How to Insulate a Concrete Slab: Under Slab Insulation Installation Guide

Insulating beneath a concrete slab can help reduce heat loss into the ground, control moisture and improve the comfort and energy efficiency of the finished building.

If you're wondering how to insulate a concrete slab, the important distinction is where the insulation is installed. For new concrete construction, under slab insulation is installed beneath the concrete before the slab is poured.

This guide explains how to install insulation under a concrete slab using Prodex Total Under Concrete, including applications with radiant floor heating.

Use: Prodex Total Under Concrete or Reflective Bubble White Poly.

Why Insulate Under a Concrete Slab?

Concrete itself provides relatively little resistance to heat flow. When a slab is placed over the ground without adequate insulation, heat can transfer between the concrete and the soil beneath it.

Installing insulation under the concrete slab creates a thermal break between the slab and the ground.

Depending on the insulation system and application, under slab insulation can help:

• Reduce heat loss through the slab
• Improve floor and indoor comfort
• Reduce downward heat loss from radiant floor heating
• Control ground moisture and vapor
• Reduce condensation and dampness
• Improve the overall energy efficiency of the building

For a detailed discussion of insulation types, benefits and applications, see our complete under slab insulation guide.

Where Does Insulation Go Under a Concrete Slab?

For this type of installation, the insulation is positioned between the prepared ground or base and the concrete slab.

With Prodex Total Under Concrete:

The foil/reflective side faces DOWN toward the ground.

The white side faces UP toward the concrete.

The wet concrete is poured over the white side. This orientation places the insulation beneath the slab where it can provide a thermal break while also helping control moisture between the ground and concrete.

How to Insulate Under a Concrete Slab

The exact preparation of the site, slab and radiant heating system should follow the plans and requirements for your particular project. For the Prodex insulation portion of the installation, follow these steps.

1. Prepare the Base

Prepare, level and compact the soil, gravel or other base specified for the concrete slab. The surface should be reasonably smooth so the insulation can lie flat without unnecessary punctures or large gaps beneath it.

2. Unroll the Under Concrete Insulation

Unroll Prodex Total Under Concrete over the prepared area.

Install it with:

• Foil side facing down toward the ground
• White side facing up toward the concrete

Continue laying the insulation until the area beneath the slab is covered.

3. Overlap the Seams

Overlap adjoining sections of insulation by approximately 2 inches. Maintaining continuous coverage helps prevent gaps in the insulation and moisture-control layer.

4. Seal the Seams

Seal the overlapping seams with Under Concrete Tape. Properly sealing the seams helps create a continuous barrier across the slab area.

5. Extend the Insulation Up the Perimeter

Allow the Prodex to extend approximately 6 inches up the walls or slab perimeter. This additional material allows for movement during the concrete pour and provides enough material to finish the insulation at the edge of the completed slab. After the concrete has been poured and finished, excess insulation can be trimmed as appropriate for the project.

6. Install Wire Mesh and Radiant Floor Piping

Install the wire mesh, reinforcement and any radiant floor heating system according to the applicable plans and manufacturer's recommendations. The radiant tubing or other heating components are positioned above the under slab insulation as required by the radiant heating system design.

7. Pour the Concrete

Once the insulation, reinforcement and any radiant heating components are properly positioned, pour and finish the concrete. The Prodex remains permanently beneath the slab.

Radiant Floor Insulation Under a Concrete Slab

Insulation becomes especially important when a concrete slab contains a radiant heating system. Radiant floor systems are intended to heat the floor and occupied space above it. Without insulation beneath the slab, some of that heat can travel downward toward the ground instead. Installing radiant floor insulation beneath the concrete helps reduce this downward heat loss and keeps more of the heating system's energy where it is intended to go.

Prodex Total Under Concrete is designed for under-slab applications and is ideal for radiant heat floors.

Why Use Prodex Total Under Concrete?

Prodex Total Under Concrete is specifically designed for installation beneath concrete slabs. It combines closed-cell polyethylene foam, reinforced foil and polyethylene backing in a lightweight roll that is easy to position across a prepared slab area.

Benefits 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 of less than 0.1
• Helps eliminate dampness and condensation in concrete
• Provides a thermal break
• Functions as a radon barrier
• Seals around nails
• Lightweight and easy to install

Each roll of Under Concrete Prodex Total 5M measures 4 ft x 175 ft and covers 700 sq ft.

What Do I Need to Install Under Slab Insulation?

The insulation portion of the project requires relatively few tools and materials.

You'll need:

• Prodex Total Under Concrete
• Utility knife or sharp scissors
• Under Concrete Tape

Under Concrete Tape is used to seal the overlapping seams.

How Much Under Concrete Insulation Do I Need?

Determine the square footage of the slab area that will be insulated and allow for the required overlaps and perimeter installation. Multiply the width times the length to determine the square footage of insulation required. One roll of Prodex Total Under Concrete covers 700 sq ft before allowances for overlaps, cuts and project-specific requirements.

