How To Waterproof Styrofoam (2 Effective Methods)

Styrofoam is a versatile, lightweight material for insulation, packaging, and crafts. However, it is not naturally waterproof and needs to be treated to protect it from moisture.


How To Waterproof Styrofoam

Styrofoam is often used in craft projects and as packaging material, but it’s not always the most durable.

If you want to ensure your Styrofoam project can withstand wet conditions, you’ll need to waterproof it. Luckily, there are a few different ways to do this.


Coat It With A Layer Of Polyurethane

One way to waterproof Styrofoam is to coat it with a layer of polyurethane. You can find polyurethane in spray form or as a brush-on solution.

Apply the polyurethane to the Styrofoam and allow it to dry according to the manufacturer’s instructions. This will create a durable, water-resistant barrier on the surface of the Styrofoam.

Seal It With A Layer Of Grease

Another way to waterproof Styrofoam is to seal it with a layer of grease. This method is best for projects exposed to water for only short periods, as the lubricant can eventually degrade and allow moisture to penetrate the Styrofoam.

To use this method, apply a layer of grease to the surface of the Styrofoam, enabling it to dry.


What Is Polystyrene Foam?

Polystyrene foam is a plastic type often used for packaging and insulation. It is made from a substance called styrene, a liquid that can be turned into a solid foam.

Polystyrene foam is lightweight and has an excellent strength-to-weight ratio, making it ideal for many applications. It is also an excellent insulator, both thermally and acoustically.

Polystyrene foam can be made into many different shapes and sizes, and it can be easily cut, molded, and glued. It is also resistant to most chemicals and does not absorb water.

Polystyrene foam is often used in packaging, insulation, and construction applications. It is also frequently used as a material for modelmaking and sculptures.

Are Polystyrene Foams Waterproof?

Polystyrene foams are not waterproof, but they are water resistant. This means they will not absorb moisture from the atmosphere or make contact with liquids, but they will allow some water to pass through their pores.

Because of their low thermal conductivity, polystyrene foams are often used as insulation in buildings and appliances. They are also used in packaging and the food service industry.

How Well Does Styrofoam Hold Up Against Water?

As it turns out, Styrofoam is quite resistant to water. Many experts believe it can take up to 30 minutes for the water to penetrate the material.

This means that if you’re looking for a material to use in a waterproofing project, Styrofoam could be a good option.

Of course, there are some caveats to this. For one, Styrofoam is not entirely impervious to water. It will eventually soak through if left in liquid for long enough.

Additionally, the material is not very strong when wet, so it’s essential to take care when using it in a waterproofing project.

Overall, though, Styrofoam is a reasonably good option for waterproofing. It could be a good choice if you’re looking for something that will keep water out for a short period. Just be sure to take care when using it, and don’t expect it to last forever.

How Long Is Styrofoam Expected To Last?

On average, Styrofoam can last for around 50 years. However, this number will differ depending on the circumstances in which it is stored. If Styrofoam is exposed to sunlight or heat, its lifespan will be shortened.

Additionally, if Styrofoam is regularly used and abused, it will not last as long as it would if stored in ideal conditions. With proper care, however, Styrofoam can last for many years.

How To Keep Styrofoam Waterproof

There are many ways to keep Styrofoam waterproof.

One way is to use a sealant or varnish. Sealants and varnishes are great for keeping water out of tiny cracks and crevices. They can also be used to coat the entire surface of the Styrofoam.

Another way is to use a water-repellent spray. You can also use a combination of both methods. Water-repellent sprays can coat larger areas and provide an extra layer of protection.

When using a sealant or varnish, be sure to follow the instructions on the label. Some products may require multiple coats. Water-repellent sprays can be applied liberally.


Let the sealant or varnish dry completely before exposing the Styrofoam to water. If you are using a water-repellent spray, you may need to reapply it after some time, depending on the level of protection you need.

What Can I Seal Styrofoam With?

There are many ways to seal Styrofoam, depending on your need. If you need to waterproof it, you can use a product like Liquid Nails or contact cement.

You can use packing or duct tape to make it airtight. Whatever method you choose, ensure the surface is clean and dry before you start.


If you’re sealing Styrofoam for a craft project, you may want to use something that will add colour or texture. You can paint it with acrylic paint, spray it with adhesive, or cover it with fabric. Make sure whatever you use is compatible with the type of Styrofoam you’re working with.


Sealing Styrofoam is a great way to recycle it for use in other projects. Once you’ve sealed it, you can use it for insulation, soundproofing, or even as padding for packaging. With a bit of creativity, the possibilities are endless.

How Do You Protect Styrofoam From Water?

If you’re worried about water damaging your Styrofoam, you can do a few things to protect it.


First, you can try sealing the Styrofoam with a clear sealant or coating. This will create a barrier between the Styrofoam and any water that comes into contact with it.

You can also try painting the Styrofoam with waterproof paint. This will help to repel water and keep the Styrofoam from getting damaged.

Finally, you can store the Styrofoam in a dry, cool place. This will help to prevent it from getting wet or damaged by moisture.


Can You Spray Flex Seal On Styrofoam?

Yes, you can spray Flex Seal on Styrofoam. This product is excellent for sealing any cracks or holes in your Styrofoam boards.

It can also coat the outside of your Styrofoam board to protect it from weather damage. Flex Seal is an excellent product for Styrofoam because it is straightforward to apply and dries quickly.