Under Slab Insulation: Complete Guide to Insulating Under Concrete
Under slab insulation is installed beneath a concrete slab to reduce heat transfer between the concrete and the ground, improve moisture control and create a more energy-efficient floor system. It is particularly important for heated buildings and radiant floor systems, where insulation beneath the concrete can reduce downward heat loss and help keep more heat in the occupied space.
Whether you're planning a home, basement, garage, workshop, warehouse or commercial building, understanding the different types of under concrete insulation can help you choose the right system before the concrete is poured.
Need installation instructions? See our step-by-step guide to how to insulate a concrete slab.
What Is Under Slab Insulation?
Under slab insulation is insulation installed between the prepared ground or base and a concrete slab.
It is also commonly referred to as:
Under concrete insulation
Concrete slab insulation
Slab insulation
Insulation under concrete
Insulation under a slab
Although the terminology varies, the basic purpose is the same: create a thermal and moisture-control layer between the concrete floor and the ground beneath it. Concrete is durable and strong, but by itself it is not an effective insulation material. Heat can transfer through the slab and its perimeter, affecting floor temperatures, indoor comfort and energy use. Proper slab insulation helps reduce that transfer.
Why Insulate Under a Concrete Slab?
An uninsulated concrete slab can transfer heat between the building and the ground. The effect depends on climate, building use, indoor temperature, slab design and other construction factors, but the result can include colder floors, increased heating demand and unwanted moisture movement. Installing insulation beneath the slab can provide several benefits.
Reduce Heat Loss Through the Slab
Heat naturally moves from warmer areas toward colder areas. During heating conditions, an uninsulated slab can allow heat from the building to move toward the colder ground. Under concrete insulation creates a thermal break that helps reduce this heat transfer.
Improve Floor Comfort
A cold concrete floor can make an otherwise comfortable building feel colder. Reducing heat transfer through the slab can help maintain more consistent floor and indoor temperatures.
This can be particularly valuable in:
Homes
Basements
Garages
Workshops
Warehouses
Commercial buildings
Radiant heated floors
Control Ground Moisture and Vapor
Concrete is porous, and moisture from the ground can migrate toward a slab. An under-slab system that provides an effective vapor barrier can help control moisture movement between the ground and the concrete. This can help reduce problems associated with slab dampness and condensation.
Improve Building Energy Efficiency
Floors are part of the building envelope. Insulating beneath a concrete slab helps limit unwanted heat transfer through the floor and can complement properly insulated walls, roofs and ceilings.
Under Slab Insulation for Radiant Floor Heating
Radiant floor insulation deserves special consideration because the purpose of a radiant heating system is to deliver heat into the floor and occupied space above it. Without adequate insulation beneath a heated slab, some of that energy can move downward toward the ground instead of contributing to indoor comfort. Installing insulation beneath the concrete helps create a thermal break between the radiant heated slab and the soil below.
This can:
Reduce downward heat loss
Improve radiant heating efficiency
Help maintain more consistent floor temperatures
Keep more of the generated heat within the building
Reduce unnecessary energy consumption
Prodex Total Under Concrete is designed specifically for under-slab applications and is ideal for radiant heat floors.
For installation instructions, see How to Insulate a Concrete Slab: Under Concrete & Radiant Floor Insulation.
Types of Under Slab Insulation
Several insulation materials can be used beneath concrete slabs. The appropriate choice depends on the slab design, climate, moisture conditions, required thermal performance, construction method and project requirements. Common options include rigid foam products and roll-type insulation designed for under-concrete applications.
Expanded Polystyrene (EPS)
Expanded polystyrene is a rigid foam insulation manufactured from expanded polystyrene beads.
Characteristics include:
Typically white
Available in different thicknesses
Available in different compressive strengths
Approximately R-3.6 per inch
Lightweight
Commonly supplied as rigid panels
Because it comes in rigid sheets, EPS must be positioned and fitted across the slab area, with pieces cut where necessary.
Extruded Polystyrene (XPS)
Extruded polystyrene is another rigid foam-board insulation commonly considered for concrete applications.
