How to Insulate Garage Floors With Rigid Foam and Plywood

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Insulating concrete floors is a smart idea if you plan on converting your garage into living space. If you intend to use your garage as a workshop or if you store vehicles there, you might consider insulate the garage floor. Insulating a garage floor can make it more comfortable to use in cold weather and make it more enjoyable to live in.

Insulating concrete floors can be done in two ways. Attach wood sleepers to the concrete floor and fill in the gaps with rigid foam insulation. Then, add a subfloor or finish flooring. You can also cover the slab with rigid foam insulation and add plywood layers before adding the final flooring.

Before You Begin

Concrete is porous and moisture can easily migrate through concrete flooring materials. Concrete is porous and can easily be affected by moisture. Moisture must be controlled indoors to prevent damage to flooring materials. However, it is important to maintain a garage that will remain a vehicle storage space. Even if the slab seems dry, moisture can often become a problem.

Our project uses exterior-grade plywood sheets to control moisture in the slab. These sheets are then laid over foam insulating panels and a sheet made of polyethylene plastic to act as a vapor barrier. The vapor barrier is typically laid first when insulating a slab in new construction. However, the vapor barrier must be placed on top of an existing slab to add insulation.

If you plan to install a foam insulation layer under concrete slabs, make sure to use rigid foam panels of compression grade. These panels are made for support vehicles and can bear the weight. For advice about the thickness and R value of the foam insulation layer, consult your manufacturer or supplier. Some panels are equipped with vapor barriers. This panel does not require any plastic sheeting, as the panels provide enough vapor barrier after the seams have been sealed with impermeable tape.

Rigid Foam Board Insulation Between Studs
Rigid Foam Board Insulation Between Studs

How to Handle the Thresholds

Two layers of plywood and foam insulation can increase the garage floor’s height by up to 2 inches. This could be problematic if you plan to use the garage for vehicle storage. There are many options for smoothing this transition. Manufacturers of garage flooring tiles can help you find the right transition threshold for your garage floor.

A similar problem may occur at the threshold of your garage passageway. In this case, it might be necessary to change the threshold or even reduce the height of the door to accommodate the new floor.

Safety considerations

Garage floors can be subject to moisture and puddling. Some garage floors may have floor drains that help to remove any moisture from the garage. For example, basement drive-under garages are a common feature in older homes. This can lead to dampness and puddling. It can be difficult to lay foam insulation panels and a subfloor on top of a concrete slab. Water will collect beneath the floor and encourage rot.

Before covering garage floors or converting garage into living space, consult a professional building engineer. To ensure that your garage floor is dry enough to allow you to live in safety and comfort, it may be necessary for you to use modified construction techniques or a sump pump.

What you’ll need

Equipment / Tools

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

Materials

  • Concrete patch or leveling compound (depending on the need)
  • 6-mil polyethylene sheeting
  • 12
    1 inch thick rigid foam insulation (compression Grade)
  • Tape for heavy-duty construction
  • 24
    Plywood sheets 1/2 inch exterior grade
  • Utility screws 7/8 inches in diameter (corrosion-proof)
  • Threshold strips/edging (as required)

Instructions

Make sure to clean and repair the garage floor

Before you apply the vapor barrier or subfloor, inspect the concrete garage floor. Concrete floors with minor unevenness can easily be levelled with floor leveling compound. If the slab is curved or heaved, you can use other structural methods. If the slab is in serious condition, it is not advisable to attempt to convert a garage into a living space.

Install the Vapor Barrier

The first step in insulating concrete garage floors is to cover the entire floor with 6-mil polyethylene. This will act as a vapor barrier and prevent moisture from leaking through the slab. Seal the seams with waterproof tape. The polyethylene should be run along the walls for 3-4 inches. Secure the walls with construction tape.

Lay rigid foam

Spread sheets of rigid high-density foam over the polyethylene. To cut the foam to size, use a utility knife (or a hacksaw) to do this. You should leave a gap of 1/4 inch around the perimeter. Use heavy-duty construction tape to seal the seams of foam panels.

Lay the plywood panels

Place 1/2-inch plywood sheets of exterior grade plywood on top of the foam. Make sure the sheets are parallel to the long dimension. Place the panels about 1/4 inch apart with a 1/2-inch space around the perimeter. This allows for seasonal expansion or contraction of plywood. Measure and cut the panels as needed using a circular saw.

Layer a second 1/2-inch plywood layer over the first. Place this layer perpendicularly to the first with the same spacing. Attach the plywood layers together using 7/8-inch utility screws.

Tip

To fix bowed or bouncy areas in the interior or perimeter of the space, you can drill masonry screws through plywood insulation and into concrete.

Install Threshold/Transitions

Install threshold strips of the right size if the garage is still being used for vehicle storage. The manufacturer will provide instructions on how to attach the strips. Many come with adhesives specifically for this purpose.

If necessary, use a similar technique to adjust the threshold of the smaller garage passage doors.

Install flooring

After you have insulated the concrete floor, you are free to install any type of flooring. There are many garage mats and tiles that can be used to protect your garage from the elements. You can also paint or coat plywood with epoxy.

The plywood subfloor can be used as a base for any type of new flooring or underlayment if the garage is being converted into living space.

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