something not so good up there.

In the attic? How’d that happen?

Attics are the least actively used space in most homes. Not only is it out of sight and therefore out of mind, but also nobody wants to spend very much time up there because it’s so hot in the summer and brutally cold in the winter. And that is actually part of the reason why some attics get moldy. Attics are not typically our favorite places, so we don’t dedicate much attention to them, but you can develop a very expensive issue if things get out of control while you’re not watching.

For a mold problem to develop, you have to have excess moisture. No water = no mold. Mold will not grow in a dry, well-ventilated attic. So what causes moisture to accumulation in an attic? A ventilation problem. It’s really that simple.

Let’s start with where the moisture comes from.

People indoors generate significant humidity simply through living. Respiration (breathing), transpiration (sweating), cooking, bathing, cleaning and numerous other activities pump water into the air. Having lots of houseplants can add a lot of humidity. Damp basements and crawlspaces often manifest into a moldy attic. Unused or misused dryer vents and improperly installed bathroom exhaust fans are very common culprits too.

Here’s where I’m going to throw you a curve ball. Most people think it has to be warm and wet for mold to grow, and while that’s partially true, certain molds can grow in very low temperatures, but they still require moisture. Have you ever seen black mold growing on the gasket around a refrigerator?  This is usually a very common mold called Cladosporium. It doesn’t mind the cold so much. It also very commonly grows in attics where there’s a moisture trap. But here’s the key, most mold problems in attics are a wintertime phenomenon, not a summertime one. I know it’s counter-intuitive, but I’ll explain.

As we all remember from eighth-grade science class, warm air rises in a building. In a case where there’s a lot of moisture in that warm air, when it finds its way into a cold attic, the water in the air will bead up on the cold interior surfaces of the roof like it would on a glass of iced tea on a hot summer day. During really cold periods, this condensation will actually freeze, making some attics an unintended winter wonderland.

In such circumstances, the exposed nails will transform into icicles overnight, and when the sun comes up, the roof warms, melting the icicles, causing it to drip rusty water droplets onto the floor. This cycle of moisture accumulation on the dusty wooden surfaces of the attic is enough to create an environment conducive to mold growth.  Sometimes this takes decades, sometimes only one season. Depending upon how severe the problem is, the damage can range from some minor surface mold, which can be easily cleaned, to complete rot and degradation of the sheathing, requiring a new roof to be installed. Not fun.

Now let’s talk about the ventilation.

Contrary to what you might think, an attic fan is not the solution, as attic fans are designed to operate during the summertime to remove excess heat. Also, the vents on opposite ends of many attics, called gable vents, do nothing. They are worthless for the purpose of moisture control. I’ve seen homeowners try every type of shortcut possible to avoid doing what needs to be done, but in essence, there’s only one solution which works flawlessly: natural ventilation with ridge and soffit vents.

So, what is natural ventilation, you ask? Good question.

When warm, moist air rises into the attic, a well-ventilated attic will allow it to escape through the peak of the roof, through something called a ridge vent. In order to make up for the air escaping through the peak, makeup air needs to come from outside, but in a very specific way, from a very specific place: the eaves through soffit vents. In summary, through natural ventilation, warm moist air from the house escapes through the attic peak, rather than being trapped, while the vents in the eaves allow air to come in from outside, keeping the surface temperature of the inside of the roof closer to that of the outside of the roof. This prevents condensation in the wintertime and also helps keep the roof cooler in the summer, extending the life of asphalt shingles.

No fans. Nothing to remember to do. It occurs naturally and flawlessly, but here’s the key. The ridge and soffit vents have to be continuous and integrated. What that means is that they have to go from one end of the roof to the other end, and they don’t work without each other. Building codes here in New Jersey require new roofs to have ridge vents, but they say nothing about retrofitting the soffits with vents. In essence, the ridge vents are worthless without soffit vents.

There is a specific amount of ventilation needed for every attic. Owens Corning has an interesting tool to perform the calculation.  You should definitely refer to it if you plan on making any changes. Here's the web address:


The other element is to make sure the vents in the soffits are not blocked by insulation. Many a weekend warrior will install new insulation in the attic to bring down the utility bill, stuffing every bit of insulation possible into the far corners of the attic, including the eaves, only to find that they inadvertently blocked the vents, rendering a functional natural ventilation system worthless.  A few hundred dollars in annual savings becomes thousands of dollars in mold remediation before you know it.

Another common mistake is venting bathroom exhaust fans into the attic instead of through the roof to the outside. When doing bathroom renovations, please don’t make this mistake. It’s a costly one. Similarly, the absence of an exhaust fan in a bathroom used for bathing sends a ton of water into an attic. Get a fan installed and use it very time you shower or take a bath. Some of my clients have connected the light switch to the exhaust fan so that they know the kids are using it without thinking about it. It works like a charm.

So, if you have a problem, what do you do next?

First of all, you should engage a professional, such as 1-800-GOT-MOLD?, who specializes in diagnosing mold and moisture problems. What they will first do is track down the source of excess moisture, assess the extent of the damage to the structure, and determine where the ventilation failed and what needs to be done to correct it.

In all cases where a visible mold problem exceeds 10 square feet, the US EPA recommends you use a professional to clean it up. A quick word of caution here: You should never hire a company that performs both diagnostics/testing and remediation. This is a blatant conflict of interest since they will often be testing and inspecting their own work.

If the sheathing has not been damaged to the point where it is delaminating and losing its structural integrity, you can usually get away with surface cleaning. In this case, a qualified professional mold remediation firm would isolate the attic from the rest of the house as described in the IICRC S520 Mold Remediation Standard, bag and remove all insulation, then clean all exposed surfaces with HEPA vacuums and damp wipes.

Afterward, before final funds are released to the contractor, they would submit themselves to a third-party clearance inspection where samples would be collected for analysis to ensure that the job was complete. Also, the inspector would look to see that the appropriate repairs have been made, including the installation of requisite ventilation. After the inspector provides the green light, the insulation can be replaced. Do yourself a favor and get the formaldehyde-free insulation. And make sure not to block the vents in the eaves!

If the sheathing has deteriorated and the plywood has started to delaminate, you will need to replace the roof. I know this is bad news, but it’s the truth. Once that’s done, then you can proceed with the instructions above, although I’ve seen most of the cleaning of rafters and removal of insulation occur with the roof off, which makes a lot of sense but requires coordination of the roofing contractor, mold remediation firm and Mother Nature. Regardless, make sure the roofer understand what continuous ridge and soffit ventilation is all about and that he/she installs them that way. Also please make sure all of your exhaust fan vents are directed through the roof to the outdoors. 

Most people don’t want to spend as much time in their attic as you just did reading about it, but this happens far more commonly than you might think. Hopefully your new-found awareness and knowledge will help you avoid having to call us. But if you do have questions, please feel free to email inquiries@1800gotmold.com or call us at 1-800-GOT-MOLD?.

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Why Attics Get Moldy
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