Characteristics include:
Rigid board construction
Typically blue, pink or green depending on manufacturer
Approximately R-4.7 per inch
Available in various thicknesses and compressive strengths
More expensive than EPS in many applications
As with other rigid boards, seams and penetrations require attention to maintain continuous coverage.
Prodex Total Under Concrete
Prodex Total Under Concrete 5M is a roll-type insulation designed specifically for installation beneath concrete slabs. It consists of closed-cell polyethylene foam, reinforced reflective foil and polyethylene backing.
characteristics include:
Designed specifically for under slab applications
Less than 9% compression in under concrete applications
Ideal for radiant heat floors
Can reduce heat loss by 56%
Vapor barrier with a permeance rating below 0.1
Thermal break
Helps eliminate dampness and condensation in concrete
Radon barrier
Resistant to moisture
Seals around nails
Lightweight and easy to install
Each roll measures 4 ft x 175 ft and covers 700 sq ft. Unlike rigid foam boards, Prodex is supplied in large flexible rolls, allowing installers to cover substantial slab areas with relatively few seams.
Reflective Bubble White Poly Insulation
Reflective Bubble White Poly is another option for under-concrete applications. It consists of a polyethylene bubble center with a metalized low-emissivity surface on one side and polyethylene on the other.
Characteristics include:
Acts as a vapor retarder
Provides a thermal break
Lightweight
Flexible
Easy to install
Economical
Comparing Under Slab Insulation Options
When choosing insulation beneath concrete, don't evaluate the material solely by one characteristic. Consider the complete floor assembly and what you need the insulation system to accomplish.
| Consideration | EPS | XPS | Prodex Total Under Concrete |
|---|---|---|---|
| Form | Rigid board | Rigid board | Flexible roll |
| Under-slab application | Yes | Yes | Yes |
| Thermal break | Yes | Yes | Yes |
| Moisture resistant | Varies by product | Varies by product | Yes |
| Built-in vapor barrier | Depends on system | Depends on system | Yes |
| Radiant floor application | Yes | Yes | Yes |
| Large roll coverage | No | No | Yes |
| Seams required | Yes | Yes | Yes |
| Easy to cut around penetrations | Requires board cutting | Requires board cutting | Flexible material |
| Radon barrier | Depends on assembly | Depends on assembly | Yes |
The best choice depends on your project's structural, thermal, moisture and building-code requirements.
What to Look for in Under Concrete Insulation
Before choosing insulation under a concrete slab, consider several important performance characteristics.
Compression Resistance
Insulation beneath concrete must be suitable for the loads associated with its intended application. Prodex Total Under Concrete has less than 9% compression in under-concrete applications.
Moisture Resistance
Under-slab materials are installed close to the ground, where moisture is an important consideration. Look for insulation that will maintain its performance in the intended environment and provide the moisture-control properties required by the slab assembly.
Vapor Control
Ground vapor can migrate toward the concrete. Prodex Total Under Concrete has a vapor barrier permeance rating of less than 0.1, providing an integrated vapor-control layer.
Thermal Performance
The purpose of slab insulation is to reduce unwanted heat transfer between the slab and ground. The appropriate insulation system will depend on climate, building use and the design of the floor assembly.
Installation
Consider how the material will be transported, cut, positioned, joined and sealed across the slab. A large roll product can simplify installation over large areas compared with fitting individual rigid boards.
Under Slab Insulation and Slab Edge Heat Loss
The center of the slab isn't the only area that matters. The perimeter of a concrete slab can also be an important path for heat transfer because slab edges may be exposed to greater temperature differences than areas farther from the perimeter. The appropriate slab-edge insulation method depends on the building design, climate and applicable building requirements. When planning concrete slab insulation, consider the complete slab assembly rather than only the material directly beneath the center of the floor.
How to Install Under Slab Insulation
Under slab insulation is installed before the concrete is poured.
With Prodex Total Under Concrete, the basic process is:
Prepare and level the base.
Unroll the insulation over the prepared area.
Install it with the foil side facing down toward the ground and white side facing up toward the concrete.
Overlap adjoining sections approximately 2 inches.
Seal the seams with Under Concrete Tape.
Allow approximately 6 inches of material to extend up the perimeter.
Install reinforcement and radiant heating components, when applicable.
