Do You Need To Put Rigid Insulation Under Concrete Slab? [Explained] - Today's Homeowner (2024)

Whether you wish to insulate your concrete slab with rigid insulation, it’s crucial to know the relevant information, including the different insulation materials used, the building codes, costs, and how to insulate concrete floors properly.

Here’s a detailed guide on everything to know about rigid foam insulation under concrete slabs.

What Insulation Typically Goes Under Concrete Slab?

Concrete slab insulation is essential because it helps maintain constant indoor temperatures, enhances moisture control, improves energy efficiency, and saves energy bills.

One of the most common forms of insulation used under a concrete slab is rigid foam insulation, such as expanded polystyrene (EPS), extruded polystyrene, and Polyisocyanurate (Polyiso).

Expanded polystyrene (EPS), also known as beadboard, is a lightweight, rigid, closed-cell foam insulation. It is made from synthetic materials called tiny polystyrene beads. They go through thermoplastic polystyrene that applies steam or heat with pentane to the beads to soften, expand, mold, and fuse together.

EPS Insulation is also made of 2% plastic and 98% trapped air with an average R-Value of 3.6 to 4.0 per inch. This type of rigid foam insulation comes in several compressive strengths to withstand back-fill forces and load, ensuring flexibility, durability, and dimensional stability. It’s also the least expensive rigid foam insulation type and more resistant to moisture.

However, EPS under slab insulation is permeable by water molecules and has minimal water absorption, but it’s within the American Society for Testing and Materials (ASTM) standards. It’s also the most vapor-permeable of the insulation types and doesn’t lose R-value over time.

Extruded polystyrene is a thermoplastic polymer foam board insulation with a closed-cell structure that comes in colors pink, blue, and green. It is made from solid polystyrene crystals or resin manufactured through an extrusion process where it is mixed and melted with additives into an extruder to create a viscous fluid. The liquid form is then expanded using a blowing agent and then extruded into the desired shape through a die.

XPS insulation has an average R-Value of 4.5 to 5.0 inches, indicating high levels of insulation effectiveness. This type of rigid foam insulation also has compression and frost resistance. It also doesn’t decay quickly and has more moisture resistance than EPS insulation. However, XPS under slab insulation is more expensive, has less water-absorbing properties than EPS, and loses R-value over time.

Polyisocyanurate, also known as PIR, is a thermoset, closed-cell, rigid foam board insulation. It is made of two liquid chemicals: isocyanurate and polyol, which are combined by a catalyst that rearranges and fuses the molecules together. Then a blowing agent is used to create closed cells and flame retardants to form a rigid, polyisocyanurate insulation board sandwiched between two facers.

Polyiso insulation has an average R-value of 6.5-6.8 inches that doesn’t lose over time, the highest R-Value per inch out of all the three rigid foam insulation types. It provides high energy efficiency and increases structural and thermal performance, providing excellent insulation

It also helps reduce water condensation, air exfiltration, and infiltration, decreasing heat loss. Moreover, Polyiso under slab insulation offers moisture and fire resistance and high control of vapor.

Building Codes For Rigid Insulation

Building codes for rigid insulation requirements may vary, depending on where you live. In general, the International Energy Conservation Code (IECC) is the one that designates minimum energy-efficiency requirements for buildings using prescriptive and performance-related provisions. For example, it requires rigid foam insulation if the surface is less than 12 inches below grade.

It also mandates in IRC Section R402.2.10 a minimum of R-10 insulation in Climate Zones 4 to 8. For Climate zones 1 to 3, installing rigid foam insulation under a concrete slab is optional, but it’s best to have one to improve energy efficiency. If you’re installing heated slabs in zones 1 to 3, the rigid foam insulation to a depth of the footing or 2 feet.

Since it depends on your location, it’s best to check your local building codes for more precise rigid foam insulation requirements under a concrete slab.

Are You Required to Use a Vapor Barrier Under Concrete Slab?

A vapor barrier under a concrete slab is essential as it enhances the foundation of the concrete slab. It will prevent moisture from penetrating the concrete slab as water tends to soak into the rigid foam insulation over time.

The American Concrete Institute requires the vapor barrier to be a minimum of 10-mi thick (A mil is one-thousandth of an inch) to tolerate heavy construction activity over concrete subfloors. A higher range may also be needed if the material has sharp angles. In addition, the vapor barrier should also have a moisture permeance rate of 0.3 perms, meaning only 0.3 grains of water vapor can pass through the barrier.

Moreover, according to IRC, in subsection R506.2.3, a vapor barrier with a minimum of 6-mil is required for concrete slabs that will be installed in heated structures. It’s also best to consult your local building codes for a more accurate requirement.

How Much Does Rigid Insulation Cost?

The accurate estimate of rigid foam insulation cost may greatly vary depending on the factors, including rigid foam insulation material, location and size, price of labor, difficulty level, and whether the house is a new one or an existing home.

In general, an average rigid foam insulation costs around $1.50 to $5.00 per square foot. The total cost may range from $2,500 (low cost), $3,000 to $10,000 (average), and $24,000 (high cost) installation process.

The rigid foam insulation material will also significantly impact the actual cost of the installation, and the price may vary depending on the thickness of the board and its R-value.

In general, an EPS insulation may cost between $0.25 to $0.35 per square foot, while XPS insulation ranges from around $0.40 – $0.50 per square foot. Polyiso, the most expensive of the three types, ranges from approximately $0.40 – $0.60 per square foot for the foam board itself.

How To Insulate Concrete Floor Before Pouring

Before insulating the concrete floors, it’s best to consult your local weather bureau about your area’s heating degree days (HDD) or the number of days a building demands energy to heat it. It’s also used to determine the appropriate insulation depth and R-value.

The first step is to measure the width and length of the slab and multiply them to determine the number of needed rigid foam insulation to cover the concrete slab. Don’t forget to measure around the slab’s perimeter.

Next is to excavate the area 12 inches below the slab for the insulation and fill material and place 4 inches of gravel or crushed stone, raking the gravel in the excavation across the entire floor area.

Lay the chosen vapor barrier material until the area is fully covered, using duct tape or other construction tapes to hold the material together. Don’t forget to add insulation on the edges for additional insulation, protecting the concrete floor from cold air infiltration.

Then, you may also add 2 inches of sand, raking it on top of the insulation. Finally, pour the concrete into the insulated area until it is entirely covered. Use a rake or shovel to level the poured concrete.

Insulating Basem*nt and Garage Floors With Rigid Foam

The different insulation materials you can use for insulating under a concrete garage floor include Rigid foam, Fiberglass, Radiant Barriers, Reflective Insulation, Cellulose, and Spray foam.

First, it’s also important to remember to fill the air gaps in your uninsulated garage before starting the garage insulation project.

The rigid foam insulation materials commonly used for basem*nt floors include EPS, XPS, Polyiso, and closed-cell polyurethane spray foam. Before insulating the basem*nt floors, it’s vital to consider the following factors: insulation requirements, structural problems indications, finishing details, and plumbing and wiring upgrades.

Ensure there are no more air leaks and provide necessary repairs. After that, place the base gravel, evenly spreading and keeping it 1 inch lower. Next is to place extruded polystyrene foam board, taping the joints with a builder’s tape to create a tight insulation layer. Then, it’s time to set the chosen rigid foam insulation material over the board, ensuring no holes, tears, or gaps. Lastly is to tape the joints with a builder’s tape.