Why Should You Put Foam Under Concrete? Everything You Need To Know

Put Foam Under Concrete

If you are fairly new to construction, then you have likely seen contractors and builders place rigid insulation under concrete. It may seem like a rather odd thing to do, but there are a variety of different reasons why this is done. In this article, we are going to be answering exactly that – why put foam under concrete?

Simply put, the main reason why foam is placed under concrete is to create insulation. It also provides stability by filling up any gaps between the concrete and the ground. Generally, expanded polystyrene, or EPS, is used for this purpose.

Did you know that in 2015, it was reported that more than 50% of a household’s annual energy consumption in the USA is mainly attributed to space heating and air conditioning? However, this leads to a lot of energy loss. And the main culprit behind it is poor insulation. It has been estimated that approximately 10-15% of a building’s total heat loss is mainly caused by uninsulated foundations. 

With all that said, let’s find out what are some of the primary reasons why you should put foam under concrete.

Why Put Foam Under Concrete? Top Reasons Explained

There are three key reasons why contractors put foam under concrete. Below we have elaborated on these points.

Water Absorption

It’s common knowledge that foam absorbs water. That is why most people prefer to use EPS under concrete, as it does not absorb as much water as regular foam. Otherwise, if water comes into contact with concrete, then it can cause algae and fungus formation around it, which can severely affect the reinforcement strength of the concrete block. 


To further expand upon the previous point, bugs and insects love wet, damp places. This can potentially lead to infestations within your structure, which can be a potential cause for loss of foundational integrity. For this, you must ensure that no water comes into contact with either the foam or the concrete, which we will discuss further later. 


Last, but not least, one of the primary reasons why people put foam under concrete is to improve insulation when living in cold countries. Without proper insulation, the difference in internal and external temperatures can cause a rapid loss of heat from the inside of your home. EPS helps to reduce the total heat loss in residences, cold storage units, manufacturing plants, warehouses, etc., by creating a barrier through which heat cannot escape. 

How Should You Layer Concrete Slabs?

According to experts, you should layer them from the bottom up. First, place the crushed stone, then put the EPS on top of it, next comes the polyethylene, and finally, the concrete.

Put Foam Under Concrete
How Should You Layer Concrete Slabs?

Why Do You Need Polyethylene?

As time passes, water from the soil slowly gets absorbed into the rigid foam. This can cause the structural integrity of the concrete to deteriorate slowly over time. This is where polyethylene comes into play as it acts as a vapor barrier between the concrete and the rigid foam, thus, preventing any moisture from coming directly into contact with the concrete blocks.

How to Put Foam Under Concrete?

  • Wait for warm, dry weather to prevent any moisture from getting trapped within your insulation.
  • Dig as deep as per your requirement and smooth out the excavated area while filling out the low spots with dirt. 
  • Spread a layer of gravel over the dirt that is around 8 inches deep.
  • Smooth out the gravel to prevent any jagged edges from sticking up. 
  • Then, install the rigid panels of insulation across the foundation.
  • Leave a vapor barrier between the concrete and the rigid foam by using polyethylene.
  • Add a non-permeable membrane across the perimeter to prevent bugs from entering the structure.
  • After ensuring everything is smooth, pour the concrete. 

And there you have it! Hopefully, this article answered your questions relating to why put foam under concrete, how to carry out the process, how you should layer them, and the importance of polyethylene. So, the next time you see a contractor do this, you’ll know exactly why it is being done.

Leave a Comment