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How to Insulate an Existing Concrete Slab

Garages tend to be one of the most versatile spaces in a home. We use them to protect our cars, as a workshop, and even for storage. Often, as our family expands and needs more space, we also convert them to a living space.

However, because the space is not designed to be lived in, we are often faced with solving potential problems before they start. One of those issues is how to insulate an existing concrete pad floor.

Today we will discuss what insulating a concrete pad involves, why we do it, and what may be needed to accomplish the task successfully.

What Does it Mean to Insulate an Existing Concrete Pad?

Insulating an existing concrete pad refers to adding an insulating layer on top of the pad and adding a new surface. This process will vary depending on the intended use of the space. For example, sometimes concrete pads are insulated as part of a remodel to create more living space.

Generally speaking, garages are not part of the “thermal envelope” of a home, which refers to the spaces that are permanently heated, like the interior. Therefore, if the garage is to be heated certain adjustments must be made to make the space heatable. Among these adjustments is insulating the concrete pad.

Why Would I Want to Insulate a Concrete Pad?

There are a number of benefits surrounding concrete pad insulation. Among them is the ability to heat the pad, especially in an enclosed garage. Sometimes the pad is located in geographic regions that experience long, harsh winters.

Insulating an existing concrete pad can make the space usable year-round as a place to store our cars and valuables, as a workspace, or even living space. However, since the majority of concrete pads are not designed to be insulated or heated, we usually have to retrofit some type of flooring.

Insulating the pad is critical if the space is to be heated because otherwise any heat will simply be drawn into the ground by convection. It may seem counterintuitive to concern ourselves with insulating a floor when we know that heat rises.

However, with existing concrete pads, the concern is as much as retarding moisture as it is with retaining heat. In modern construction, concrete pads are often poured directly onto a compacted substrate like gravel, with just a thin sheet of plastic separating the concrete from the ground. In the case of older homes, even this barrier to moisture may be missing. For this reason, we should assume that there is none present.

What Is Involved In Insulating an Existing Concrete Pad?

When the pros tackle a project like insulating an existing concrete pad, the process is essentially the same as if the pad were being poured for the first time. If a concrete pad was insulated before the initial pouring, a vapor barrier, along with compression grade foam panels is likely present. Insulating an existing pad is done in much the same way, however, there are often caveats to deal with, such as the additional transitions required due to the raised floor. 

In most applications, insulating an existing concrete pad will involve a vapor barrier, foam insulation designed for concrete, special insulation tape, and a new subfloor. Here we will describe the steps commonly required to insulate an existing pad and offer a few tips the pros use to make the project cost-effective and successful.

Step 1. Prepare the Pad

This step can seem unnecessary considering that the pad will be completely covered anyway. However, as a rule, concrete pads in garages are designed to slope towards the garage door. This is usually a building code requirement because water entering an open garage door must be able to drain away or it will accumulate towards the interior, causing water damage.

If the garage is to be used as a living space this can be somewhat awkward, so some builders will first correct this slant. This is often done with a self-leveling floor compound, which is a very thin cement product. The product usually contains chemicals to help it bond to existing concrete, preventing a “cold joint” from occurring.

A cold joint describes new concrete that does not chemically bond to old concrete, causing it to flake off over time. In addition, this compound does not contain the gravel found in concrete and can be troweled out to form a layer as small as ⅛” thick. This allows the installer to level a floor (using multiple applications of the compound) as needed to prepare the surface for new materials.

If the pad does not require leveling, the first step is often to ensure that the pad is intact and will support the weight of the new floor. In most cases, even if the pad is cracked the gaps can be sealed with a concrete crack sealer. The most important part of this step is to confirm that the pad is level, will not move, and is as flat as possible.

Step 2. Install a Vapor Barrier

Installing a moisture barrier is very important because the most common issue with insulated concrete pads is moisture. Again, it may seem odd to be concerned about moisture if the pad is inside an enclosed garage. However, it should be noted that concrete is porous, so it does not repel water even after it is completely cured.

