How do I insulate a garage floor for conversion to a heated space in Riverview NB?

Insulating a garage floor for conversion to heated living space in Riverview requires addressing two problems simultaneously — thermal insulation to keep the floor warm, and moisture management to prevent dampness from the concrete slab. Most garage slabs in Riverview were poured without a vapour barrier underneath and without perimeter insulation, which means ground moisture wicks up through the concrete and the slab acts as a massive thermal bridge to the cold ground below. Both issues must be solved before any finished flooring can be installed.

The most common and effective approach is a dimpled membrane and rigid foam system. Start by thoroughly cleaning the existing concrete slab and repairing any cracks larger than 1/4 inch with hydraulic cement or polyurethane caulk. Then lay down a dimpled polyethylene membrane (such as Delta-FL or a similar product) directly on the concrete, dimple side down. This creates a small air gap between the concrete and whatever goes on top, allowing any moisture that comes through the slab to drain to the perimeter without contacting the insulation or flooring above. The dimpled membrane costs approximately $1.00 to $2.00 per square foot for materials.

On top of the dimpled membrane, install rigid foam insulation board — either extruded polystyrene (XPS, the blue or pink board) or expanded polystyrene (EPS, the white board). For Riverview's climate, where winter temperatures regularly drop to -20 degrees Celsius and below, a minimum of 2 inches of XPS (R-10) is recommended, though 3 inches (R-15) provides noticeably warmer floors and better energy efficiency. XPS is preferred over EPS for below-grade and slab applications because it absorbs less moisture and maintains its R-value better in damp conditions. Rigid foam board costs $1.50 to $3.00 per square foot for materials depending on thickness.

On top of the rigid foam, you have two options for creating the finished floor surface. The first option is a plywood subfloor — lay 3/4-inch tongue-and-groove plywood directly on top of the foam, fastened with construction adhesive (not screwed through to the concrete, as this would create thermal bridges). The plywood provides a solid surface for any finished flooring — hardwood, laminate, vinyl plank, or carpet. The second option is engineered click-together subfloor panels (such as DRIcore or similar products) that combine a moisture barrier, insulation, and an OSB surface in one panel. These cost $3.00 to $5.00 per square foot but simplify the installation considerably and are a good DIY-friendly option.

One critical consideration in Riverview is the floor height change. Adding a dimpled membrane (1/4 inch), 2 inches of rigid foam, and 3/4-inch plywood raises the floor by approximately 3 inches. If your garage slab is already 4 to 8 inches below the main house floor level — which is typical for attached garages in Riverview — the insulated floor system actually helps reduce the step between the garage and the house, though you may still need a transition ramp or small step. Conversely, adding 3 inches to the floor height reduces your ceiling height by the same amount, which could be a problem if your garage ceiling is already close to the 2.3-metre (7 foot 6 inch) minimum required by the NB Building Code for habitable rooms.

Perimeter insulation is equally important. The edge of the concrete slab where it meets the frost wall is a major source of heat loss and can create a cold strip around the perimeter of the room. If possible, install rigid foam insulation on the interior face of the frost wall from the top of the slab down to the footing. This is easiest during the framing stage when the walls are being insulated — the wall insulation and floor insulation should connect to create a continuous thermal envelope.

For the floor insulation itself, this is a project that many competent DIYers can handle — laying dimpled membrane, cutting and fitting rigid foam, and installing plywood subfloor are straightforward tasks that require basic tools and careful attention to detail. Budget approximately $4 to $8 per square foot for a complete floor insulation system including dimpled membrane, 2-inch XPS, and plywood subfloor, or roughly $1,600 to $3,200 for a 400-square-foot two-car garage floor. However, if your conversion involves modifying the slab height, cutting concrete for plumbing, or addressing significant moisture problems, those tasks require professional help. Find experienced contractors through New Brunswick Garages — a free matching service through the New Brunswick Construction Network.