Not just any insulation will work under your concrete slab foundation. You’ll need to make sure you’re choosing an insulation type and grade that is compatible with this special application. Special attention is required when it comes to compressive strength for under slab insulation, since it will be bearing a whole lot of weight from the concrete on top.
Compressive strength is measured in pounds per square inch (psi). What psi is needed for under slab insulation? Read on to find out.
Compressive Strength of Insulation
Compressive strength is the measure of how much load a material can withstand in compression without deforming and degrading performance. It’s typically measured over an area (such as a square inch) in order to provide a more realistic picture of how a material performs under pressure.
Let’s compare the compressive strength of some common insulation types:
Under Slab PSI Requirements
As a bare minimum, 16 psi is the lowest contractors will go on compressive strength of under slab insulation. It’s best to use a factor of safety and settle with a slab insulation with a compressive strength in the 20-25 psi range.
Options for Under Slab Insulation
As you select the optimal material for your underslab insulation, you can narrow down your selection only to products that fit above the acceptable minimum compressive strength. That means you’ll be left choosing XPS, EPS, or polyiso.
Here are the compressive strengths for XPS insulation by type.
XPS Compressive Strength by Type
EPS also has various densities, which affect its compressive strength. The table below shows the compressive strength of different types of EPS.
EPS Compressive Strength by Type
Finally, let’s compare the compressive strength of Type II Polyiso Options as classified by the ASTM C1289 standard in the table below.
Polyiso Compressive Strength by Type
The Benefits of Polyiso Insulation
Polyiso is an ideal material for under slab insulation and has many benefits in this application. Let’s cover why in more detail.
Compressive Strength
Of the options listed above for under slab foam insulation, polyiso is the only one that meets the compressive strength requirements for under slab applications (above 16 psi) for all classes and grades. The inherent closed-cell structure gives polyiso excellent dimensional stability and compression strength to ensure that it can withstand the weight and pressure of a concrete slab. By selecting an insulation with sufficient PSI, you ensure that your slab will not settle or buckle over time from insulation deformation.
Thermal Performance
Polyiso is one of the best performers in thermal resistance. It has the highest R-value per inch when compared to other foam insulations like extruded polystyrene (XPS) and expanded polystyrene (EPS). For example, Rmax Thermasheath® boasts R6 per inch. Since the IECC building code recommends R10 continuous insulation under slab for most climate zones, and more for heated slabs—code compliance can be achieved with just over 1.5” of polyiso.
Check out Polyiso R-Values per thickness in the table below.
Polyiso R-Values for Popular Thicknesses
Moisture Resistance
Polyiso excels as an under slab insulation due to its inherent moisture resistance properties, including low water vapor absorbance and low vapor permeance. It performs better than XPS in both categories and is comparable to EPS. See the numbers for yourself below.
Moisture Performance Comparison: XPS, EPS, Polyiso
An under slab insulation that resists moisture helps your foundation function better by eliminating condensation, inhibiting mold growth, and delivering long-term performance.
Select the Best Under Slab Insulation with Rmax
Finding the right under slab insulation is critical to ensuring a stable and durable foundation for the longevity of your building. Lay the right slab insulation to get started off on the right foot.
Reach out to the experts at Rmax to make sure you’ve got the right type of insulation slated for the job!