Skip to Content

Home Efficiency Guide is an affiliate for companies including Amazon Associates and earns a commission on qualifying purchases.

Can Foam Board Insulation Get Wet?

Can Foam Board Insulation Get Wet?

When it comes to thermal insulation, foam board insulation is regarded as a great alternative to the fiberglass batt, which cannot compare in terms of R-value. Foam board insulation is compact, durable, easy to install, and serves to control a building’s energy use efficiently, which essentially lessens energy cost. 

Foam board insulation products are generally considered water and mold resistant but expanded polystyrene foam boards (EPS) have air bubbles that can collect moisture and become wet. Moisture makes it hard to prevent mold from growing, making the foam board also prone to mildew growth. 

Nonetheless, since foam boards are made of different materials and sizes, read on to find out what other alternatives you can use and what strategies you can implement when your foam board gets wet. 

Types of Foam Board Insulation

There are three types of foam board insulation, according to the U.S Department of Energy, such as:


This is the most expensive type of foam board insulation. It is versatile, fire-resistant, durable, compatible with most solvents used in construction adhesive, and environmentally friendly (source).

Polyiso foam boards have the highest R-value (R-7.0 – R-8.0) per inch thickness. R-value refers to the material’s ability to resist the transfer of heat and insulate a structure from heat loss and moisture. Since polyiso insulators have the highest R-value they have the best insulating power and waterproof capabilities. 

This insulation also contains a reflective foil facing on both sides. This foil facing works as a radiant barrier making it a great insulator for radiant heat. Polyiso foam boards are rigid, meaning they won’t collapse or lose their thermal performance in the course of time.

In addition to providing quality insulation, Polyiso foam boards add a clean appearance to space, making them suitable for wall insulation, sheathing purposes, or roof insulation.

Extruded Polystyrene (XPS)

This foam board has an R-value of 4.5 to 5.0 per inch thickness. It has a smooth plastic surface and comes in two colors, including pink and blue. Extrude polystyrene foam board is easy to work with and lightweight, making it easy to transport. It is also reasonably priced.

This foam is widely used in residential construction and is deemed ideal for insulating basement walls, foundations, and cement slabs.

Expanded Polystyrene

The Expanded polystyrene foam board has the lowest R-value (R-3.6 to R-4.0), making it the least preferable type of insulation in the market.

It is commonly used for ICFs, also known as insulated concrete forms, and sometimes it is used on commercial buildings for wall and roof panel insulation that is fitted between light gauge metal.

It has air bubbles that are good for stopping heat transfer, but they can also collect moisture making the foam board ineffective against mold and mildew growth.

This is the cheapest type of rigid foam insulation.

What Can You Do When a Foam Board Gets Wet?

Foam board insulation used on the foundation inside a basement, on the exterior foundation, and on the outside of the house has a high chance of getting wet. For this reason, the procedure to follow when your insulation gets wet will vary also depending on the material or type of insulation used. 

Nonetheless, this doesn’t mean that your insulation is damaged.

To salvage a wet foam board insulation, start by assessing the area with the issue. Determine whether the insulation is wet due to moisture build-up, condensation, a leak, or flooding. 

Depending on the extent of structural dampness, you can dry the moisture with a towel or leave it to evaporate. Polyiso foam board, which also has a closed-cell structure, doesn’t absorb water; hence it can be wiped down.

A dehumidifier or fans placed against the wet area can also hasten the drying process. 

Essentially the extent to which the foam board is wet will determine whether you’ll remove the foam boards to remedy it.

Can You Still Use Wet Foam Board Insulation?

Wet insulation is prone to pests, mold, mildew, and eventually, it can cause damage to your house. It can change the air quality of the home, efficiency of the insulation, and the structure, which can lead to more costs.

Prevention measures are a great way to keep such costs at bay.

Use caulk or tape to seal up and to prevent air movement. According to CMHC, air flowing through gaps is 30 times more likely to transfer water vapor through building structures than through the simple diffusion of water vapor. 

Foam board insulation is available in a variety of facings, thickness, lengths making it easily customizable to fit any gap, cavity, or hole in your basement, attic, or crawl space. 

How Can You Waterproof a Foam Board?

Foam board insulation doesn’t absorb water, which essentially limits the amount of moisture build-up in your insulation. Polyiso and extruded polystyrene foam boards have the strongest waterproof properties.

Foam boards that are treated with a foil vapor barrier are the best in terms of preventing moisture from reaching surfaces

EPS is susceptible to crumbling and absorbs more moisture. However, it can be treated with air barriers that prevent moisture from transferring through air currents. 

What Is the Difference Between Vapor Barriers and Air Barriers?

Vapor barriers and air barriers are both used on insulations for treatment purposes. However, both are used in different circumstances and for different reasons.

  • Vapor barriers limit the movement of water vapor by diffusion.
  • Air barriers control the migration of moisture-laden air by allowing it to disperse.

Air barriers are used where vapor barriers cannot be efficient. 

For instance, since vapor barriers have holes, more water might pass through as the humid air passes. This means the vapor barrier could allow more water to flow to a structure instead of stopping it.

In such circumstances, air barriers are deemed more effective than vapor barriers. Nonetheless, both can be used together to increase energy efficiency and the overall life of a house. 

Which One Should You Choose: Air Barriers or Vapor Barriers?

To decide which treatment option will be ideal for your insulation, constructors advise that you consider these three options:

  • Climate
  • Your home 
  • The location of the surface you’re insulating

Vapor barriers are often applied on the warm side of your insulation to inhibit warm, humid air from condensing on a cold surface inside the wall.

Therefore, if you happen to live in a predominantly cold climate, use a vapor barrier on the inside of the roof, ceilings, walls, attics, or crawl spaces to stop the moisture from condensing. 

Water on such areas could lead to mold and mildew growth. In addition, prolonged moisture can result in wood rot. If you live in a hot climate, the general rule is to apply the vapor barrier on the exterior of the surface instead of the inside. 

Air barriers can be located on the interior or exterior of the surface as they prevent the movement of air. 

Air barriers and vapor retarders often contain foam board insulation or more materials such as Builder’s foil or polyethylene plastic sheets. 


In addition to helping you maintain the desired temperature in your home, foam board insulation allows you to cut down on energy costs.

Foam board insulations also have a high R-value and low perm rating since they do not allow water to diffuse easily through the material.

This makes foam board insulators such as XPS effective vapor retarders. 

Both vapor barriers and vapor retarders are good options for moisture control and are placed around the perimeter of a structure to increase its longevity and thermal regulation.

Related Reading:

Leave a comment

    American Home Shield provides warranty coverage for your essential home appliances and systems. Compare all plans.