Most common causes of mold in attics of NW homes

With the exception of summer, the Seattle and Puget Sound area have a climate with plenty of rain and high relative humidity. This type of climate makes it difficult to keep attic sheeting dry in your attic. Mold growth requires a source of moisture. So in order to prevent mold growth in your attic, you need to control the moisture.

Here are some of the most common reasons for mold in your home's attic.

Mold due to inadequate ventilation.

The first and most common cause of mold growth in an attic is inadequate venting. Venting is the most important way to keep moisture levels normal and prevent mold growth. The type and amount of venting need to be strong enough to keep the attic dry by preventing condensation.

The lack of adequate venting could have been the fault of the builder. Builders are not all venting experts and keep in mind that a builder only has 7 years of liability. So if you don’t identify the problem in the first 7 years, the builder is not liable in any way. Also, the building code for attic venting may work in most of the country but not necessarily in our northwest climate.

If your builder did not vent the attic properly, you may not find out until it’s too late if you don’t either look in your attic or have it inspected by a professional. I have spoken to countless homeowners who had no idea about the attic mold issue until it was too late to get the builder to fix it. On the rare occasion, I have been paid for making attic repairs by the builder.

Mold growth resulting from changes made to the home.

Another way that can lead to inadequate venting is any changes that are made during the life of the house. One change that can modify the venting is installing a composition roof where there was previously a cedar shake roof. The shake roof tends to allow the attic to breathe but when you install new sheeting and a composition roof, the roofer will have to provide more venting to allow for adequate airflow and moisture control.

Another big change that can damage proper venting is adding a ridge vent without adding the appropriate amount of intake venting. Ridge vents are typically more powerful than standard roof vents so if you increase the exhaust venting, you need to increase the intake venting in order to maintain a balance which is how passive venting works.

Mold in attics caused by large trees near the home.

The NW is famous for its huge native trees and lots of homes have big trees growing right next to them. The shade from these trees on the roof,  especially on the south or west side of the house will increase the risks of mold growing in your attic.

When you reduce the sunshine on your roof, you can make the attic colder and more susceptible to condensation.

Improper ductwork.

If the ductwork from either kitchen or bathroom fans is not properly installed, it can lead to the perfect condition for mold growth in your attic.

The current building code requires any interior fan to either be properly ducted through a wall or through the attic. In the case of attic ductwork, this would be a 3, 4, or 6-inch duct (think of this as a tube or hose) that connects to the fan and then the other end connects to a flapper vent that is installed into the roof itself. That way, when you turn the fan on, the moist air will be blown right out to the outside.

Sometimes these ducts get disconnected from either the fan or the flapper vent and need to be reconnected. All that warm, moist air is now getting dumped into the attic space and provides conditions for mold growth.

A rather serious problem that can cause mold growth is when the dryer vent ductwork gets disconnected in the attic. Given how much moisture comes out of a dryer, this can cause quite a mess. There will be a lot of lint in the attic but also quite a lot of moisture so the mold growth
can be quite aggressive and extensive. Luckily most dryers are vented through a wall but with the popularity of upstairs laundry rooms, ducting through the attic is becoming more common.

Finally, the least common but perhaps most problematic cause of mold growth in attics is a broken B-vent. These vents B-vents are the ductwork from your furnace that goes through the attic and
terminates with a cap to keep the rain out and allow the warm moist air to escape.

The furnace generates a lot of moisture and on a rare occasion that B-vent can either rupture because of poor quality or just get rusted through. If something like this happens, a lot of moisture gets into the attic and of course, can lead to aggressive mold growth.

Is the observed attic mold due to a current or past issue?

A homeowner may want to know if the moisture issue is a current one or a past issue. If you are inspecting the attic during the wet months (approximately October through April) you can use a moisture meter to check the moisture level. Any reading above 16% is a problem because 17% is about where mold starts to grow on wood. However, the mold should be removed regardless of whether it's caused by an old or current issue.

If you pay for attic mold removal and the mold comes back, the moisture issue hasn’t been resolved. If you remove it and it doesn’t come back, your problem is solved. Hire a company that offers is a 10-year warranty. This will put the liability to resolve any future moisture-related issues on the company.

The bottom line with attic mold issues is that you need to find a company that has extensive experience inspecting attics, identifying moisture-causing problems, and can make all the necessary repairs in order to ensure the mold issue does not come back.

This article was written by Peter Kakoczky with Seattle company Mold Solutions NW. He has seen it all when inspecting over 4,000 Puget Sound area attics. His company focuses on both mold removal and improving ventilation so that the mold doesn't come back again.