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Wet Insulation | Will it Dry Out? (All Types Covered)

Have you ever had to deal with wet insulation? It’s a common problem that can happen for a variety of reasons, from leaks to flooding. The important thing to know is what to do with the insulation when this happens.

The type of insulation material is one of the most important factors when it comes to if and how well insulation will dry after it has become wet.

Cellulose and natural fiber insulation are hygroscopic and will take the longest to dry. Fiberglass, plastic fiber, and mineral wool spray will take between two days to a few weeks to dry out. Spray foam and foam board insulation are water resistant and will not absorb moisture.

Cellulose Insulation

Cellulose insulation is made from recycled paper products, including newspapers, cardboard, and other paper waste. It is characterized by its loose and fluffy texture, which helps it to conform to any space and fill in any gaps or voids.


Because of its high flammability, it is treated with fire-retardant chemicals and insecticides to increase its durability and resistance to moisture. Cellulose insulation is typically installed by blowing the material into attics, wall cavities, or other areas to be insulated.

Cellulose insulation comes in two main forms: loose-fill (mainly used in unfinished attics) and dense-packed (for filling spaces between walls and ceilings). There’s also batted cellulose, which is a newer option for wall insulation. 

Cellulose insulation is known for its excellent insulation properties, being eco-friendly, and its affordability compared to other insulation materials. It can provide a high R-value per inch, making it an effective solution for reducing energy consumption and costs.

Even though it’s a great way to recycle and save the planet, cellulose is hygroscopic, meaning it easily soaks up and holds water. And unfortunately, cellulose takes a very long time to dry as a result of this.

Cellulose can get wet from condensation, air leaks, or water leaks in the home. The slow drying time of the insulation after it becomes wet causes it to deteriorate and settle (or compress). This can make the cellulose heavy and potentially cause your ceilings to sag.

What’s even worse is moisture causes the chemicals used to fireproof cellulose insulation to become highly corrosive. This can cause your electrical wires and plumbing pipes to rust if they’re in contact with the wet cellulose for an extended period of time.

Moist cellulose can also provide a fantastic nesting and breeding ground for pests such as rodents and insects. It is also the perfect food source for mold, and the moist environment provides the ideal conditions for mold growth.

Cellulose can be treated with a mold-resistant chemical, but it will only work up to a certain point. The EPA recommends replacing cellulose insulation once it has been wet. Other insulation materials are different.

Fiberglass Insulation

Fiberglass insulation is created using fine glass fibers that are heated and combined into a mat-like material. This mat is then cut into different forms to meet various insulation needs. The two main forms of fiberglass insulation are loose-fill and batts. 


Loose-fill fiberglass insulation is made of small fibers that are blown into the empty spaces of an attic or wall cavity to fill it up. Batts, on the other hand, are pre-cut pieces of fiberglass insulation that can be easily installed between walls, floors, and ceilings to provide a barrier to heat flow and sound

Unlike cellulose insulation, fiberglass does not retain moisture, making it less prone to mold and other moisture-related problems. However, if fiberglass insulation does get wet from a water leak or similar, it can temporarily lose its insulating properties.

If it has a chance to dry and hasn’t become compacted, it will regain its insulating properties.

If the insulation gets wet and stays that way for a few days, it can lead to a reduction in its insulating quality and the risk of mold and wood decay of the structures in contact with the fiberglass. 

You may need to open your wall to check for wetness and allow the insulation to dry out completely. If the moisture source is internal, such as a leaky pipe in the wall, and the insulation doesn’t dry within two to three days, it should be removed.

Depending on the level of compression, dirt, or contamination that the fiberglass insulation has been subjected to, it may be reused.

If the fiberglass insulation has been exposed to dirty flood water and is thoroughly saturated, the fibers may clump together due to dirt and become compressed. 

In this state, the fiberglass insulation will not be effective in providing insulation, and it is recommended to replace it. If, however, the insulation is assessed to be in good condition, you can reuse it.

Natural Fiber Insulation

Natural fiber insulation refers to insulation made from natural, renewable materials, such as sheep’s wool, cotton, hemp, and cork.

Because of the natural fibers’ flammability, the insulation is treated with chemicals like borax to minimize fire risk, insects, and mold growth. 

Natural fiber insulation is available in a variety of forms, including bales, batts, boards, and loose-fill. All the forms of natural fiber insulation have their own unique benefits, are environmentally friendly, sustainable, and provide effective insulation.

Cotton insulation is very similar to cellulose insulation in that it is hygroscopic and susceptible to mold if it does not dry out quickly enough. It will also sag and lose its insulating properties when it gets very wet.

Frost King CF1 "No Itch" Natural Cotton Multi-Purpose Insulation, 16 x 1 x 48-Inch

Sheep and hemp wool, on the other hand, have a very unique property in that they can absorb up to a third of their weight in moisture and remain dry to the touch while maintaining their insulating properties. So if they do get wet, they will continue to insulate your house.

Another added bonus of using sheep’s wool is that the fibers of wool are made up of keratin. Keratin does not support the growth of mold, which makes your sheep wool insulation resistant to mold growth if it gets wet. 

Hemp wool, although not made from keratin, is also resistant to mold, making it a great environmentally friendly option for your insulation.

However, consistent moisture or excessive water can leach the fire retardant chemicals out of the wools, making them more prone to fire risk. It is best to bring in a professional to assess your insulation if you have had any major flooding.

