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Can Mold Grow on Wet Insulation? (All Types Covered)

Mold is not welcome in our homes. Whether of the green, black, or white variety, mold needs to be addressed quickly, which means you need to know where it might appear. So, do you need to worry about insulation?

Insulation comes in many forms and is an integral part of most homes, even though we don’t see it. This material is often in the danger zone for getting wet from flooding, leaking ceilings, or leaking or burst pipes and ducts. This can combine with other factors to foster the growth of mold.

Cellulose, spray foam, mineral wool, and foam board are mood-resistant. This is not a guarantee and mold can also grow on surrounding structures because of wet insulation. Fiberglass and natural fiber insulation are susceptible to mold. Always let wet insulation to dry.

Cellulose Insulation

Cellulose insulation is made from wood-based materials like recycled paper. It can be purchased in loose fill, dense-packed, and batted forms and is generally used for attics, ceilings, and walls.

While this insulation is treated for water and mold resistance, it’s not foolproof.

Unfortunately, if it does get very wet from leaking or flooding, the paper-like material retains moisture well. It will take a long time to dry out, which is a mold concern.

Even though the insulation itself is not likely to grow mold, it will sit against the structure of your home, creating a dark and damp environment. So, you want to deal with wet cellulose as fast as possible.

If only a portion of the insulation got wet, you might be able to clean the mold and let the material dry out.

This might require opening floorboards or removing sections of drywall so that the insulation can air for 24 hours or until dry. If it’s insulation for a new construction, this is much easier to do.

However, if it was a case of extreme flooding or it was thoroughly soaked, you may need to replace the insulation. Not only will you need to check for mold in the cavity, but cellulose insulation can settle and sag under the weight of the water, displacing it and reducing your R-value.  

Surprising Reasons Why Insulation Gets Wet

Fiberglass Insulation

Fiberglass insulation is made from synthetic and natural fibers that are made into long glass strands. It comes in batts, boards, rolls, and loose-fill, and is typically used in ceilings, attics, floors, walls, and to insulate piping and ducting.

Types of fiberglass insulation, batt, loose-fill and board fiberglass.jpg

Fiberglass insulation can be prone to mold. The dust and residue that are left behind once it gets wet and resin from a paper backing (if present) facilitate the growth of mold. Thankfully, it can be exposed and left to dry.

If the insulation was utterly drenched (particularly with dirty water), the fibers can end up clumped and compromised, so there is no point to drying it. In this scenario, you will need to replace the insulation to achieve the proper protection, so don’t waste your time trying to save it from mold.

Besides, if the insulation is very wet, waiting for it to dry could create mold growth in your home’s structure.

Natural Fiber Insulation

Natural fiber insulation is created from plant and animal materials and can be found in batts, bales, boards, and loose-fill forms. It is used to fill various cavities, floors, walls, and ceilings.

This category of insulation covers a variety of materials: wool, cotton, straw, cork, hemp, wood fiber, and even cellulose is often included. As with most insulation, if these get wet, you might end up with mold growth (aside from the cellulose as covered above).

The way you would deal with mold, for the most part, with natural fiber insulation is to replace the damaged material. Natural fibers are easily compromised by water exposure, so if your insulation is mold, you need to replace it.

Plastic Fiber Insulation

Plastic fiber insulation is made from recycled plastic that is available in blanket sheets, rolls, or spray foam. This type of insulation can be used almost anywhere since you can choose the best form for different locations.

Spray foam insulation is part of this category of insulation, and it is exceptionally durable and water-resistant. Mold can grow in your home, but it will not grow on this insulation. I will detail this type of insulator in a later section.

I have not seen reports of mold growing on other plastic fiber insulation. So, it makes sense to consider that this insulation shares a similar problem: even if it doesn’t grow mold, the structures and materials around it might.

Plastic insulation may be able to dry, but if you are concerned about the material being exposed to water, you should call a professional to assess the insulation.

Mineral Wool Insulation

Mineral wool insulation is made from stone or ore byproducts. It comes in batts and rolls and is versatile. You can use it almost anywhere you need insulation, provided the form matches the space.

Mineral wool insulation is a good choice for most places since it is water-resistant. Not only does it preserve the function of the insulating material against water damage, but it also helps prevent issues with mold.

Man wearing gloves installing mineral wool in the roof

Since the mineral wool doesn’t absorb water like some other insulation types, it doesn’t create that moist environment in which mold thrives.

Of course, this insulation is water-resistant and not waterproof, so it might eventually grow mold if there are persistent moisture troubles. So, ensure you give the cavity a chance to dry and check that no insulation is displaced or missing.

Polyurethane Insulation

Polyurethane, or spray foam, insulation is plastic-based insulation. It comes in different densities (high, medium, and low) and structures (open/closed-cell) that affect the R-value.

Spray foam insulation is not only known for its durability but it can also be used in a variety of places.

Spray foam insulation is actually very airtight and creates a solid moisture barrier, too, considering it is very water-resistant, even when submerged. So, you aren’t likely to experience mold problems with well-installed polyurethane insulation.

The rigidity of closed-cell spray foam means that it would take a lot of water before you have trouble with damage.

Open-cell spray foam dries very well since it is a bit more porous. This sounds slightly counter-intuitive, but although some moisture can pass through the material, it will easily evaporate.  

Spray foam insulation may take anywhere from one to seven days to dry, depending on how much water it was exposed to and how much airflow there is around the material.

However, while spray foam is very water-resistant, your home’s wooden structure is not. Mold can still grow on the vulnerable wooden and drywall around the insulation.

Another point I will address is this: you must clean mold from the cavity before installing the spray foam. While the insulation won’t grow mold, it cannot solve an existing mold problem either.

Foam Board Insulation

Foam board insulation is made from closed-cell plastic fibers in the form of an incredibly rigid board. Although effective and durable, its form limits where it can be used. It can be used for walls, floors, ceilings, and anywhere that can fit sheets, but it cannot be bent or rolled.

Pink Insulation Foam 2" Thick, Two Pieces (2.5 sq ft)

The closed-cell, rigid nature of this material makes it resistant to water, much like the closed-cell spray foam insulation.

So, while it is possible that mold may grow around this insulation, it will not grow on it, and there will be little water trapped against the structure nearby, provided the insulation is correctly installed and is allowed to air.

This type of insulation is commonly used for basements, which puts it in the line of flooding zones and will often offer a nice warm environment to foster mold spores.

Now, it is not like spray foam that expands into all the tiny spaces, so it may not be a watertight barrier against submersion. Ensure you are allowing the space under the sheets a chance to dry if it gets wet.

Wet Insulation | When to Replace and Why


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