Use of Expanded Polystyrene Under Concrete Floor Insulation

Insulation under a concrete slab acts as a strong barrier against everything that may come through the ground, including toxic gases, unwanted creatures, cold temperatures and moisture. It also controls and controls thermal discrepancies and transfers as well as muffles noise from outside.

Rigid floor insulation is typically installed on a gravel base and poly vapor diffusion retarder, with a sheet of XPS or EPS insulation on top. Choosing the right rigid foam insulation for your project can save a lot of time and money.


The use of expanded polystyrene under concrete floor insulation is a cost-effective way to meet new energy efficiency requirements. This type of insulation helps you reduce your energy costs while meeting prescriptive R-values for your climate zone.

EPS is the cheapest rigid foam insulation available. It is also easy to install, making it an economical option for many projects. In addition, EPS has long-term R-value stability, meaning that its thermal performance doesn’t degrade over time. This is a benefit that other types of insulation do not have.

Another advantage of using EPS under slab insulation is that it can absorb the pressure of heave, which can occur over time due to heat, humidity and wet weather. This can save you from having to dig internal trenches. This is a great advantage in regions with wet soils. It also means that minimal earthworks are required for your building project. This can also help you avoid the costly costs associated with soil remediation and disposal.


When you install rigid insulation under concrete slabs, it prevents heat transfer and reduces the amount of energy needed to warm or cool a building. It also helps protect concrete from frost heave.

Many contractors use extruded polystyrene (XPS) for below-grade and under-slab insulation. However, expanded polystyrene (EPS) offers similar or better performance across key below-grade and under-slab insulation attributes such as moisture retention and compressive strength.

EPS is made by placing beads of foam in a mould and heating them to fuse together. One cubic metre of EPS contains 10 million beads each counting approximately 3,000 closed off cells. Its dense construction resists heat loss and transfers and acts as an effective moisture barrier to prevent mould development and mutes external noises.

A study conducted by the U.S Department of Energy found that XPS retained less than half its R-value after 15 years in contact with moisture. On the other hand, EPS retained 94% of its original R-value in the same study and is one of the few insulations that resists moisture permeability.

Moisture Resistance

When used for concrete floor insulation, a layer of expanded polystyrene underfloor insulation prevents cold from migrating through the slab and provides a moisture/vapor barrier that helps to maintain a dry space below-grade. This prevents mold, mildew, and unwelcome creatures from settling in and also reduces energy consumption.

Rigid insulation is a good choice for floors as it is durable and will not degrade over time. It is normally installed over a damp proof membrane (DPM) to avoid capillary action of water through the floor. It is important to use a DPM that meets ACI specifications.

Some insulation manufacturers offer XPS rigid foam insulation, which offers a compression strength of 300kPa. However, EPS insulation performs just as well and is significantly cheaper. Installing an insulation that is rated for a higher compressive strength than required can significantly increase costs. EPS is the best rigid insulation for floors because it can handle the weight of the concrete slab and foundation backfill and still provide adequate insulation.

Noise Reduction

EPS insulation is an effective way to reduce noise in a new build. It is available in a range of sizes and thicknesses, and can be custom-cut for specific job specifications. EPS is a great option for below-grade and slab insulation due to its low moisture absorption and strength.

It is also highly resistant to moisture ingress. However, it is not completely impervious to water vapor ingress. It is important to know the vapor resistance of the insulation you choose before you use it in your home.

While EPS is an effective thermal insulator, it does not provide as much sound control as other types of insulation. Its acoustic performance can be improved by using acoustic barriers with it.

EPS is a lightweight building material that is easy to install in walls. Its low density helps it decouple the layers in a wall and absorb sound. EPS is molded into different shapes and can diffuse noise frequencies between 600 Hz and 5,000 Hz.

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