Concrete requires some form of barrier to ground water or it will act as a sponge, soaking up not only moisture but cold from the ground. To solve this potential problem, a 6mm or thicker sheet of plastic is laid atop the pad (even if the existing pad already has one) and extended up the adjacent walls by at least four inches. This will deflect ground water away from the pad and greatly reduce the potential for mold and mildew. 

Step 3. Add the Insulation

In most situations, the pros will use compression-grade foam sheets to insulate the pad from the finished flooring. This insulation can be any thickness, but in most installations, the goal is a balance between enough insulation while raising the floor as little as possible. In most cases, the insulation will be between 2” and 4” thick to achieve an R-value of 6 or greater. The first piece is usually cut in half so that the seams of subsequent courses do not align and cause a gap that might transfer moisture to the finished floor.

Installation of the foam begins with calculating the amount needed. Most of the time these sheets will contain 32 square feet of area, so a typical garage 12’ wide by 20’ deep will need about eight sheets. These sheets are laid edge to edge until the entire area is covered, then the seams are closed using insulative tape designed for the foam. 

This will prevent any additional moisture that permeates the vapor barrier from contacting the finished floor. After the foam has been installed and taped, the subfloor can be installed over it.

Step 4. Install the Subfloor

The subfloor usually consists of specially treated particleboard, chipboard, or exterior grade plywood. These products often include a tongue on one edge and a groove on the other. This allows each piece to connect together and provide support without the need for mechanical fasteners like nails.

Often the first step is to apply subfloor adhesive to both of these edges before tapping them together with a rubber mallet. As with the insulation, the pros will start with a half sheet to ensure that adjacent seams do not touch. 

As such, it is important to install the subfloor 90 degrees to the insulation layer. As an example, if the insulation was installed east to west, the subfloor will be installed north to south.

Since the goal is to prevent moisture from penetrating upwards to the finished floor, this method ensures that no two seams can overlap and potentially cause a gap. The subfloor is often left unattached to the insulation as well to allow it to also float. A gap of ¼” is typically built-in all around the perimeter of the subfloor (where it would contact the walls) to allow for expansion and contraction of the wood. After the subfloor has been installed, a floating floor can be installed over it.

What Is a Floating Floor?

A floating floor refers to a finished flooring material that is not physically attached to the subfloor. This type of floor is often constructed with interlocking laminates or engineered hardwood. These floors “float” above the subfloor, which allows for the expansion and contraction of the material as the temperature changes.

These materials are designed to attach to each other mechanically, often with some form of interlocking edges, or tongue and groove designs using adhesives. 

When these are installed over an insulated concrete pad, the entire floor can move slightly as needed to prevent splitting and cracking.

Additional gaps of ¼” are often required around the perimeter to prevent the finished floor from abutting the adjacent walls, causing puckers in the floor as the temperature rises and the material expands. Once the floor is installed, baseboard and quarter-round trim are typically installed to hide this gap and improve the overall appearance.

Floor Heating As an Alternative?

Those interested in insulating a concrete pad might also consider heating the floor. Hydronic and electric floor heating systems are available to keep a concrete pad at a constant temperature. Hydronic systems incorporate plastic tubing into the pad, into which warm air or water is circulated, warming the floor. Adding one of these systems is common during an existing concrete pad project because there is no better time to install one of these systems than when the pad is being insulated.

2 Responses

  1. Good article. But it doesnt address if pressure-treated wood sleepers are used to support the plywood subfloor over the rigid insulation. The anchors for the sleepers and interior partition sill plates would penetrate the vapor barrier to embed in the existing slab. What does this do to the insulation approach, if anything? Thank you.

  2. We cannot pull up a new floor. Their is a vapor barrier and a heat reflective barrier underneath the flooring on top of the slab. Our concern is the concrete that is above grade. In the NE it gets below zero so the cold penetrates in about 2 feet. How do we insulate the exterior concrete .

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