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Can Mold Grow on Fiberglass Insulation?

mold on fiberglass insulation

If you’ve lived in or owned a home for any length of time, you probably know that mold is bad news. Mold is a fungal growth most often caused by persistent moisture or humid conditions and can cause or worsen any number of health conditions.

One of the worst things about mold is how sneaky it is – You may notice it growing out in the open especially in a bathroom, but mold often grows in unseen, forgotten areas of a house. The basement, the attic, crawl spaces, inside your walls – All these can be prime breeding grounds for mold.

Those types of areas tend to be heavily covered in insulation. So, the natural question is: Can mold grow on insulation? Specifically, does mold grow on fiberglass insulation, the most popular type of insulation?

The short answer is yes, mold can indeed grow in and on fiberglass insulation. And it’s important to understand how it can happen, how to spot it, and what to do to get rid of it.

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Can Mold Grow on Fiberglass Insulation?

First, it’s necessary to understand what fiberglass insulation actually is.

Fiberglass insulation is a composite of glass and plastic fibers. These fibers are blown out into a matrix of shards and strands, creating a network of pockets and cavities.

Neither glass nor plastic are good conductors of heat, and air gets trapped in the numerous gaps within the insulation. This means that cold or hot air trying to move into an area of higher/lower temperature experiences resistance. This is what makes fiberglass insulation such a great insulating material.

What Does Mold Need to Grow?

Mold needs three things to grow.

  • A source of moisture or water.
  • Temperatures ranging between around 45 degrees to 120 degrees F.
  • A food source.

If you can keep an area dry, you’ll keep it mold-free. But that’s usually easier said than done, especially in many climates around the country/world. The same goes for the temperature – Most regions tend to have temperatures in the mold range pretty much year-round.

The last ingredient is a food source, which for mold means just about any organic matter. But plastic and glass aren’t organic materials. That should mean that mold won’t grow on fiberglass installation, right? Unfortunately, it’s not quite that simple.

Why Does Mold Grow on Fiberglass Insulation?

There are a couple of main reasons why mold can grow in and on fiberglass insulation. The first and most likely reason is due to dirt, dust, and other grime and particles that collect over time.

While the fiberglass insulation itself isn’t a mold food source, if that insulation is covered with a food source the mold will grow just like it would anywhere else. Just about anything can qualify as a source of nutrients for mold, and that includes most dirt and dust that accumulate in houses.

In the visible areas of a home, we regularly clean dust and other particles. But in most places where insulation is installed, it’s not even possible to clean. And in rooms like an attic or crawl space, it’s often months between visits. During that time, grime can build up on just about any exposed surface.

Dust and dirt build-up both within fiberglass insulation and on its surfaces over time. That grime is a fertile breeding ground for mold. When you combine appropriate temperatures, dirt and dust, and a persistent source of moisture (such as humid air), you’ll inevitably get mold.

The other likely way mold can grow on fiberglass insulation has to do with how fiberglass insulation is finished. Many types of fiberglass insulation are ‘faced’ or covered with paper. The paper itself can act as a food source for mold.

Is Fiberglass Insulation Mold Resistant?

It’s accurate to claim that fiberglass insulation is mold resistant. As we’ve discussed, the main materials from which it’s created are not a food source for mold. In theory, a paper-free installation of fiberglass insulation would not support mold growth.

But the problem is that, in the real world, conditions aren’t quite that perfect. Fiberglass installation tends to be put in areas impossible to clean, and once it gets dirty it is no longer mold resistant.

The bottom line is that mold can and does grow on fiberglass insulation, even though the primary materials don’t directly support mold. Given enough time and the presence of moisture, fiberglass insulation is almost certain to become infested with mold growth.

How Do You Know When You Have Mold in Your Insulation?

The biggest and most reliable indicator of mold on your fiberglass insulation is visual. If you see dark blackish discolorations in and on your insulation, this may be mold.

But it’s worth understanding that not all discolored insulation necessarily means that mold is growing on that insulation. Dark spots on insulation may be grime and dust rather than mold growth.

If the dark spots on fiberglass insulation smear when pressed, or if they have a slimy or fuzzy appearance, this is likely mold growth.

There are a couple of further things to keep in mind, though.

Number one, it can be difficult to inspect fiberglass insulation, depending on where it’s located.

Number two, both insulation and mold can be hazardous to handle if you’re untrained.

Often, your best option is to schedule an inspection with a licensed professional to give a thorough assessment as to whether any mold is growing and what might need to be done.

How to Get Rid of Mold on Your Insulation

Once a section of insulation has begun to grow mold, you’ll need to get rid of that insulation. The bare minimum is to replace the insulation, as there’s no practical way to clean the mold off the insulation.

But one very important thing to keep in mind is that if mold is growing on insulation in an area of your house, it’s almost certain to be growing in other areas in that space. As we discussed, the materials of fiberglass insulation are naturally mold-resistant. If mold has managed to grow on the insulation, it’s very likely to be growing elsewhere in the vicinity too.

And once mold has begun to grow, you need to get rid of all of it, otherwise it will quickly return to the area. Mold emits mold spores into the air, that they can settle on nearby surfaces and spread further.

Not to mention, mold is a hazard to your health and the health of your family. Mold aggravates allergies, leads to respiratory problems, and can cause a handful of other health conditions.

This means that usually, you’ll need to go further than simply getting rid of the mold-infested insulation. You’ll need to also thoroughly dry, scour, and clean the walls and surfaces in the area where you’ve discovered mold.

Removing moldy insulation, cleaning the room or area, and installing new insulation can be a lengthy and tricky process. You’ll need specialized safety gear both to handle the fiberglass insulation and to be in the presence of mold.

And of course, you’ll also want to make sure you’ve installed the new insulation properly, as incorrectly installed insulation leads to increased heating and cooling bills and a host of other problems.

If you’re an experienced Do-It-Yourself-er, you may be able to handle the removal of moldy fiberglass insulation and the installation of new insulation. But in many cases, this is a project where it’s wisest to bring in a mold removal professional.

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