What Is The Best Insulation For Under A Concrete Slab

What Is The Best Insulation For Under A Concrete Slab

Some areas of the house undergo extreme temperatures, depending on the time of the day and location. Basement and attic are some of these spaces that can be extremely hot or cold. To manage the room temperature in these areas, proper insulation is necessary. If your basement is made up of concrete slab, you might be wondering what is the best insulation for this area. 

Importance Of Proper Home Insulation 

Insulating the concrete slab spaces in your home comes with three main benefits – heat loss prevention, moisture prevention, and providing comfort to the residents.

Insulating your basement is beneficial for you and your family in many ways. It can prevent heat loss in your home. During the coldest season of the year, one of the challenges of any homeowner is to retain the heat inside their homes. If some areas of their home are not insulated, the heat easily escapes, causing their HVAC system to work harder. This could spike their electricity bill, as a result. A basement is a place in your house that accounts for almost one-third of the heat loss in the house. A good insulation system is important to reduce heat loss, elevate home comfort, and conserve electric energy. 

Insulation also helps in protecting your concrete from moisture. Although concrete slabs look solid and strong, it is porous. The moisture from the ground can easily seep into these pores, causing unwanted molds, mildew, or water damage. Any material coverings of the concrete slab, like carpets or vinyl, will also be affected or even damaged by this moisture. 

Additionally, insulation provides a comfortable feeling to the occupants of the house. If you want to the highest level of comfort, you may install radiant heating mats before attaching your finished floors. 

Insulating Under A Concrete Slab 

Insulating areas under a concrete slab like the basement of your home or building is important as heat is also lost through these areas. Placing an insulation barrier and a vapor barrier between the concrete slab and the ground underneath is great in maintaining the warm air and protecting the space from unwanted moisture. 

How To Select the Right Insulation For Your Space 

So which insulation is best for buried foundations and under concrete slabs? Read the following pointers on how to choose a cost-effective and high-quality insulation type for your project. 

Look For Long Term Thermal Performance

Expanded polystyrene (EPS) and extruded polystyrene (XPS) are two of the strongest foam insulations that are most popular in below grade and under concrete slabs. Both of these insulations are closed-cell, yet their performance differs over a long period. 

Extruded polystyrene (XPS) posses a higher initial insulating R-value and has the same density and thickness as EPS, but its R-value depletes over time. R-value refers to the resistance value or the capacity of insulating materials to resist temperature flow. An insulating material that has a higher R-value has a higher insulating capacity. 

On the other hand, expanded polystyrene (EPS) is a more consistent type of insulation material because it does not undergo any thermal drift. Its reported R-value also stays the same over the years. 

R-value is one of the critical factors in choosing the right insulation for your space. A decreasing R-value translates to lower thermal capacity over time, therefore expanding the heating and cooling energy, and eventually the cost for the electricity. 

The best way to check the insulation’s long term thermal performance is to read the warranty. Expanded polystyrene (EPS) manufacturers usually cover the warranty 100 percent of published R-value for 20 years. While most extruded polystyrene (XPS) commonly includes only up to 90 percent of the published R-value in the warranty. This is to consider the R-value degradation that will occur in the long run. 

Verify Minimal Long Term Moisture Absorption 

Aside from the R-value factor, long term moisture absorption is another important thing to consider in foam insulations. Rigid foam insulations vary in their rate of moisture absorption and drying capacity. If the insulation material is wet, it degrades over time and provides less thermal protection.  This is something to consider since most insulation materials are installed below grade and often in contact with wetted soils. 

When it comes to moisture absorption, EPS performs better because it can dry quicker. This feature helps the EPS to be protected from repeated moisture exposure. But XPS posses a stronger capacity for moisture absorption. There are studies found that XPS insulation installed below grade for several numbers of years had absorbed 67 percent of ground moisture. That’s more significant capacity for moisture absorption compared to the EPS.

Look For The Right Compressive Strength

Ensure that the rigid foam insulation is not over-engineered before installing it under concrete slabs. This can help you save on costs over time. The issue with over-engineered insulation foam is that engineers frequently use an extremely conservative method for insulation under concrete slabs. These people assume that point loads applied to a slab distribute to the insulation in a triangular load path, but in actuality, concrete slabs load more uniformly. Therefore, the insulation does not necessarily need a high compressive resistance.

XPS product has an overly conservative design approach, while an EPS is more cost-effective and carries adequate strength. Considering the XPS is more expensive than EPS per inch, this is wasted money for any project. 


Expanded polystyrene (EPS) insulations outperform extruded polystyrene (XPS) in terms of long term thermal resistance and long term moisture absorption. Aside from that, EPS has a different variety of compressive strengths applicable to almost every construction project. Lastly, EPS has the highest R-value per dollar making it a more cost-effective choice.