What Causes Attic Mold?

Most people typically think of mold as something that affects drywall and basements. But it turns out that attics are on the frontline in the battle against mold too. One of the reasons for this is that lofts are out of sight and out of mind, meaning that attic mold has plenty of time to become established before the owner notices.

Mold in attic materials is a problem. Mold not only damages roofing materials but can also get into the insulation, requiring the whole lot to be removed and replaced at an extraordinary cost. Suffice to say; you’ll want to avoid attic molds if you can.

Avoiding attic mold, however, is easier said than done. In order to prevent it, you need to know what causes it. Unfortunately, the reasons behind what causes attic mold are many. It’s not just a case of one perennial problem causing issues: the reasons for attic mold are all over the map.

In this article, we’re going to look at some of the most common causes of attic mold growth. It’s important to point out, however, that at the root of all attic mold problems come from excessive moisture of one kind or another. Even if none of the causes highlighted below apply to you, it’s worth bearing that in mind.

Identify The Source Of The Mold Issue

Before we start delving into specific causes of mold, it’s important to note that resolving an attic mold problem requires you to deal with the root problem. If you don’t, then the mold problem will come back again and again in the future.

Mold problems in the attic generally fall into one of three categories: leaks in your roof, excessive moisture levels from lack of ventilation, or issues with your plumbing and vent outlets.

Roof Leaks

A leaking roof is one of the most common causes of attic mold. Leaks can develop practically anywhere and are usually the result of storm damage or degradation of roofing materials over time.

To rule out a roof leak as the source of your attic mold issue, you’ll want to perform an internal and external visual inspection of your roof. Check your roofing materials and sections systematically in the following order:

Step 1: Check For Problems With Insulation And Wood

Wood and insulation both provide an ideal habitat for mold. Damaged wood and insulation can harbor moisture mold uses to grow and multiply. Check your sheathing, joists, rafters, and fascia boards for signs of damage and water infiltration.

Step 2: Check Your Roof Valleys

The point at which two roofs join is notoriously prone to developing leaks. Check roof valleys for damage and ensure that any sheeting that lies below your tiles extends to the valley edge.

Step 3: Inspect Roof Features

Roof features, like chimneys, skylights and attic windows all rely on proper sealing to keep moisture out. Gaps can quickly form, however, when two different materials come into contact (because of the way that each warps and evolves).

Step 4: Check Your Vapor Barrier

Some roofs have a vapor barrier. You’ll want to check this at regular intervals to make sure that it’s not collecting condensation. It’s best to monitor after cooking, or following use of the shower, just in case daily household activities are leading to mold issues.

Step 5: Check Attic Plumbing Stacks

If you can’t see any places where rain could get in or any reason for moisture buildup, then you could have a plumbing stack problem. Check for leaks and call your local plumber if you find any.

Vents And Exhaust Fans

The purpose of vents and exhaust fans is, as you might expect, to remove humid air from your home. Keeping humidity low in the house is vital because it starves the mold of the moisture that it needs to survive. However, if the humidity level in your home is above 60 percent consistently, then mold can establish a foothold.

Building regulations dictate the exhaust fans from showers, and stoves should exhaust humid air outside of the home. But sometimes builders don’t follow the rules and pump moisture into the attic instead.

As you can imagine, pumping water-filled air into the attic isn’t a good idea. Warm, humid air comes into contact with cool roofing materials, condenses, and provides the ideal environment for mold to grow.

If you notice exhaust fans and exhaust-spewing humid air into your attic space, hire a local tradesperson to relocate the terminal outside your home.


Many homes have a vapor barrier designed to prevent moisture from damaging the fabric of the building. But to work correctly, a vapor barrier needs adequate ventilation in the form of vents. If your vents are close to your roof, then you need about one square foot of ventilation for every 300 square feet of floor space in your attic. If they’re further away, then you need double that.

Inadequate ventilation can lead to other moisture problems in the attic. Because heat rises, any steam created when cooking or bathing will inevitably make its way upwards towards the rafters. Kitchens and bathrooms should both have extraction fans to prevent this, but sometimes, these don’t function as intended.

To resolve this problem, you may need to ask a building engineer to install more powerful extraction fans or include passive ventilation throughout your home. Ideally, you want to remove high humidity air from your property within 30 minutes.

Finally, many homeowners inadvertently cover up their vents without realizing the critical role that they play in preventing the buildup of mold. Vents are usually just small plastic grids that cover up a section of the wall where a brick is missing (though they can take many forms). Vents are a vital part of your home’s infrastructure, protecting you from excessive moisture buildup around the clock.

If you find mold in the attic, the key is not to panic. Attic mold issues are often easily rectified. In most cases, you can do attic mold removal quickly, without risking your health in the process. Small changes to your home can make a big difference.

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