How to Insulate an Existing Concrete Slab?

(A Complete Guide)

In this guide we will answer the question, “How to insulate an existing concrete slab?”. This article will provide a step by step instruction, tips, and several points to remember to successfully install insulation to an existing concrete slab. A discussion also on several types of insulation is presented in this article.

How to insulate an existing concrete slab?

The simplest way to insulate an existing concrete slab is by adding the insulation layer above the existing concrete slab. For concrete floor slab, it will raise the floor level that might cause problems that may violate building regulations such as unequal step heights at staircases and elevating the step heights at external doors. Furthermore, it will require doors to be reduced in height from the bottom door and will require skirtings and radiators to be reduced and be adjusted in accordance with the new floor elevation.

Thus, adding insulation for existing concrete slab, it is necessary to undergo an assessment and preparation of your existing floor level, affected building requirement and existing house architecture.

Tools and materials

Step by step insulation of existing concrete slab

 Here are the step by step instruction for insulating existing concrete slab [2]:

  • Prepare the insulating floor. Remove dirt to fully expose the edges of the existing slab.
  • Measure the distance between the slab.
  • Install vapor barrier. Cover the entire floor with 6-mil polyethylene. Using waterproof construction taper overlap the seams about 6-8 inches.
  • Install the rigid foam. Lay sheets of high-density rigid foam over the polyethylene. Make sure to leave a ¼-inch gap around the perimeter especially space perpendicular to the wall.
  • Install plywood panels. Add the exterior-grade plywood sheets above the foam. Leave space for the panels about ¼ inch apart with ½-inch gap around the perimeter. This gap is to account the seasonal expansion and contraction of the plywood.
  • Install Threshold/Transitions. Smoothen the transition between the elevated floor after installation of insulating material using appropriate threshold strips size.
  • Finish the flooring material. Plywood can be painted or coated with epoxy. Floating floors can also be installed to allow the flooring to expand and contract. One example of floating floor is laminate flooring which are planks joined together along edges.

Assessment and preparation of the building

Building Regulations

U-values or the minimum thermal performance values affects the amount of insulation required set by the building regulations in your area. U-values is a numerical value that determines how quickly the heat travels through the floor expressed in watts per square meter per degree Kelvin (W/m2K). The lower the U-value, the slower the heat transfer through the insulator, thus the better the insulator performance.

For example, 0.25 W/m2K or less would require at least 70 mm of high-performance foam insulation or 150 mm of mineral wool depending on the type of insulation material used, floor type and construction.

Summarized below are the required minimum U-values set by Kingspan for domestic and non-domestic buildings [1].

CountryBuilding TypeDomestic (W/m2K)Non-domestic (W/m2K)

England (April 2014)
New: Best starting point (fabric only)0.110.18
Existing: Extension0.220.22
Existing: Refurbishment0.250.25

Wales (31st July 2014)
New: Best starting point (fabric only)0.150.22

Scotland (1st October 2015)
New: Best starting point (fabric only)0.130.15
Refurbishment and extensions(where existing dwelling’s walls and roof U-values are worse than 0.70 W/m2K in the walls and worse than 0.20 W/m2K in the ceiling)0.15 
Refurbishment and extensions(for other extensions, upgraded existing thermal elements, non-empty conservatories, and conversion of unheated buildings)0.180.20
Conversion of heated buildings0.250.25

Controlling moisture

Concrete is fairly porous and ground moisture can easily migrate through the slab damaging the surface flooring materials. Build-up moisture from crawl space can wet the insulation reducing the insulating performance. Crawlspace is the space raising the building above the ground to provide access to electrical wirings, pipeworks and so on [3]. 

Crawlspace needs to be ventilated to prevent moisture accumulation. The vents or sometimes called foundation vents are simple openings in the bottom floor to allow natural ventilation, usually created by installing a suspended timber floor suspended above the subfloor of the foundation. However, these vents can allow cold air into the crawl space creating moisture. Thus, a vapor barrier needs to be installed.

Crawl space.jpg

Laying over exterior-grade plywood sheets over the foam insulating panels and adding another layer of polyethylene plastic as a vapor barrier to obstruct moisture from ground is one method of controlling moisture. Usually for new construction this vapor barrier is laid down under the concrete slab, but for existing slab this vapor barrier is placed over the top before the foam insulating panels.

Dealing with the edges

The addition of foam insulation and other layers of layers of panel can add up 2-3 inches of height to the ground floor. For garage conversion for example, this raised slip at the garage doorway can be a problem. Thus, edging strips or thresholds are helpful to smoothen the garage floor transition of elevated doorway. It is important to note the transition threshold that will fit the height of insulated concrete slab.

