Polystyrene Under Concrete Slab

Rigid foam insulation under a concrete slab helps lower energy bills, reduces moisture and meets modern building codes. It also protects the foundation from toxic gases, unwanted creatures, and cold temperatures.

The most popular types of Polystyrene under concrete slab floor insulation are EPS and XPS rigid foam boards. Both have high compressive strengths, allowing them to resist structural loading.

Structural Strength

When insulating slab foundations, the strength of the insulation is crucial. Without sufficient strength, the concrete slab can slowly collapse over time and destroy the foundation of a home.

While many contractors think the highest strength insulation makes the most sense for a slab foundation, that’s not always the case. Often, products with lower compressive strengths are more than adequate and can save a significant amount of money for the contractor.

One option for under-slab insulation is polyiso, which provides incredible strength while also blocking cold, heat, moisture and mold and unwelcome creatures like radon. It’s produced from two liquid chemicals that are combined with a blowing agent and formed into rigid foam boards. This closed-cell insulation is also a highly effective barrier to moisture and vapor, increasing thermal performance and reducing energy consumption. In addition, polyiso is made from recycled materials, making it a greener choice for your next construction project.

Moisture Resistance

When it comes to below-slab insulation, moisture resistance is very important. Wet products quickly lose their R-value, which increases the amount of energy a building will need to heat or cool it. EPS and XPS both provide excellent moisture resistance. XPS tends to perform better in laboratory short term moisture absorption tests, but real world long term results show that EPS performs just as well and retains its R-value over time.

Using foam insulation under the concrete slab prevents thermal discrepancies, air transfers, and loss, reduces heating and cooling costs, acts as a vapor barrier to control moisture, mold and radon gas and mutes noise. It also helps to prevent structural problems like heaving and buckling that can result from an inadequate vapor barrier.

Another benefit of EPS under slab insulation is that its compressive strength – or the ability to support weight – aligns with below-grade foundation load requirements. In contrast, XPS insulation requires a higher compression strength to meet below-grade requirements.

Fire Resistance

A layer of polystyrene under your foundation can protect your building from unwelcome critters, mold and soil gases like radon. It also helps keep moisture from seeping into your concrete slab and causing damage.

A recent experimental study compared the structural behavior of lightweight foam concrete flat slabs to normal-weight concrete flat slabs when exposed to fire. The crack patterns, load-deflection curves and steel strains of the tested specimens were studied. The results showed that the maximum load was lower in the foam concrete flat slabs compared to the normal-weight concrete ones.

Contractors typically install XPS rigid insulation under slabs, but expanded polystyrene (EPS) performs just as well and is significantly less expensive. It is important to confirm with the designers that they are using accurate calculations when determining how much compression the under-slab insulation needs. Over-specifying an XPS product that requires high compressive strength can add to the cost of your project. EPS offers a range of compressive strengths that will meet back-fill forces and load requirements, making it the most cost-effective option for under-slab applications.

Energy Efficiency

Under slab insulation is required by modern building codes to keep energy costs low and protect the foundation from moisture. It’s also an important part of any construction project because a poorly-insulated foundation is the main source of heat loss in new buildings.

Rigid foam under a concrete slab significantly reduces energy bills, keeps the building warm and dry, complies with code, and helps to prolong the life of the concrete slab. It’s also a great option for projects using radiant heated flooring.

Molded polystyrene is a resilient, closed-cell rigid foam insulation that offers high thermal efficiency, moisture and fire resistance. It’s also environmentally-friendly, because the majority of EPS is recyclable.

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