Skip to Content

The Dangers of Wet Insulation | Risks, Smells and Efficiency Loss

Storms, roof leaks, and humidity are just some of the ways that insulation can become wet, smelly, ineffective, and potentially dangerous. It’s important to know what to look out for and what it means for your home.

The health risks, smells, effects on efficiency, and solutions will vary based on which type of insulation you have. Not all smells are related to moisture, but moisture always needs to be addressed when it occurs, and often insulation needs to be treated or replaced.

Moisture can damage the structure of a home, grow mold and bacteria, and reduce the effectiveness of insulation by 40%. The risks depend on the type of insulation, with cellulose insulation being the most vulnerable.

Risks Associated With Wet Insulation

Structural Unsoundness

Insulation itself is not structural. However, it rests against the house frame and below the roof’s membrane. When wet, it will hold moisture against beams, fasteners, and steel studs. Over time, that moisture can cause these structural elements to warp, rot, and rust. 

The wetter the insulation gets, the heavier it will become—as much as 275% heavier. Extra weight will be an extra burden for your walls and roof, which can lead to structural damage. 

The risks depend on the existing condition of the wood and fasteners, for example, if the house is very old or poorly maintained.

It also depends on what type of insulation you have.

Types of insulation, spray foam insulation, stone wall insulation, fiberglass insulation and cellulose insulation

Spray foam insulation is a moisture barrier, so it’s less likely to become soaked.

Stone wool repels water. But water will still be present near the surrounding wood and can cause damage.

Fiberglass insulation cannot itself absorb water, but the air pockets between the fibers will fill with moisture, which can transfer to the structure of the house. A professional will likely need to remove the insulation to dry, but it can often be reinstalled unless it is contaminated by dirty water.

Cellulose insulation will hold the greatest amount of moisture, therefore posing a higher risk to wood and fasteners. It’s made mostly of recycled newspaper and absorbs water like a sponge. If a substantial amount of cellulose insulation gets wet, it will need to be replaced entirely.

In addition, the sagging beams will have to support the water-logged, and thus, heavier insulation.

Growth of Bacteria

In the damp environment created by water-damaged buildings, insulation serves as the perfect food for bacteria.

The phrase Sick Building Syndrome refers to the fact that some airborne microbes associated with building materials can cause headaches, nausea, eye irritation, chest tightness, and more.

One common type of bacteria is actinomycetes, which will grow in wet insulation and then enter the indoor air supply. They have been associated with lung infections and a lung condition called hypersensitivity pneumonitis.

Another type, called mycobacteria, can irritate the respiratory system and exacerbate allergies.

Sick building syndrome, girl experiencing headache inside the house due to airborne microbes

While not bacteria, another air contaminant associated with insulation is VOCs (volatile organic compounds), which can cause eye, nose, and throat irritation. Fiberglass, polyurethane foam, and polystyrene are the most common insulation types that off-gas VOCs.

Bacteria are a risk regardless of insulation type. Still, cellulose insulation gets wetter faster and stays wetter longer, which means that bacteria are likely to grow sooner.

Fiberglass insulation is less likely than cellulose insulation to grow harmful bacteria and mold (more on mold later), but that depends on how long it has been wet and where the water initially came from.

Formaldehyde Off-Gassing

Fiberglass, mineral, or wool insulation typically contains formaldehyde. When wet or hot and humid, formaldehyde will deteriorate and start to off-gas. Formaldehyde off-gas is corrosive, which poses additional risks to your roof fasteners and studs.

It also poses health risks. Fiberglass insulation can emit 32 PPM of formaldehyde off-gas, which can lead to eye irritation, upper respiratory irritation, and pulmonary effects.

Formaldehyde off-gasses for long periods of time once wet and the levels of formaldehyde in the air will never return to zero.

The risks associated with formaldehyde are why there is a push to eliminate its use in furniture and other household products.

Mold and Mildew Outbreaks

It can take only a few days for mold to start growing. Mold decreases indoor air quality and can lead to headaches, throat irritation, coughs, itchiness, and worsening of pre-existing respiratory issues, like asthma.

Mold, like other types of fungi, produces mycotoxins that are spread through the air. Mold can also produce allergens and spores that irritate those who breathe them in.

Water damage is associated with black mold, which is particularly dangerous.

Cellulose will grow mold fastest. The EPA emphasizes that cellulose-containing building materials must be dried within 24-48 hours. 

