Does Styrofoam Absorb Water?


Are you thinking about doing insulation? If you are thinking about making it water resistant, your task requires waterproof materials. Are you wondering what could be suitable materials? If you are searching for a good answer, styrofoam could be the insulation material. 

Styrofoam is Polystyrene foam mainly used for insulation and cushioning because of its design and properties. This polystyrene foam is made of more than 95% air. It is used for various applications, including thermal insulation. This article will also answer if styrofoam insulation is waterproof and its other characteristics. Keep reading to know more about styrofoam.

Is Styrofoam Waterproof?

Styrofoam is not fully waterproof. A Material is called waterproof if it’s wholly impervious or impenetrable to water. This means water cannot pass through that material and also not absorb any water in the process.

So, for example, styrofoam insulation can be water-resistant but not fully waterproof. This is because styrofoam does not absorb moisture from the air but can absorb water if it directly interacts with water. 

Although polystyrene is a waterproof plastic, styrofoam isn’t wholly waterproof or moisture resistant. This applies to styrofoam and other expanded and extruded polystyrene foams.

They’re made of polystyrene molecules linked together to form a huge network of bonded pellets. However, as all these materials have a closed-cell foam construction, some “interstitial gaps” remain between the expanded closed-cell pellets. These gaps are too small to let water pass through easily to the other side but not small enough to prevent it entirely.

Is Styrofoam the Same as Polystyrene?

Styrofoam is the expanded version of polystyrene plastic. Polystyrene is a plastic that is hard, solid, and clear. Still, it can be made into a foam-like material called Extruded Polystyrene (XPS) or Expanded Polystyrene (EPS), depending on the nature of the material.

The trademark name for closed-cell extruded polystyrene foam is Styrofoam (by The Dow Chemical Company). Still, the word “Styrofoam” is often used as a synonym for polystyrene foam and is also widely accepted as an alternative term to describe expanded polystyrene foam (EPS). 

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Polystyrene is a highly versatile type of plastic used in various projects and construction and has many practical applications in multiple fields. The material is incredibly lightweight but has a pretty stiff structure for construction.

Does Styrofoam Float in Water?

Yes, styrofoam floats in water. This depends on an object’s density or how closely packed its matter is. A thing that is less dense than water will float in water. Since the density of water is much greater than the density of Styrofoam, it will float. Styrofoam is not very tight, which makes it suitable for buoyancy

Generally, the material with less density has a stronger buoyant force. However, the strength of the buoyant force may not be the only thing required here. At the same time, the air is very light and, when trapped, can provide powerful forces. Therefore, styrofoam works well as it is comprised mostly of trapped air. 

As per Archimedes’ principle, the upward buoyant force on an object equals the weight of the water it displaces. This should balance the downward force of your weight when you stand on it and the weight of the material you are using to stand on. Therefore, that material needs to be strong enough to not collapse when you stand on it while it sinks into the water and is light enough.

Predict what will happen when you put the Styrofoam piece in the water. Will it float? Continue reading to discover more.

Extruded Polystyrene (XPS) Vs. Expanded Polystyrene (EPS)

The key difference Between Expanded and Extruded Polystyrene is that expanded polystyrene or EPS is a thermoplastic foam material produced from solid polystyrene beads.

In contrast, extruded polystyrene or XPS is a foam material made from solid polystyrene crystals. Coming to the production processes, the expanded polystyrene has a less harmful effect on the environment than the extruded polystyrene.

Expanded polystyrene is lightweight, rigid and closed cell insulation. The best part is excellent thermal insulation, damping properties and extremely lightweight. As 98% of expanded polystyrene is air, it is one of the lightest packaging materials that make transport costs minimum. These properties make it useful as building materials, packaging materials, model planes, etc. 

Extruded polystyrene is another foam made from polystyrene whose trademark name is Styrofoam. It has a uniform closed-cell structure, smooth skin, and superior moisture resistance.

