Photo of dirty pink fiberglass insulation with cartoon microbes and the caption "Can mold grow on insulation?"

There are many things to think about when buying insulation–cost, installation, R-value. But what about mold? The last thing you want is to have deadly mold spores spreading in your walls.

We investigated seven insulation types to see which ones can grow mold, and we were surprised at the results. Do you think organic or inorganic insulation is better at preventing mold growth? Keep reading to find out which is the best, and what you can do to prevent mold growth!

Can It Grow Mold? Seven Types Of Insulation Examined

No one wants mold growing in their home. It’s nasty, smelly, and can damage your health. When we see surface mold or mildew, we immediately clean it. But what about where we can’t see? Is it possible that mold is lurking inside our walls?

It is possible that mold can grow on your insulation. If you have leaks or condensation in your walls, it creates a moist environment for mold to grow. The question is, how likely is that to happen in your home? Mold grows more easily on some insulation than others. That’s because mold needs organic matter to eat and spread. (Mold isn’t then only problem you can expect if you have wet insulation.)

Here is a quick chart showing seven different types of insulation and their mold resistance

Insulation At A Glance

FiberglassInorganic, naturally mold-resistant materialCan attract organic dust that can grow mold
CelluloseOrganic material, chemically treated to prevent mold growthCan trap moisture if wet and create a climate for mold to grow on wood studs
Mineral WoolInorganic, naturally mold-resistant materialCan attract organic dust that can grow mold
Spray FoamInorganic, naturally mold-resistant materialCan be used as a seal to control moisture and help prevent mold growth
Sheep’s WoolOrganic, naturally mold-resistant materialHelps control humidity and prevent mold growth naturally
Rigid CorkOrganic, naturally mold-resistant material

So let’s take a closer look at these different types of insulation.  

Fiberglass

Fiberglass is the most common insulation out there. Pink and fluffy fiberglass insulation resembles cotton candy. And they’re made in similar ways. Cotton candy is heated sugar that is spun into a cloud of yummy goodness. Fiberglass is recycled glass that is heated and melted and spun into a cloud of insulation. (The pink color is artificially added in both of them). But while cotton candy and sugar are delicious, you wouldn’t think anyone would want to eat spun glass—even mold.

So can mold grow on fiberglass insulation, even though it’s inorganic?

A picture of pink fiberglass in the wall, with a man installing it on the right side.

The answer is yes. The mold doesn’t actually eat your insulation. Like I said, though, fiberglass insulation is like a cloud, and it’s full of nooks and crannies. Those nooks and crannies trap organic dust. And then mold can feed on that. Also, certain types of fiberglass insulation are faced with paper products. This paper can also become food for mold and a place for mold spores to breed.

Cellulose

Popular because it is an affordable and green insulation option, cellulose is made from recycled newspaper. Old newsprint is shredded into small fibers and then chemically treated with borates. The borates help make all those plant fibers more fire retardant, pest resistant, and mold resistant.

So if cellulose is treated to be mold resistant, does that mean it can’t grow mold?

And the answer is, not exactly. Here’s the problem. Cellulose doesn’t hold up well to damp and water. It’s just paper, after all. Think about what happens to a wet newspaper. The pages all start to stick together and get clumpy, right?

Photo of a crumpled, wet newspaper page on wet concrete. Wet cellulose insulation is like wet newspaper and it can support mold growth.
This is not the situation you want inside your walls.

Well, the same thing happens with cellulose insulation. If it gets wet from a leak or condensation in the walls, it can start to clump and fall. And it traps in that moisture, just like your newspaper.

Now, the cellulose insulation has been treated to resist mold, but it is touching the wooden studs of your house. And the wood is not mold resistant. So the moisture in the insulation can contribute to mold growth inside your walls.

Keeping this in mind, it’s best to avoid cellulose insulation in damp locations.

Mineral Wool

When it comes to mineral wool or rock wool, you will find it behaves much the same as fiberglass insulation. These insulation materials are all manufactured the same way–by spinning molten material into fibers. The difference is all in what is being spun, just like with the cotton candy. Mineral wool and rock wool come from basalt and post-industrial slag.

a pair of hands with white gloves on them installing rockwool mineral wool insulation in between two studs in a wall
Close-up of worker hands in white gloves insulating rock wool insulation staff in wooden frame for future walls for cold barrier. Comfortable warm home, economy, construction and renovation concept.

As with fiberglass, mold doesn’t want to eat the inorganic material. The problem comes with the open cell design of the insulation. Organic material gets trapped in those holes. And if it gets moist, it can become a breeding ground for mold. Further, because the material itself is inorganic, the manufacturers don’t treat it with mold retardants.

Spray Foam

If you are looking for the best insulation to fight mold growth, spray foam is right up there. It is made from petrochemicals, so it is completely inorganic. Mold certainly doesn’t want to eat it. Yuck!

If you choose closed-cell spray foam, there aren’t any little holes to trap organic dust like you would find in open cell insulation. So that’s one more way spray foam helps cut down on mold growth.

