How to Insulate a Garage Floor With Plywood and Rigid Foam

Open Garage With Concrete Driveway
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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 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 even if you are still storing vehicles in it. An insulated floor can make a garage much more comfortable to work in during cold weather, and it can make a garage conversion much more pleasant to live in.

There are two ways to handle insulating concrete floors. 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

Any time a flooring material is laid over concrete, it is important to control moisture bleeding through the concrete. Although it looks like solid rock, concrete is actually fairly porous, and ground moisture easily migrates through the slab to affect any surface material laid over the concrete. It's especially important to control moisture in indoor locations to avoid damage to surface flooring materials, but it's important to control moisture even if the garage will continue to be a vehicle-storage space. Moisture very often becomes an issue, even if the slab appears to be dry.

In our project, controlling moisture in the slab is done by using exterior-grade plywood sheets laid over the foam insulating panels, and also by laying down a sheet of polyethylene plastic as a vapor barrier to block moisture. When a slab is being insulated in new construction, the vapor barrier is usually laid down first, before the slab is poured. But when adding insulation to an existing slab, the vapor barrier is laid over the top of the slab before the foam insulating panels are installed.

For the foam insulating layer, be sure to use compression-grade rigid foam panels, which are designed for installation beneath concrete slabs and will bear up under the weight if the new floor will continue to support vehicles. Check with your supplier or the manufacturer for advice on the best thickness and R-value to use. Some insulating panels come with attached vapor barriers. With this type of panel, no plastic sheeting is required since the panels themselves provide a sufficient vapor barrier once the seams are sealed with impermeable tape.

Handling the Thresholds

A layer of foam insulation and two layers of plywood can add as much as 2 inches of height to the garage floor. This can be a problem if the garage will continue to be used to store vehicles since it leaves you with a substantially raised lip at the doorway to the garage. Various edging strips or thresholds are available to help smooth this transition. Check with manufacturers of garage flooring tiles for 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, where it may be necessary to install a different threshold and perhaps even shorten the door to accommodate the new height of the floor.

Safety Considerations

Some garage floors are subject to frequent moisture and puddling and may even have floor drains to help remove moisture that enters the garage. Some older homes have basement drive-under garages, for example, where dampness or puddling may be a fact of life. Laying foam insulating panels and a new subfloor over such a concrete slab can be problematic, since water will be trapped beneath the floor, encouraging rot and the mold.

In garages where moisture is an ongoing issue, it's best to consult a building engineer before covering the floor and converting the garage to living space. Adapted construction techniques or a special drainage system, such as perimeter drain tile and a sump pump, may be necessary to ensure that the garage floor remains dry enough to maintain a safe, healthy living space.

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

  1. Clean and Repair the Garage Floor

    Inspect the concrete garage floor, thoroughly clean it, and repair any notable problems with the slab before you add 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 major heaving or leaning of the slab. Never attempt to convert a garage to living space if there is a major problem with the slab.

  2. Lay the Vapor Barrier

    Insulating concrete garage floors begins with covering the entire floor with 6-mil polyethylene, which will serve as a vapor barrier that prevents moisture from bleeding up through the slab. Overlap the seams about 6 to 8 inches and seal the seams with waterproof construction tape. Run the polyethylene up the sides of the walls 3 to 4 inches, and secure to the walls with construction tape.

  3. Lay Rigid Foam

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

    • Lay the Plywood Panels

      Lay 1/2-inch exterior-grade plywood sheets over the foam, running the sheets perpendicular to the long dimension of the foam, so that the plywood seams do not align with the foam seams. Space the panels 1/4 inch apart, with a 1/2-inch gap around the perimeter. This perimeter gap allows for seasonal expansion and contraction of the plywood. 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.

    • Install Threshold/Transitions

      If the garage will still be used to store vehicles, 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.

    • After insulating the concrete floor, you can install whatever flooring surface you choose. If the garage will remain a utility storage space for vehicles, a variety of garage tiles or mats are available for this purpose. Or, the plywood can be painted or coated with epoxy.

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