Furthermore, the elevated floor level will also require floor covering skirtings on the wall, radiators, and electrical fitting to be removed and re-fixed. The height of the doors needs to be reduced as well. Thus, it is important to conduct architectural assessment to your floor area before proceeding to the project.

It is also necessary to install slab-edge insulation after laying the rigid foam insulation to allow inside surface temperature of the slab perimeter to more closely control the interior conditions reducing the potential of vapor formation and elevated relative humidity.

Types of Insulation

The performance of insulation can also be determined by referring to R-value aside from the U-value. R-value measures the resistance of heat transfer. This is the mathematical reciprocal of U-value (U = 1/R).

Listed below are the 5 common type of insulation for your home [4]:

  • Blanket Batts and Rolls

An inexpensive and DIY-friendly insulation material. This is typically constructed with fiberglass, cotton, mineral wool, and plastic fibers. For roll variety, cutting the insulation to proper length with a utility knife is a must.

Blankets and Batts MaterialR-value per inch of thickness(m2·K/W / in)
Standard fiberglass2.9 – 3.8 
High performance (medium density and high-density)  fiberglass3.7 – 4.3
  • Spray Foam Insulation

Spray foam adheres to the struction sealing leaks and gaps inside existing walls. This comes in two constructions: open cell and denser closed-cell foam.

Spray Foam TypeR-value per inch of thickness(m2·K/W / in)
Closed cell foam6.2
Open-cell foam3.7
  • Blown-in insulation

This type of insulation is applied using a machine that blows a paper-like material into the slab to be insulated. Blown-in insulation can be made of fiberglass, rock wool, or reclaimed cellulose.

Blown-in Insulation MaterialR-value per inch of thickness(m2·K/W / in)
Dense cellulose (recycled newspaper or cardboard)3.8
  • Foam Board or Rigid Foam Panels

Rigid foam panels are best for unfinished walls and work well in insulating everything from foundation walls and basement walls. The common insulation materials are polyurethane, polystyrene, or polyisocyanurate.

R-value per inch of thickness(m2·K/W / in)
Foam board and Rigid foam panels4 – 6.5
  • Reflective or Radiant Barrier

This type of insulation works differently, thus its effectiveness is not measured by R-values. The purpose of reflective insulation is to reflect the heat away from home  to prevent heat gain. This is typically placed in the attic for houses located in warmer climates.


This guide answered the question, “How to insulate existing concrete slab”. Insulating existing concrete slab can be challenging, however, following the instruction and guidelines provided in this article can help you to successfully deliver your project. These guidelines include assessment and preparation of the building in compliance to building regulation and considering moisture control and edges, tools and materials, and types of insulation that can be used depending on the purpose of your project.

For any questions and suggestions about this article, please feel free to submit your thoughts in the comment section below.

 FAQs: How to insulate existing concrete slab?

Should I insulate under my concrete slab?

The best practice for insulating basement floors is inserting the insulation layer and vapor barrier between the concrete slab and the ground underneath. Since heat flows to cooler places, heat escapes the concrete slab during winter. This is when the ground is cooler than the basement temperature.

Is it worth insulating a concrete floor?

Yes, it is worth it to insulate your concrete floor since concrete has a lower insulating value making it cold. Adding insulation in your floor improves the heat retention in your concrete floors.

Do you put plastic under concrete?

Since 1950s, polyethylene plastic had been used as vapor barrier, however, recent studies proved that addition of this layer is seldom effective because of two main reasons: (1) this allows water vapor to pass through; (2) plastic can get damaged during installation of reinforcement and concrete, creating holes. Therefore, this plastic layer cannot stop the vapor but it can still slow it down making plastics a vapor retarder instead of vapor barrier.

What is the best insulation for under a concrete slab?

The two common rigid foam insulations for below-grade applications are expanded polystyrene (EPS) and extruded polystyrene (XPS). For under-slab applications, rigid foam insulation should be installed over a gravel base with vapor retarder between the gravel and insulation.

Can you use spray foam under a concrete slab?

You can use spray foam under a concrete slab. Insulating under foundation walls and basement slabs using spray foam insulation or also known as spray polyurethane foam (SPF) offers advantages over the more common method, the rigid foam board. SPF adheres to the structure and is mold and mildew resistant. It also has the highest R-value.


[1] Floor insulation. (2020, September 04). Retrieved from:

[2] Beneke, J. (2019). How to insulate garage floors with plywood and rigid foam. The Spruce. Retrieved from:,then%20add%20the%20finish%20flooring.

[3] Crawl space (2020, November 3). Retrieved from:

[4] Schwartz, D.B., Vila, B. (n.d.). All you need to know about types of insulation. Retrieved from:

[5] Housing retrofit: Ground floor insulation (2021). Retrieved from:

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