About the Product
These Rockal 3" XPS shiplap boards deliver R15 of continuous insulation in a single pass, built for assemblies where higher thermal targets or below-grade exposure call for a thicker panel. The interlocking shiplap edge locks board-to-board, closing the gap at every seam so air movement and thermal bridging are minimized without specialty tapes. The closed-cell extruded polystyrene structure repels water, holds dimensional stability under load, and retains its R-value year after year, even in damp or buried conditions. At 3" thick and 300 kPa of compressive resistance, these boards suit foundation perimeter use, under-slab applications in colder climates, and exterior continuous insulation jobs where a single thicker layer replaces stacking two thinner ones.
For solo installers, the 24"×48" (2×4') board size is the easier choice to handle and set alone compared to larger formats — it's lighter and more manageable one-handed while fastening, though it does mean more seams and more tape or sealant to close them. Larger boards cut down on seams and labour for crews working in pairs, but are harder to manage solo. Score and snap with a utility knife, fasten mechanically through to framing, and finish with most common cladding and waterproofing systems.
XPS Boards FAQ
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XPS is the right call anywhere the insulation will face moisture, ground contact, or sustained load — below-grade foundation walls, under-slab applications, exterior continuous insulation over sheathing, and any assembly where the material might get wet or bear weight over time. Its closed-cell structure resists water absorption and holds its R-value in damp conditions, while its compressive strength stands up to backfill pressure or slab loads without crushing. For above-grade interior walls or areas with no moisture or load exposure, other insulation types (fiberglass, rockwool) are often more cost-effective — XPS earns its price where water and pressure are actually a factor.
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It comes down to your target R-value and how much continuous insulation your assembly calls for. A single 3" board delivers R15 in one layer, where 2" board only gets you R10 per layer — so if your wall or foundation assembly needs to hit a higher continuous-insulation target, 3" gets you there without stacking two layers of 2" board. Continuous insulation requirements vary by assembly type and application under the Ontario Building Code (SB-12) and NBC energy provisions, so it's worth confirming your specific target R-value with your designer or building department before choosing thickness — we're happy to help work through the numbers with your drawings.
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The main difference is compressive strength and water performance. XPS has a tighter closed-cell structure that gives it higher compressive strength (300 kPa in our boards), so it holds up better under sustained load — this matters for under-slab and foundation applications where the material may be supporting soil backfill or slab weight over the long term without crushing or losing thickness. XPS also absorbs less water over time, so it holds its R-value better in damp or buried conditions. EPS is typically less expensive and still performs well for many above-grade applications, but XPS is the stronger choice anywhere ground contact, moisture, or sustained load is a factor.
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In practical terms, it's how much sustained load the board can bear before it starts to compress and lose thickness (and R-value) over time. For under-slab or foundation-perimeter installs, this matters because the board is going to have soil, concrete, or structural loads pressing on it for the life of the building — a higher compressive rating means it holds its shape and thermal performance under that load instead of gradually flattening out. For interior or non-load-bearing wall applications, compressive strength matters much less.
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Not really — thickness doesn't change handling much on its own. Handling difficulty comes down to board size (the 24"×48" format is manageable for one person), not thickness. Where 3" does add a bit of work is fastener length and sometimes furring depth, since you're penetrating and building out past a thicker panel.
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The difference is how adjacent boards meet at the seam. Square-cut edges butt flush, edge to edge, in a straight line — simple to manufacture and easy to trim on site, but the seam is left as a straight gap that needs taping or sealing to perform well. Shiplap edges have a stepped lip on one side and a matching groove on the other, so boards overlap at the seam instead of butting flush — this closes the gap that a butt joint leaves, cutting down on air movement and thermal bridging, and reduces or eliminates the need for seam tape. Shiplap also helps keep boards flush and aligned as they're fastened, since the interlock resists shifting. Square-cut is simpler and more flexible to trim; shiplap does more of the sealing and alignment work on its own, which is why it's the standard edge on our XPS line.