Under Slab Insulation Installation Tips

Before pouring concrete:

• Make sure the base is properly prepared and reasonably smooth.
• Cover the intended slab area continuously with insulation.
• Keep the foil side facing the ground and white side facing the concrete.
• Maintain the approximately 2-inch seam overlaps.
• Seal the seams with Under Concrete Tape.
• Allow approximately 6 inches of material to extend up the perimeter.
• Follow the concrete, reinforcement and radiant-heating specifications established for the project.

Once the concrete is poured, correcting an insulation installation problem becomes difficult. Inspect the insulation and sealed seams before proceeding with the pour.

Customer Installation Experience

One Prodex customer provided useful feedback about why allowing approximately 6 inches of material to extend up the perimeter can be helpful during installation (see bottom of page).

After his 4-inch concrete floor was poured, approximately 1 to 3 inches of excess Prodex remained around the perimeter. He observed that the material moved very little during the pour, but the additional height provided enough allowance to accommodate variations in placement. The excess material could then be trimmed around the finished floor. This illustrates why allowing extra insulation at the perimeter is preferable to cutting the material exactly to the anticipated finished slab height before concrete is poured.

Insulating Under a Concrete Slab vs. Above a Concrete Floor

There are two different ways to insulate concrete floors, depending on whether the concrete has already been poured. This guide covers insulation installed UNDER a new concrete slab before the concrete is poured. Prodex Total Under Concrete is placed over the prepared base with the foil side facing the ground and the white side facing up toward the concrete. If the concrete floor already exists, Prodex can also be installed ABOVE the concrete using a different installation method and different Prodex products.

For an existing concrete floor, see our step-by-step guide:

How to Insulate Above a Concrete Floor

That installation method uses spacers to create an air space, with Prodex installed between the spacer layers before the finished flooring is installed.

Frequently Asked Questions

Should insulation go under a concrete slab?

Insulation can be installed beneath a concrete slab to create a thermal break between the concrete and the ground. It is particularly useful when reducing ground heat loss, controlling moisture or improving the efficiency of a radiant heated slab is important.

Which way does Prodex Under Concrete face?

Install Prodex Total Under Concrete with the foil/reflective side facing down toward the ground and the white side facing up toward the concrete. The concrete is poured over the white side.

How do you insulate under concrete?

Prepare the base, unroll the under concrete insulation, overlap the seams approximately 2 inches, seal the seams with Under Concrete Tape, extend the insulation approximately 6 inches up the perimeter, install the reinforcement and radiant tubing if applicable, and then pour the concrete.

How do you insulate under a concrete slab?

For a new slab, insulation is installed over the prepared base and beneath the concrete before it is poured. Prodex Total Under Concrete is unrolled across the slab area, overlapped and taped to create continuous coverage before the reinforcement and concrete are installed.

What insulation should I use under a concrete slab?

Several materials can be used beneath concrete depending on the project requirements. Prodex Total Under Concrete is specifically designed for under-slab applications and combines a closed-cell foam core with reinforced foil and polyethylene backing. For a comparison of different options, see our Under Slab Insulation Guide.

Can Prodex be used under radiant heated concrete floors?

Yes. Prodex Total Under Concrete is designed for under-slab applications and is ideal for radiant heat floors. The insulation beneath the slab helps reduce heat loss toward the ground.

Do the seams need to be taped?

Yes. Overlap the seams approximately 2 inches and seal them with Under Concrete Tape.

Why does the insulation extend 6 inches up the wall?

The additional material provides allowance for movement and variations during the concrete pour. After the slab has been poured and finished, excess material can be trimmed as appropriate.

What if my concrete floor has already been poured?

If you need to insulate an existing concrete floor from above rather than install insulation beneath a new slab, use a different installation method. See our How to Insulate Above a Concrete Floor installation guide for the recommended Prodex products and step-by-step instructions.

Learn More About Under Slab Insulation

This page focuses specifically on how to install insulation beneath a concrete slab. For information about the benefits of slab insulation, different insulation materials and choosing an insulation system, read our complete Under Slab Insulation Guide. Ready to select the insulation for your project? See Prodex Total Under Concrete 5M for product specifications, coverage and ordering information.

Customer suggestion:

In my situation, after the 4” floor was poured we trimmed 1-3” off around the edges all the way around. Insulation really didn’t move any when floor was poured, just that contractor didn’t lay all the strips even. Had some running up the wall 5” an some 7”. If they would’ve had it all 6” evenly placed everywhere it would’ve required 2” trimmed all the way around before power toweling the floor.

So my theory on the advice of running 6” up the wall is for allowance for slight movement in the prodex so your end result is to have the cut off prodex edge right at the top of the finished floor on the outside walls. If gravel were used instead of packed fill sand like I used, I’m sure there would be even more movement from pouring, so the 6” recommendation is universal advice for both gravel and sand.

Hope this helps should any other customers inquire on the reasoning of running the prodex up 6” on the walls.
Thanks again for a GR8T product.

Tim Wright