If you are using cork as insulation, you’re in luck. Cork does not get wet. It will not absorb moisture, and it will stay dry through any flood.

Think about what cork has been used for before; wine bottles, buoys, and even life jackets. And because mold needs moisture to grow, your cork insulation is resistant to mold growth.

Sheep wool insulation , hemp wool insulation and cork insulation

Plastic Fiber Insulation

Plastic fiber insulation is made from fibers of various types of plastic, such as polypropylene, polyethylene, or polyester.

The fibers can be left loose to form loose-fill insulation or bonded together using binders that are then processed into various forms, such as foam boards, spray foam, or rigid panels.

The most common type of plastic fiber insulation is polystyrene. These are molded into foam boards or other shapes (packing peanuts are not one of these forms).

Polystyrene insulation is commonly used in walls, roofing, flooring, and as foundation insulation.

Woman installing a polystyrene insulation

Plastic is a hydrophobic material, meaning it is resistant to water and does not absorb moisture easily. This means if it does get wet, it will dry fairly quickly and will not be subjected to much damage.

However, moisture can become trapped within the polystyrene if it is not properly sealed or if there is a gap in the insulation where air and moisture can penetrate. This trapped moisture can lead to the growth of mold if the moisture problem is not addressed quickly.

Over time, moisture can cause polystyrene to deteriorate and lose its insulation properties, which is why it’s important to ensure that polystyrene insulation is properly installed and protected from moisture.

Mineral Wool Insulation

Mineral wool insulation is a type of insulation made from rock or stone and slag, which is a byproduct of steel production. It is commonly known as rock wool or slag wool insulation. 

The fibers are treated with a binder to give them structure, and they are processed into a range of insulation products, including loose-fill insulation for open cavities and batts for walls, attics, floors, and ceilings. 

Because of the mineral wool’s inorganic composition, it makes the insulation naturally resistant to moisture. Making it a great insulation in very humid and wet areas. If mineral wool gets wet from a leak or flooding, it’ll take a short time to dry out, and there shouldn’t be any concern for mold growth.

One of the other properties of mineral wool is that it is fundamentally fireproof, being able to withstand temperatures of up to 1,200 °F. This means that they don’t need to use a fire retardant on it.

So, if your insulation has come into a lot of contact with water due to a constant leak or flooding, you don’t need to be worried about the fire retardant being leached out. You can be sure your insulation will still be fireproof.

The only concern you may have after a long-term flooding event is over time, constant moisture can decrease mineral wool’s insulating properties. So, as soon as you notice moisture, you should deal with it immediately. 

Man cutting and installing mineral wool insulation in the attic

You can remove your insulation to dry it out fully while you deal with any damp issues in your walls or ceilings. Once that is repaired, you can return your wool insulation to your walls or attic.

Polyurethane Insulation

Polyurethane Insulation, more commonly known as spray foam insulation, comes in a liquid form that is then sprayed onto surfaces and into crevices. The foam starts to expand and fill in all the gaps where it dries and hardens.

Spray foam comes in two types. Open-cell and closed-cell. Open-cell foam is porous with interconnected cells that are filled with air. This type of foam has a lower density and is known for its soft, spongy feel.

On the other hand, closed-cell foam has a dense, compact structure where each cell is completely sealed. This makes closed-cell foam stiffer, stronger, and more resistant to moisture and air infiltration compared to open-cell foam.

Closed-cell foam can provide a moisture barrier to prevent bulk water from passing through, making it ideal for insulating basement and crawl space walls. In this respect, closed-cell foam won’t need to dry out as it won’t absorb moisture.

Open-cell foam, on the other hand, is more permeable and will allow moisture to pass through. This can allow moisture to get trapped inside the foam.

It can take several days to several weeks for open-cell foam insulation to dry out completely, depending on the amount of moisture and the surrounding environment.

Over time, if open-cell foam is continually subjected to moisture, it can degrade the effectiveness of insulation as the moisture will reduce its thermal resistance. Long-term exposure to moisture can also provide a breeding ground for mold.

It can be difficult to replace foam insulation, as it often requires removing and disposing of the old foam and then installing new foam. The process can be time-consuming and disruptive to the surrounding area, especially in areas that are difficult to access.

Foam Board Insulation

Foam board insulation is a closed-cell, polystyrene, polyisocyanurate, or polyurethane foam board. They come in large sheets or panels, which can have a laminated layer.

Laminated foam board insulation typically consists of a foam core sandwiched between two layers of paper, aluminum foil, or other material. 

Pink Insulation Foam 2" Thick (2.5 sq ft)

The laminated layer provides additional strength and stability, as well as protection from moisture and other environmental factors. That being said, non-laminated boards are still considered to be water-resistant.

Because of its closed-cell structure, foam board insulation has a dense, compact structure, which makes it resistant to water. The cells are filled with a gas, typically a hydrofluorocarbon, that gives them their closed-cell structure and acts as a barrier to moisture

This means that moisture cannot penetrate the foam board and cause damage to the insulation properties. If it gets wet, it will be dry as soon as the water evaporates, as it will not absorb any of the water. You can accelerate the drying time by using a towel to wipe down any moisture.

Foam board insulation is perfect for basement and crawl space insulation where moisture is at its highest. 

Because foam board is impermeable to water, it could cause water to be trapped behind it. Therefore, if you do notice any moisture or condensation around your insulation, you will need to access and repair any leaks you may discover.


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