What rigid foam board insulation thickness is needed under a slab-on-grade floor in New Brunswick's climate?

For slab-on-grade floors in New Brunswick, you need a minimum of 2 inches (R-10) of rigid foam insulation at the slab perimeter, extending either 24 inches below grade or 48 inches horizontally under the slab edge. However, for optimal performance in NB's Zone 6 climate with frost depths reaching 4-5 feet, many builders are installing 3-4 inches (R-15 to R-20) of XPS rigid foam.

The NB Building Code requires R-10 minimum at slab perimeters, but this is truly a minimum standard. With NB's 4,800-5,200 heating degree days per year and ground temperatures that can drop well below freezing, under-insulated slabs become massive heat sinks. A poorly insulated slab edge can account for 15-25% of a home's total heat loss, making floors uncomfortably cold and driving up heating costs significantly.

XPS (extruded polystyrene) is the material of choice for under-slab applications in New Brunswick. Pink or blue board XPS provides R-5 per inch, maintains its insulating value when exposed to soil moisture, and won't degrade from freeze-thaw cycles. EPS (white beadboard) costs less but absorbs more moisture over time. Polyiso should never be used under slabs in NB because its R-value drops dramatically when cold and wet.

Proper installation extends the insulation both vertically and horizontally. The vertical portion runs from the top of the slab down to either 24 inches below grade (minimum code) or ideally to the footing level. The horizontal portion extends inward under the slab edge for 24-48 inches. This creates an insulated "thermal break" that prevents the concrete slab from conducting heat directly to the cold ground and frost zone.

For radiant floor heating systems, increase the insulation to R-15 or R-20 (3-4 inches of XPS) under the entire slab area, not just the perimeter. Without full under-slab insulation, radiant systems heat the ground instead of the house — an expensive mistake that can double or triple heating costs. The insulation also needs a vapor barrier (6-mil polyethylene) on top to prevent ground moisture from reaching the concrete.

Installation timing matters in NB's climate. Rigid foam must be protected from UV exposure and physical damage during construction. Install the foam after excavation and grading are complete but before placing gravel and concrete. In winter construction, protect foam from ice buildup and ensure concrete doesn't freeze during curing.

Professional installation is recommended for slab-on-grade projects because proper grading, vapor barrier placement, and insulation continuity are critical. A gap in the thermal envelope at the slab edge creates a permanent "thermal bridge" that wastes energy for the life of the building. The cost difference between R-10 and R-15 under-slab insulation is minimal during construction but expensive to retrofit later.

Need help finding a professional insulation contractor experienced with slab-on-grade applications? New Brunswick Insulation can match you with local contractors through the New Brunswick Construction Network who understand NB's climate requirements and building code standards.