But one of the key factors that helps prohibit mold growth with spray foam insulation is how well it seals a space. When the foam is sprayed on, it makes contact and seals to the surface of wood, ducts, or whatever it is you are insulating. This seal becomes impervious to moisture. That tight bond helps inhibit mold growth.

A man in full protective gear is sitting on the attic floor while he sprays spray foam insulation on the underside of the attic ceiling.

In general, spray foam is one of the most effective insulators out there and works very well at sealing out moisture, so it helps prevent mold growth.

Sheep’s Wool

If you are concerned about mold and indoor air quality, you may not want to go with insulation like spray foam that is made from chemicals. But don’t worry–there are natural insulation options available. And they are even mold-resistant.

One naturally mold-resistant insulation material is sheep’s wool. Think about it. Have you ever had a wool sweater that you wore in the rain and then stashed away while it was still damp? You find it again two weeks later and think it is sure to be ruined. But wow! It’s a miracle–no mold or mildew. Your sweater is fine. That’s thanks to the natural mold-repelling qualities of wool.

And I bet you didn’t know that sheep’s wool can actually help keep your home at constant humidity. It’s true! Sheep’s wool will absorb moisture from the air when it is damp (but still feel dry). Then when the humidity drops, the wool releases that moisture back into the air. Isn’t Mother Nature amazing?

Now, natural products do come with a higher price tag, unfortunately. You will find sheep’s wool insulation to be more expensive than other products on the market.

Rigid Cork

Another natural insulation choice that is gaining popularity is rigid cork. That’s right, cork, like you find in your wine bottles. Cork is used to seal wine bottles because it doesn’t absorb liquids, and it’s naturally mold and mildew resistant. The same properties that help it preserve a fine vintage of wine make it great insulation for your home.

Rigid cork insulation is crafted into sheets that can be used just like polystyrene boards. But instead of being made from harmful petrochemicals, these insulating boards are made from the outer bark of an oak tree, commonly called the cork oak. Completely sustainable, the harvesting of cork doesn’t harm the tree in any way. The outer bark can be harvested, and the tree can continue to live for another hundred years or more, producing further harvests of cork.

So if you are looking for a natural, mold resistant insulation, cork can be a good option. In fact, when we went looking for the healthiest insulation options, cork was one of our top picks. Cork is not as expensive as sheep’s wool, but you will find it more costly than fiberglass or cellulose.

How To Prevent Mold Growth

While mold can grow on your insulation, properly installed insulation can actually help inhibit mold growth. The key is understanding how mold grows.

a pair of hands with teal blue colored gloves cleaning mold on a white wall with a sponge and a spray bottle

Mold needs the following conditions to thrive:

  • A temperature range between 40-120 degrees Fahrenheit
  • Humidity level above 50%
  • Organic material to eat

Now inside your walls, where you can’t see, you have all these pipes and ducts that bring you water and air condition your home. These create a space where warm air meets cool air, causing condensation. Now that condensation, combined with the toasty temperature inside your home, creates the ideal environment for mold to thrive. All it needs is food.

But what if we change the conditions? Then mold can’t grow.

The easiest way to change the conditions is to control the humidity. And insulation can help with that. The EPA recommends insulating your cold water pipes to help prevent condensation and discourage mold growth. You can also insulate all your ductwork. 

Photo of hands placing insulation around copper pipes between wall studs with insulation. Preventing condensation with pipe insulation helps prevent mold.
The insulation on these pipes will prevent condensation from forming and soaking into the insulation touching them.

It is also important that you choose the correct insulation for the space and have it installed correctly. You want to choose an insulation that performs well with moisture when you are insulating a damp basement, for example.

Properly venting your appliances can also help control the humidity level in your home to inhibit mold growth. Check your clothes dryer and stove to ensure they aren’t adding to your home’s humidity level. If you live someplace really humid and nothing else is working, you may want to invest in a dehumidifier or even an air conditioner.

But reducing your humidity isn’t the only way to prevent mold growth. The National Insulation Association also recommends increasing the filtration on your ductwork. By filtering the air better, you improve indoor air quality and reduce the number of mold spores in the air. While we may never be able to eliminate the spores completely, fewer spores means that mold is less likely to grow in your home.

Final Thoughts

Mold and mold spores are everywhere. And nothing we can do will completely remove them from our environment. Unfortunately, there is always the risk that mold will grow in your home. Even inorganic insulation options like fiberglass and mineral wool can be at risk. However, some natural options are surprisingly mold resistant.

Cork and sheep’s wool are both natural options that are resistant to mold growth. If you are concerned about chemical sensitivities or looking for a more sustainable insulation option, these are good choices (if you have the money).

Otherwise, spray foam will be the best choice if you are concerned about mold growth. Closed-cell insulation provides few places for organic material to collect. This limits mold’s ability to eat and spread.

Spray foam can also be used to help prevent mold growth. By effectively controlling humidity and properly sealing your home, you can minimize the risk for mold growth. Properly venting your appliances can help prevent mold growth, as well.

While mold is everywhere, we don’t have to create an environment for it to thrive. Proper prevention, combined with the correct insulation, can go a long way to prevent mold growth in your home and on your insulation.

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