How to insulate an existing concrete slab

Finishing a basement is a stellerine way to enhance resale value to your home. The easiest way to insulate an existing slab is by adding the insulating layer above the existing concrete slab.  An insulated floor makes the space comfortable to reside in during cold weather and lowers utility bills. Thermal insulation is usually installed below the concrete floor to provide continuous low-level heating. Existing floors with no insulation often get cold during winter enhancing the need for a heater. Although insulating an existing concrete slab is far less tangled than it sounds, most people still can’t grasp how to insulate an existing concrete slab.

To insulate an existing concrete slab first, prepare the floor by leveling and removing the dirt on the slab’s exposed edges and measuring the distance between the slabs. Install the vapor barrier and then lay the rigid foam sheets over the polyethylene. Install the plywood panel and allow about ½ inch gap for expansion and contraction. Install the threshold and finally the flooring material.

An image of a concrete floor being prepared for insulation

How to insulate an existing slab

The additional layer in the concrete slab will raise the floor level, violating building regulations like unequal step heights at staircases and elevating step heights at external doors. It may require readjusting the doors with new floor elevation by reducing the height from the bottom door, which requires reducing the skirting and radiators.

Required tools and materials

  • Drill driver
  • Circular saw
  • Utility knife
  • Tape measure
  • Rigid foam insulation
  • Edging strips
  • Exterior grade plywood sheets
  • 6mil polyethylene sheeting
  • Outdoor grade (corrosion-proof) utility screws
  • Construction tape

Step 1. Prepare the insulating floor

Clear and remove dirt and other debris to expose the edges of the existing slab. The slab surface should be free of any item that can puncture the panels or the vapor barrier.

Step 2. Measure the distance between the slabs

Use the heavy-duty tape measure to measure the distance between the slabs and mark.

Step 3. Install vapor barrier

Use the 6-mil polyethylene to cover the entire surface and waterproof construction tape to overlap the seams by about 6-8inches.

Step 4. Install the rigid foam

Lay high-density rigid foam sheets over polyethylene. Leave about ¼ inch gap around the perimeter, especially the space perpendicular to the wall. The wood strapping fastens the vapor barrier to the slab and holds the weight of the subflooring.

Step 5. Install plywood panels

Above the foam, add the exterior grade plywood sheets leaving space for panels of about ¼ inch apart with a ½ inch gap around the perimeter for seasonal expansion and contraction of the plywood.

Step 6. Install threshold

Smoothen the transition between the elevated floor after installation of the insulating material using appropriate threshold strip sizes.

Step 7. Finish the flooring material

Paint or coat the plywood with proxy. You can also install floating floors to allow the expansion and contraction of the flooring. Laminated flooring with planks joined together along edges is an example of a floating floor. 

The video below illustrate how to insulate an existing concrete slab

Common types of insulation methods and materials 

Insulation keeps your home warm during the winter season and cools during summer, thus saving you money on electricity bills. Due to numerous insulation methods, choosing insulation methods and materials can be overwhelming.

Insulation Methods

Spray foam insulation

It is an economical and energy-efficient method of insulating your home and the process entails spraying a polyurethane or latex spray foam. The foam is sprayed to fill the cracks and small gaps in areas to prevent air from escaping. The foam insulation is used around doors, electric outlets, plumbing pipes, vents, and windows. The foam insulation also prevents the growth of mold and mildew. There are two types of spray foam; close cell and open-cell foam.

  • Closed-cell foam

It is a dense and rigid type of form with a consistency resembling glue. It doesn’t expand, and air cannot pass through it. Its airtight nature makes them better at regulating temperatures. Besides being a good insulator, it adds strength and impact resistance to structures.

  • Open-cell foam

It is soft and flexible with spongy like consistency. The foam expands rapidly, it’s easy to install and can be applied in areas hard to reach, but the insulation power is not as strong as the close cell foam.

Blown-in/loose-fill insulation

It entails the use of specialized pneumatic tools to blow fuzzy fiberglass strands, cellulose, or mineral wool into the area. Blown in/loose-fill insulation is a highly versatile insulation method installed in enclosed or open wall cavities, and attic floors. A major setback of this method is its reduced effectiveness as time passes due to material settling.

Batts and roll insulation

These forms of installation come in precut rolls of fiberglass, flexible plastic, and natural fiber. It is popular for DIY attic and exterior wall insulation projects. It aids in sealing in the course of installation, its fire resistant and acts as vapor barrier.

Reflective insulation

Involves placing reflective materials like foils, papers, or film in the attic, ceiling, floors, or unfinished walls. Reflective systems reflect heat away from home, making it efficient in hot climates. This form of insulation prevents heat transfer from warming cooler areas, and it’s majorly installed in rafters and joists.

Foam board insulation

It’s the installation of rigid insulation panels in homes to lower conveying heat via the structural elements. The foam boards are mostly made of polystyrene or polyurethane and can insulate the floors, ceiling, and unfinished walls.

Types of insulating materials


It is the regularly used insulation comprising of thin glass strands and plastic filaments fabricated into fiber to form an efficacious insulating material. The fiberglass is inexpensive, non-flammable, and reduces the flow of sound and heat through an area.


