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Wet Insulation | When to Replace and Why

Trying to keep your insulation completely dry is sometimes almost impossible. We all know how frustrating and damaging wet insulation can be. Knowing how to handle it—well, that’s another matter.

In this article, I’ll look at how moisture can affect different types of insulation and see if it’s necessary to replace your wet insulation. Let’s take the plunge and learn all about the damp situation with insulation, when to replace it, and why it’s so important.

Cellulose and natural fiber insulation take the longest to dry and are more likely to need to be replaced if exposed to moisture. Fiberglass, mineral wool, and plastic fiber will need to be replaced if subjected to long-term moisture. Foam insulation is water-resistant and will last 10 years or more.

Factors to Consider

How Wet the Insulation Is

The extent of moisture in insulation is a significant factor in determining whether it needs to be replaced or not.

If the insulation is only slightly damp, it may be possible to dry it out and save it. However, it may have to be removed and replaced if it is thoroughly saturated.


Moisture can significantly reduce the insulating properties, making it less effective at preventing heat transfer. In addition, wet insulation can provide ideal conditions for mold growth, which can cause health problems and structural damage. 

It is important to monitor your insulation for signs of moisture to ensure that it is functioning properly and that your home is protected.

Is It Likely to Dry Quickly?

The likelihood of insulation drying quickly depends on several factors, such as the type of insulation, the amount of moisture absorbed, the ventilation in the area, and the weather conditions. Weather, in particular, can play a significant role in the drying process. 

In cold and rainy conditions, the insulation may take longer to dry out than it would in warm and dry conditions. High humidity levels can also slow the evaporation process, making it more difficult for the insulation to dry out completely.

If the insulation remains wet for an extended period of time, it can become damaged and lose its insulating properties, as well as become a breeding ground for mold and bacteria.

The amount of moisture absorbed by insulation is a critical factor to consider when evaluating its overall effectiveness and durability. Water conducts heat much more efficiently than air does, so wet insulation does not provide the same level of insulation as dry insulation.

What Happens if Insulation Gets Wet During Construction?

Type of Insulation

Different types of insulation will react differently to moisture exposure. Some insulations, like spray foam or foam board insulation, are less likely to absorb moisture and, thus, less likely to be affected by it. They can be dried with a simple wipe down of a towel.

Hygroscopic insulation, on the other hand, absorbs moisture like a towel. They soak up and hold onto water, making it difficult for them to dry.

The moisture absorption can cause the insulation material to become heavy, sag, or even collapse, affecting its performance and reducing its ability to insulate effectively. 

Insulation that absorbs water can become an excellent breeding ground for mold and bacteria. As not only do they provide the perfect environment, but they can also be an excellent food source for mold.

Mold and bacteria can degrade the insulation and potentially cause health problems. 

Location of the Insulation

The location of the insulation in your home can also play a role in how it reacts to moisture. For example, insulation located near water sources, such as leaky roofs or plumbing, will be more likely to get wet and retain moisture. 

Crawl spaces and basements tend to be damp, which can shorten the lifespan of the insulation to 10 to 40 years. 

Fiber insulation installed in a crawl space

On the other hand, insulation in well-ventilated areas, such as properly-constructed attics, will be less likely to retain moisture. Proper ventilation, often achieved in attics through the installation of baffles, helps prevent moisture buildup and helps the insulation dry more quickly if it does get wet. 

If insulation covers electrical wires or junction boxes and becomes wet, it increases the risk of a fire hazard. Moisture can cause electrical circuits to short out, leading to sparks and potential ignition. If the insulation remains wet for a prolonged period, the risk of fire increases. 

When and Why to Replace Wet Insulation


Cellulose insulation is made from recycled paper products and is treated with a fire retardant. Cellulose is hygroscopic, which means it can easily soak up and hold water. This means it will take a long time to dry. Wet cellulose gets heavy quickly and tends to settle, which can cause your ceilings to sag. 

Cellulose insulation installed in wooden walls

Moisture can also make the fireproofing chemicals highly corrosive, affecting electrical wires and plumbing pipes.

Wet cellulose also provides an ideal environment for pests, mold growth (although it is treated to try to prevent this), and potential health hazards.

When a small section of cellulose gets wet, you may be able to remove it and allow it to dry completely before placing it back. If the moisture damage is severe or there was a huge flood, it is best to replace the cellulose insulation as soon as possible.

Cellulose also has a life expectancy of around 20 to 30 years. So, if your cellulose insulation hasn’t been changed since then, it’s probably a good idea to replace it as it would not be as effective in insulating your home as it used to be.


Fiberglass insulation does not retain moisture, reducing the risk of mold and other moisture-related problems. If it does get wet, it can temporarily lose its insulating properties but regain them if it dries completely. 

If it stays wet for a few days, the fibers can absorb water and cause them to clump together. The fibers lose their fluffiness and shape, reducing its insulating properties and effectiveness. Dirt and other contaminants can also contribute to the clumping and compaction of wet fiberglass insulation.

Wet fiberglass insulation should be removed if it doesn’t dry within two to three days. To aid the drying process, you can use dehumidifiers or fans directed towards your insulation.

Fortunately, once dried, and if it is still in good condition, fiberglass insulation can be reused. However, If it’s thoroughly saturated with dirty flood water, it is best to replace the fiberglass insulation immediately, as the dirt can get trapped within the fibers and promote mold bacterial growth.

