What type of insulation should I use under a basement floor slab in New Brunswick to prevent cold floors?

Rigid extruded polystyrene (XPS) foam board is the best insulation to install under a basement floor slab in New Brunswick, providing both thermal resistance and moisture protection against the cold, damp ground conditions found throughout the province. If you are pouring a new slab or replacing an existing one, sub-slab insulation is the most effective way to eliminate cold floors and reduce heat loss into the earth below.

For new construction or full slab replacement, the standard approach is to lay 2 inches of XPS rigid foam (R-10) directly on top of a compacted gravel drainage layer and beneath the concrete slab. The installation sequence matters: compacted gravel base (minimum 4 inches for drainage), followed by the XPS foam boards with staggered joints and taped seams, then a 6-mil polyethylene vapour barrier over the foam, and finally the concrete slab on top. The XPS serves a dual purpose — it insulates against the cold ground (which stays around 4°C to 8°C year-round in New Brunswick) and resists moisture absorption. XPS has a compressive strength of 15 to 25 psi, which easily supports a standard 4-inch residential concrete slab and normal basement loads.

The critical distinction is XPS versus EPS (expanded polystyrene). While EPS is cheaper at roughly $0.60 to $1.00 per square foot for 2-inch thickness, it absorbs more moisture over time and loses R-value in wet conditions. XPS costs $1.00 to $1.50 per square foot for 2-inch boards but maintains its rated R-5 per inch even in damp soil contact — a significant advantage in New Brunswick where groundwater and soil moisture levels are consistently high due to the Maritime climate.

If you are not replacing the slab, you have two above-slab options for addressing cold floors. Dricore subfloor panels are engineered OSB panels bonded to a raised plastic membrane that creates a 3/4-inch air gap above the concrete. They cost $3 to $5 per square foot and provide roughly R-1.7 plus moisture separation. For more thermal performance, you can install 1-inch or 1.5-inch XPS foam boards directly on the existing slab, then cover with 3/4-inch tongue-and-groove plywood as a subfloor, adding R-5 to R-7.5. This assembly costs $4 to $7 per square foot installed but delivers noticeably warmer floors.

In New Brunswick's climate, the ground temperature beneath your slab stays cold year-round, and concrete is an excellent thermal conductor — meaning without insulation, your basement floor constantly pulls heat out of the room. In Moncton and Saint John where sandy and clay soils hold different moisture levels, sub-slab insulation also helps control moisture wicking, where water vapour migrates upward through the concrete and damages flooring materials.

Before installing any sub-slab or above-slab insulation system, test the existing concrete for moisture using the plastic sheet method (tape a 2-foot square of clear plastic to the slab for 48 hours and check for condensation). If significant moisture is present, address the drainage situation first — a sump pump and interior drainage system may be needed before insulation makes sense. Also, test for radon before sealing up the slab, as New Brunswick has elevated radon levels in many areas, and sub-slab insulation combined with a vapour barrier is actually part of an effective radon mitigation strategy when paired with a vent pipe.

For most existing New Brunswick basements, the Dricore panel system offers the best balance of cost, ease of installation, and performance. For new construction, always specify 2-inch XPS beneath the slab — it is a modest cost during the pour that pays dividends in comfort and energy savings for decades.

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• Thirty Four Renovations
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