It is eco-friendly, made from recycled materials like cardboard and paper. Has a loose form but after installation it becomes compact. It contains almost no oxygen once compact, and the lack of oxygen effectively reduces fire damage.

Polyurethane foam

It is a flimsy insulation form with high insulation prowess. The foam is placed using a spray-in area that has not been previously insulated, and it’s fire-resistant.

Mineral wool

It is made from recycled glass, rock wool formed from basalt. It’s not fire-resistant and shouldn’t be installed in areas experiencing extreme heat. Mineral wool prevents the growth of mold and mildew.

What kind of insulation goes under the slab?

The most used rigid foam insulation under the slabs is expanded polystyrene (EPS) and extruded polystyrene (XPS). Though both are enclosed cell insulations, they perform differently over time. The XPS has a higher initial insulating R-value than EPS of similar thickness and density, but the R-value of XPS degrades over a certain period. The R-value of EPS remains the same throughout its installed service.

Garage floor insulation tiles

The insulated garage floor makes the garage more comfortable to work in during cold weather. There are two ways to insulate the garage floor. You can attach wood sleepers to the floor, fill the gaps with rigid foam insulation and then apply subfloor and finish flooring. You can also wrap the concrete slab with rigid foam insulation, add double layers of plywood and finally finish flooring.

How to insulate concrete floor before tiling

Before applying tiles on the garage floor on the concrete, first control moisture bleeding through the concrete to avoid damaging surface flooring tiles.



  • Screw gun
  • Sawhorses
  • Circular saw
  • Tape measure
  • Utility knife
  • Concrete trowels


  • Six mil polyethylene sheeting
  • Concrete patch
  • 12 1-inch thick rigid foam insulation
  • Heavy-duty construction tape
  • 24 ½ inch exterior grade plywood sheets
  • 7/8 inch outdoor grade utility screws (corrosion-proof)
  • Threshold strips

Steps involved in insulating garage floor

Clean and repair the garage floor

Examine the concrete garage floor for any problems and repair before adding the vapor barrier and subfloor. Level small unevenness with floor leveling compound, then clear and clean all the dirt and debris.

Lay vapor barrier

Cover the entire floor with six mil polyethylene which acts as vapor proof to prevent moisture from bleeding through the slab. Overlap the seams about 6-8 inches and seal them with waterproof construction tape. Cover the polyethylene on the sides of the wall 3 to 4 inches and secure it to the walls with construction tape.

Place the rigid foam

Lay high-density rigid foam sheets over polyethylene. Use a utility knife to cut the foam to size. Leave ¼ inch gap around the perimeter, then use heavy-duty construction tape to adhere to the seams of the foam panels.

Place the plywood panels

Lay sheets of ½ inch exterior grade plywood over the foam perpendicularly to the dimension of foam so that the plywood seams do not align with foam seams. Place the panels ¼ inch apart, with a ½ inch gap around the perimeter to allow for seasonal expansion and contraction of the plywood.

Add a 2nd layer of ½ inch plywood over the first layer perpendicularly with the same spacing, then fasten the two layers of plywood together with 7/8 inch utility screws. 

Install threshold

Install threshold of appropriate size when the garage still stores vehicles, smooth the transition between raised flooring surface and outside driveway. Follow the guidelines for attaching strips that come with construction adhesives designed for this purpose.

Install flooring material

Install the floor garage tiles after insulating the concrete floor when the garage will remain a utility storage space for vehicles.

Frequently Asked Questions

Can you use spray foam under the concrete slab?

Foam insulating layers are designed for installation below concrete slabs, and they will bear up the weight when the new floor continues to support the above weight.

What is the best insulation under a concrete slab?

The commonly used rigid foam insulations below concrete slabs are the expanded polystyrene (EPS) and extruded polystyrene (XPS). Although they are both closed-cell insulations, their performance over time varies. EPS has low moisture absorption, a better long-term thermal performance, and a higher R-value.

Can you put plastic under concrete?

Installing the 6-mil polyethylene is rarely effective. It seems water-tight but allows a lot of water vapor to pass through it, and the plastic can be damaged during installation, creating holes that can allow more water vapor into the slab. 

Is it worth insulating a concrete floor?

The major benefits of insulating a concrete floor are heat loss, moisture prevention, and increased comfort. Homeowners can save money on electricity bills if they insulate their concrete floors.

Why do they put foam under concrete?

A layer of poly keeps the slab dry as it stops vapor diffusion upward from the ground. The foam layer isolates the room temperature slab from the cold under the slab. The foam moves with the building and absorbs the heaving pressure leaving the slab intact.

Can you put polystyrene under concrete?

The EPS sheets are cost-effective products for insulating under concrete slabs. They achieve R-value above most building regulations. The XPS is tolerant to damp environments and can be laid directly on the prepared ground with DPM, acting as a separating layer between insulation and concrete, thus saving time and membrane material.

Why does concrete crack when drying?

The main course of concrete cracking is shrinkage. The concrete shrinks as it dries and hardens due to excess evaporation of mixing water. The shrinking causes forces in the concrete to pull it apart.


Before installing a new floor, it is usually a smart move to begin by insulating the concrete floor. Insulation keeps your home warm during the winter season and cools during summer, thus saving you money on electricity bills.

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