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Moreover, its excellent chemical resistance and soil compatibility make it suitable for outdoor storage. It is therefore used in construction and engineering applications. Both these materials are thermoplastic and rigid. 

What Happens When You Put Styrofoam in Water?

It will float. But where? It will float to the edges first and then in the middle of the container if totally filled. However, water may pass through foam under pressure or by gravitational force. In addition, water may enter a cell or interstices of a cell if the cell is opened or the gaps are opened. 

But water cannot dissolve styrofoam as water is a polar solvent. Therefore, when removed from the water, it will no longer contain any water and will retain all its insulation capabilities.

Let’s experiment with a tall clear drinking glass, water and a styrofoam piece. Fill the drinking glass halfway with water and put the Styrofoam piece in the water. Then start filling the remaining cup portion up to the top with water. Pour the water in slowly to fill the water past the top of the lip of the cup. Keep the cup at eye level to see what the water’s surface looks like! What happens to the foam piece now? 

The first thing is that whatever is floating in the water will reach its highest point. So, when the water is halfway filled, the foam piece will always float to the edges as they are the highest water points.

If the cup is filled, a curve can be observed at the top of the water forming a bubble shape with the foam directly in the middle of the cup as it is the highest point, not the edges! A property of water called “cohesion” is applied where water sticks to other things, such as glass.  

Large foam board parts might make it difficult for any moisture gathered between the wall and the insulation to be absorbed by the wall and escape. This remaining moisture accumulates against the wall, promoting the growth of bacteria and mold and the development of wood rot.   

What Happens To Styrofoam Under the Sea?

Let’s take the example of styrofoam cups. The Styrofoam cups experience massive pressure in the deep sea. As a result, it shrinks uniformly as it is lowered into the ocean, and the air bubbles are pressed out of it. 

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Styrofoam is made up of polystyrene beads expanded by bubbles of air. During descents into the ocean, the accumulated weight of water increases, and air pressure builds up, which is about 14.7 pounds per square inch for every 33 feet (10 meters) of depth. As pressure mounts, it squeezes the air out of Styrofoam objects. As a result, it becomes a shrunken, polystyrene matrix of a fraction of the size of the original cup. (NOAA research).

Does Styrofoam Absorb Heat?

Styrofoam is made mainly of air. Therefore, it is a poor conductor of heat but a good insulator. However, it is an excellent convector. It traps the air in small pockets and blocks the flow of heat energy. This helps reduce both conduction and convection, making styrofoam a good insulator.

A styrofoam cup makes for an excellent adiabatic wall and keeps all the heat released or absorbed by the reaction inside the cup so it can be measured.

Can Styrofoam Get Mold?

No. Styrofoam does not support mold growth. Mold occurs due to excessive moisture and an accumulation of organic material(nutrients) such as dirt. By nature, styrofoam contains no properties that naturally support mold growth.

Mold needs moisture to grow, and mold spores cannot germinate on a dry surface. Moreover, like all living organisms, mold also needs food other than water. Mold can feed on any plant organism containing a nutrient called cellulose like cotton, wood and paper. In addition, it feeds on other organic materials such as hair and soil. But, styrofoam bears no organic materials; therefore, it does not support mold growth. 

When moisture penetrates a porous surface, it becomes trapped for a long time. Then it becomes ideal for enabling proper mold growth. Styrofoam is one of the most naturally mold-resistant forms of insulation, but it can still grow mold on its surface in extreme cases.

Except in intense saturation, like flooding that compromises the insulation, styrofoam does not hold water and contains enough natural water resistance to stop mold in its tracks.


Certain awareness also needs to be raised about styrofoam. First, it is a hazardous substance. We should know what’s happening to our oceans regarding plastic pollution, particularly microplastics. Polystyrene is non-biodegradable and accumulates along shores and waterways, especially in its foam form; it is one of the most common microplastics on the ocean’s surface.

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