Pour the concrete over the white side of the insulation.
For complete instructions, photographs, installation tips and radiant floor information, use our dedicated How to Insulate a Concrete Slab installation guide.
Under Concrete Insulation vs. Insulation Above an Existing Concrete Floor
These are two different applications.
Under concrete insulation is installed beneath a new slab before the concrete is poured. That is the application discussed throughout this guide. If you already have an existing concrete floor and want to add insulation above the concrete, the installation method and recommended Prodex products are different.
See How to Insulate Above a Concrete Floor for that application.
Why We Recommend Prodex Total Under Concrete
For under-slab applications, we recommend Under Concrete Prodex Total 5M.
It combines several functions in one product:
Thermal Break + Vapor Barrier + Moisture Control + Radiant Floor Insulation
The product is specifically designed to be placed beneath concrete and has less than 9% compression in under-concrete applications. It can reduce heat loss by 56%, functions as a vapor and radon barrier, helps control dampness and condensation, and is supplied in 4 ft x 175 ft rolls covering 700 sq ft. For specifications, pricing and ordering information, see Prodex Total Under Concrete 5M.
How Much Under Slab Insulation Do I Need?
Start by determining the square footage of the slab area.
For a rectangular slab:
Length × Width = Square Feet
Remember that installation requirements such as seam overlaps, cuts and perimeter material can affect the amount of insulation required. One roll of Prodex Total Under Concrete provides 700 sq ft of material before installation allowances.
Frequently Asked Questions About Under Slab Insulation
What is under slab insulation?
Under slab insulation is insulation installed between the ground or prepared base and a concrete slab. It creates a thermal break that helps reduce heat transfer between the concrete floor and the ground.
Is under slab insulation the same as under concrete insulation?
The terms are commonly used to describe the same general application: insulation installed beneath concrete before the slab is poured.
What is slab insulation?
Slab insulation refers to insulation used as part of a concrete slab assembly to reduce unwanted heat transfer. Depending on the construction design, insulation may be installed beneath the slab, around its perimeter or in other specified locations.
Why is insulation used under concrete?
Insulation beneath concrete helps reduce heat transfer between the slab and ground, improve floor comfort, control moisture and improve building energy efficiency.
What is the best insulation for under a concrete slab?
There isn't one material appropriate for every building. EPS, XPS and products specifically designed for under-concrete applications can all be used depending on structural, thermal, moisture and code requirements. For projects where a flexible roll product with an integrated vapor barrier, moisture control and radiant-floor capability is desired, we recommend Prodex Total Under Concrete 5M.
Is insulation necessary under a radiant heated slab?
Insulation beneath a radiant heated slab helps reduce downward heat loss toward the ground so more of the heating system's energy remains available to heat the floor and occupied space.
What insulation is used for radiant floors?
Different insulation systems can be used depending on the radiant floor design. Prodex Total Under Concrete is designed for under-slab applications and is ideal for radiant heat floors.
Which way does Prodex Under Concrete face?
Install Prodex Total Under Concrete with the foil side facing down toward the ground and the white side facing up toward the concrete. Wet concrete is poured over the white side.
How do you install insulation under a concrete slab?
The insulation is positioned over the prepared base before the concrete is poured. With Prodex, the material is unrolled, overlapped approximately 2 inches, sealed with Under Concrete Tape and extended approximately 6 inches up the perimeter. See our complete How to Insulate a Concrete Slab guide for step-by-step instructions.
Can I insulate an existing concrete floor?
Yes, but insulating above an existing concrete floor requires a different installation method from under-slab insulation. See our How to Insulate Above a Concrete Floor guide for that application.
Under Slab Insulation: The Bottom Line
The best time to address concrete slab insulation is before the concrete is poured. Proper under slab insulation can reduce heat transfer between the slab and ground, improve moisture control and increase floor comfort. It becomes particularly valuable with radiant floor heating, where reducing downward heat loss can improve the performance of the heating system. For under-concrete applications, Prodex Total Under Concrete 5M provides a thermal break, vapor barrier, moisture control and radiant floor insulation in one flexible roll.
Ready to install? Follow our step-by-step How to Insulate a Concrete Slab guide.