There only needs to be a small amount of moisture present for mold to start. Worse, it’s almost impossible to completely remove from the wet, loose material of cellulose.

Unlike cellulose, fiberglass is not a food source for mold and mold can be cleaned off. However, mold can cause fiberglass to degrade, leading to the off-gassing of VOCs, including formaldehyde. Still, off-gassing is unlikely to be a major concern if you take care of water damage in a timely manner.

Like fiberglass, stone wool insulation (also called mineral wool or rock wool) is inorganic and, therefore, cannot serve as food for mold. Stone wool is even more mold-resistant than fiberglass because it repels and drains water.

Man in ppe checking the damped insulation

What Does Wet Insulation Smell Like?

Smell is an important indicator of wet insulation. While the outside layers might be dry, the inside layers can still be wet and odors will let you know.

Smells can vary widely by type of insulation.

Wet cellulose insulation will have a musty odor, which can indicate that it has been wet for an extended period of time and is growing mold. 

Wet fiberglass will have a sharper, more chemical smell like formaldehyde or rubbing alcohol. This odor can be the result of off-gassing, so it’s best to use a respirator and ventilate the area when working with wet fiberglass insulation.

A distinct smell coming from fiberglass insulation doesn’t always mean that it’s wet. If your insulation smells like burnt cookies or burnt sugar, it is overbaked. This means that it’s a non-formaldehyde fiberglass insulation and that the batts were cured for too long.

Both fiberglass insulation and stone wool insulation have been accused of smelling like cat urine or ammonia when wet. That can be attributed to formaldehyde, which is a common binder for both types. 

A “cat pee” smell can be a sign of not only wet insulation that contains formaldehyde, but also the growth of black mold, so get it checked out.

Some customers have reported an ammonia smell even when stone wool or fiberglass insulation is not wet. This is likely a VOC, but not at an elevated level compared to the normal amount of VOCs in a home.

So, while an ammonia smell from dry insulation is not dangerous to your health, it can be unpleasant. People have very different senses of smell, so this comes down to personal tolerance.

Smells will also become less pungent when sealed within walls.

Effect of Moisture on Insulation Function

Water conducts heat out of a building—think of how wet hair makes your head colder. Moisture has a big impact on the function of insulation, which we call thermal performance or R-value

Wet insulation can lose approximately 40% of its original R-value, meaning that it is 40% less effective.

The loss of efficiency happens for different reasons in different types of insulation.

Cellulose insulation is particularly vulnerable. Moisture leads cellulose loose-fill to settle, which creates a gap between the insulation and the exterior wall. Air will move within these gaps, leading to a draftier home.

Cellulose insulation is expected to settle a little, which is accounted for in the installation. However, any level of added water or humidity can exacerbate the settling, even just a single leak. Cellulose insulation is a type of “blown-in” insulation. Even once dry, blown-in insulation will not re-expand.

Fiberglass insulation only temporarily loses thermal resistance if treated properly. Water replaces the air pockets between fiberglass strands, which makes it insufficiently light and fluffy.

If there was a low level of moisture, a dehumidifier could do the trick to dry fiberglass insulation. But if there was a leak or water infiltration, you may need a professional to remove and dry the insulation.

Similarly, stone wool insulation can be thoroughly dried without impacting the original R-value. However, as long as it is still damp, it will be less effective. Stone wool will absorb less moisture than cellulose or fiberglass, but it still needs to be thoroughly dried if exposed to water.

What the Reduced Effectiveness Means

Damage to your insulation can mean damage to your personal comfort, energy bills, and property.

At the most basic level, ineffective insulation can mean that your house becomes cold and drafty in the winter.

Settled cellulose insulation can leave gaps of many inches that are essentially uninsulated. These gaps can also allow cold air to enter in the winter and escape in the summer. As water conducts heat, damp insulation can also draw heat into your home in the summer.

If you can’t maintain the temperature of your home as efficiently as you once could, your energy consumption will increase. This has consequences not only for your wallet but for the environment.

Keeping a consistent indoor temperature is important to caring for many household objects. Wood expands and contracts when the temperature fluctuates, so ineffective insulation could damage flooring and furniture over the long term.

Additionally, expensive objects like musical instruments—particularly pianos—can be damaged by temperature and humidity changes.


Amazon and the Amazon logo are trademarks of, Inc, or its affiliates.