Fiber insulation installed on the roof

Your fiberglass should last you at least 80 years with minimal moisture exposure. However, if any compaction is noticeable, your insulation will not be as effective, and you will need to consider replacing it.

Mineral Wool

Mineral wool insulation is made from rock, stone, and slag. The inorganic composition of mineral wool makes it moisture-resistant. This makes it an ideal insulation material for particularly humid or wet areas. 

Unlike other types of insulation, mineral wool does not absorb moisture and is less likely to develop mold or mildew. If it does get wet from a leak or flooding, it dries out quickly and doesn’t pose a risk of mold growth.

In general, the lifespan of mineral wool insulation is long, with many manufacturers estimating it to last between 40-80 years, depending on factors such as exposure to moisture, pests, and how well it’s protected. 

However, if mineral wool is constantly exposed to moisture, it may start to lose its insulating properties over time. It is recommended to check your insulation regularly and to replace it if you notice any signs of moisture or loss of insulating properties. 

Additionally, if you have an older home with insulation that has been installed for many years, it may be worth having an insulation inspection to see if it needs to be updated or replaced.

Natural Fiber

Natural fiber insulation materials are typically made from renewable, organic sources like sheep’s wool, cotton, hemp, or cork. They are prized for their environmental sustainability, but like all insulation materials, they can be affected by moisture.

Cotton insulation is very similar to cellulose, and if it is not able to dry out within a few days, it will need to be replaced.

Sheep’s wool and hemp insulation are known for their moisture-resistance properties. They are able to absorb a large amount of moisture while still retaining their insulating properties. Additionally, sheep and hemp wool fibers do not support mold growth.


However, even though mold cannot grow on sheep and hemp fibers, if the insulation remains wet and is in contact with other surfaces like your roof or walls, it could promote mold growth on those surfaces. 

Prolonged moisture exposure can also impact the fire-retardant properties of natural fiber insulation. This is an important aspect, as these chemicals help to protect against fires, pests, and mold. If the fire retardant leaches out due to moisture, it could compromise your insulation’s effectiveness and safeness.

Unlike other natural fiber insulation, cork does not absorb moisture and will not become wet, making it resistant to mold growth. Cork insulation will most likely not need to be replaced after it gets wet.

Plastic Fiber

Plastic fiber insulation, such as polystyrene, is hydrophobic, meaning it resists water and does not readily absorb moisture. If plastic fiber insulation does get wet, it will dry fairly quickly and not be damaged.

However, there is a slight chance that if there is a gap in the insulation where air and moisture can penetrate, water can become trapped within the plastic fibers. If the moisture problem is not addressed quickly, this trapped moisture can lead to mold growth. 

Over time, moisture can also cause plastic fiber insulation to deteriorate and lose its insulation properties. It is best to have your insulation installed by a professional to ensure it is properly watertight.

If your insulation was installed correctly and after a leak or flood, you see your plastic fiber insulation is still good, i.e., there is no noticeable mold, and your home is still insulated well, then you are good to go. No need to replace your insulation.

Polyurethane Spray Foam

Polyurethane insulation, also known as spray foam insulation, is available in two forms: open-cell and closed-cell. Open-cell foam has a porous structure with interconnected cells that are filled with air, making it soft and spongy. 

Closed-cell foam has a dense, compact structure with completely sealed cells, making it stiffer, stronger, and less susceptible to moisture.


The moisture resistance of spray foam depends on the type of foam used. Open-cell foam is more permeable and allows moisture to pass through. This can lead to the trapping of moisture within the foam, reducing its thermal resistance and providing a breeding ground for mold.

Although spray foam can last up to 100 years, if you have open-cell insulation and it is continually subjected to moisture, it is best to replace it as soon as you notice any mold growth or a decline in your home’s insulation.

Closed-cell foam provides a moisture barrier, preventing bulk water from passing through. This makes it ideal for insulating basement and crawl space walls. However, it is suggested to replace your crawlspace insulation every ten years due to the constant exposure to moisture.

If you have closed-cell foam insulation in your attic, you shouldn’t need to replace it as often. And if no major leaks or pipe bursts occur, your insulation should last you a lifetime.

Foam Boards

Foam board insulation is typically made from closed-cell polystyrene and comes in large sheets or panels. The closed-cell structure of foam board insulation makes it highly resistant to moisture. 

The cells are filled with a gas that gives the foam its closed-cell structure and acts as a barrier to moisture, preventing water from penetrating the foam board. This means that foam board insulation will not absorb water.

Pink Insulation Foam 1/2" Thick (6 sq ft)

Some foam boards have a laminated layer that provides additional strength and protection from moisture and environmental factors.

Foam board, like spray foam, will last up to 100 years. And depending on its location, it may never need to be replaced. A simple wipe-down with a towel will ensure it remains dry in areas like the attic.

But, like spray foam, foam board insulation in crawl spaces or basements should be replaced every 10 years because of their long-term exposure to moisture.

If you notice moisture or condensation near your foam board insulation, it’s possible that moisture is getting trapped behind it due to its impermeability to water.

In such a scenario, it might be necessary to remove the insulation temporarily to let the area dry thoroughly before replacing your insulation.


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