How to Insulate a Garage Floor With Plywood and Rigid Foam

Make your garage more comfortable in every season

Open Garage With Concrete Driveway
imaginima / Getty Images
Project Overview
  • Working Time: 6 - 12 hrs
  • Total Time: 6 - 12 hrs
  • Yield: 20 x 24-foot garage
  • Skill Level: Intermediate
  • Estimated Cost: $600 to $750

If you are converting your garage to a living space and plan to install a new floor, it is usually smart to begin by insulating the concrete floor. You might also choose to insulate a garage floor if you plan to use the garage as a workshop, or if you'll be storing vehicles or a boat. Insulation can make your garage floor warmer during colder weather, and it's a necessity should you be converting a garage into an indoor room.

There are many ways to handle insulating a garage for living use. You can attach wood sleepers to the floor, fill the gaps with rigid-foam insulation, and then apply a subfloor and finish flooring. Or, as demonstrated here, you can cover the slab with rigid-foam insulation, add two layers of plywood, and then add the finish flooring.

Before You Begin

The best insulation to use over a concrete floor in an unheated garage is debatable, so we've outlined the pro and cons of a few materials before we get started on our method:

 Insulation Type  Pros  Cons
Rigid Foam - Great R-value
- Easy to install
- Clean look
- Expensive
- Too airtight
Fiberglass - Readily available
- Affordable
- Irritant; hard to work with
- Can trap moisture 
Cellulose - Made from recycled materials
- Fire-resistant
- Requires special equipment
- Settles over time 
- Expensive
Spray Foam - High R-value
- Built-in vapor barrier
- Structurally sound
- Expensive
- Needs to be professionally installed 
Reflective Insulation - Simple to install
- Lowers heat and cooling costs
- Expensive
- May not work in cold climates 

Anytime you try to insulate a concrete floor from the cold, it is important to control moisture. Concrete is a fairly porous material, and ground moisture can easily migrate the slab and affect any surface material laid over the concrete. Controlling moisture avoids damaging the surface flooring material, and can be an issue even if the slab appears dry.

As a moisture barrier, lay a sheet of polyethylene plastic over the slab, and then use exterior-grade plywood sheets laid over foam panels for the top. During new construction, the vapor barrier is usually laid down first, before the slab is poured. But if you're adding insulation to an existing slab, the vapor barrier is laid on top of the slab before the foam insulating panels are installed.

Be sure to use compression-grade rigid foam panels, which are designed for installation beneath concrete slabs and will bear the weight of the new floor. Check with your supplier or manufacturer for advice on the best thickness and R-value to use. Some insulating panels come with attached vapor barriers and do not need plastic sheeting, as the panels themselves provide a sufficient vapor barrier.

Handling the Thresholds

A layer of foam insulation and two layers of plywood can add 2 inches of height, or more, to the garage floor. This can be a problem if the space will continue to be used to store vehicles since it leaves a raised lip at the doorway. Various edging strips, or thresholds, are available to help smooth this transition. Manufacturers of garage flooring tiles should offer transition thresholds that fit the height of your insulated garage floor.

You may face a similar problem at the threshold of the passage door to the garage. It may be necessary to install a different threshold here, and perhaps even shorten the door, to accommodate the new height.

Safety Considerations

Some garage floors are subject to dampness and puddling, and even have floor drains to help mitigate this problem. This can occur in older homes that have basement garages at ground level. Laying foam panels and a new subfloor over a moist concrete slab can be problematic, since water will be trapped beneath the floor, encouraging rot and mold. In this instance, it's best to consult a building engineer before covering the floor and converting the garage to a living space. Adapted construction techniques or a special drainage system, like a perimeter drain tile and a sump pump, may be necessary to ensure that the garage floor remains dry.

What You'll Need

Equipment / Tools

  • Concrete trowels (as needed)
  • Utility knife
  • Tape measure
  • Circular saw
  • Sawhorses
  • Screw gun

Materials

  • Concrete patch or leveling compound (as needed)
  • 6-mil polyethylene sheeting
  • 12 1-inch thick rigid-foam insulation (compression grade)
  • Heavy-duty construction tape
  • 24 1/2-inch exterior-grade plywood sheets
  • 7/8-inch outdoor-grade (corrosion-proof) utility screws
  • Edging/threshold strips (as needed)

Instructions

How to Insulate a Garage Floor

  1. Clean and Repair the Garage Floor

    Inspect the concrete garage floor, thoroughly clean it, and repair any notable problems before adding the vapor barrier and subfloor. A concrete floor with minor unevenness can be leveled out with floor leveling compound. Other structural methods can be used if there is slab heaving or leaning.

    Never attempt to convert a garage to a living space if there is a major problem with the slab.

  2. Lay the Vapor Barrier

    Cover the entire floor with 6-millimeter polyethylene, which will serve as a vapor barrier. Overlap the seams about 6 to 8 inches, and then seal them with waterproof construction tape. Run the polyethylene up the sides of the walls 3 to 4 inches, and secure it with construction tape.

  3. Lay the Rigid Foam

    Lay sheets of high-density rigid foam over the polyethylene barrier. Use a utility knife to cut the foam to size (a hacksaw or drywall saw will also work). Leave a 1/4-inch gap around the perimeter. Tape the seams of the foam panels with heavy-duty construction tape.

  4. Lay the Plywood Panels

    Lay 1/2-inch exterior-grade plywood sheets over the foam, running the sheets perpendicular to the foam panels so that the seams do not align. Space the panels 1/4 inch apart, with a 1/2-inch gap around the perimeter. This gap allows for seasonal expansion and contraction. Where necessary, measure and cut panels to fit using a circular saw.

    Add a second layer of 1/2-inch plywood over the first layer, running this layer perpendicular to the first, with the same spacing. Fasten the two layers of plywood together with 7/8-inch outdoor utility screws.

    Tip

    If there are areas of the flooring that seem bouncy or bowed, masonry screws can be drilled through the plywood and insulation and into the concrete to help them stick.

  5. Install Thresholds and Transitions

    Smooth the transition between the raised flooring surface and the outside driveway by installing threshold strips of the appropriate size. Follow the manufacturer's instructions for attaching the strips; many come with construction adhesives designed for this purpose.

    Use a similar method, if needed, to adjust the threshold for the smaller passage door to the garage.

  6. Install Flooring

    Install whatever flooring surface you choose. If the garage will remain a space for vehicles, use garage tiles or mats, or paint the plywood with epoxy.

    If the garage is being converted to a living space, the plywood subfloor is a sufficient base for any new flooring material, provided preexisting moisture problems have been addressed.

FAQ
  • Do you need a vapor barrier when insulating a garage?

    Unheated garages need to breathe, so you should only use a vapor barrier if you plan to heat your garage. Heated garages without a vapor barrier could have issues with frost and mold.

  • Will insulating an unheated garage cause mildew?

    A vapor barrier will help control mold and mildew in an unheated garage. So will selecting an insulation material that does not retain moisture, like spray or injection foam insulation.

  • Is it worth putting a dehumidifier in the garage?

    Placing a dehumidifier in your garage can help control humidity levels during the warmer months. A dehumidifier can also help protect personal items from collecting mold or mildew.

Article Sources
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. A Brief Guide to Mold, Moisture and Your Home. United States Environmental Protection Agency.