What insulation goes under a Vancouver deck over a room?

For a deck over a room in Metro Vancouver, the insulation question is secondary to waterproofing — but when insulation is needed, closed-cell spray polyurethane foam (SPF) applied to the underside of the deck structure is the most effective solution for this climate.

The reason closed-cell spray foam is the right choice here comes down to Metro Vancouver's persistent moisture. With 1,200–2,000mm of annual rainfall and year-round humidity sitting between 60–80%, any insulation system under a deck over habitable space must handle constant moisture exposure without absorbing water, losing R-value, or promoting mould growth. Closed-cell spray foam does all three — it is vapour-impermeable, structurally rigid, and completely resistant to mould. It also air-seals the joist cavities simultaneously, which matters enormously in a climate where warm interior air meeting cold exterior surfaces creates condensation problems inside the framing.

The correct assembly, from top to bottom, is:

The deck surface itself must be a fully waterproofed system — a vinyl membrane (Duradek, Tufdek) heat-welded at all seams over a plywood substrate, or an aluminum deck system with integrated drainage. No amount of insulation below compensates for water getting through the deck surface and into the structure. The membrane is the first line of defence, and it must be installed by a certified professional. Membrane failure causes rot in the framing and water damage to the room below that costs far more to repair than the original deck.

Beneath the plywood substrate, between the joists, is where insulation lives. Closed-cell spray foam applied at 2–3 inches achieves approximately R-14 to R-21, and because it bonds directly to the wood framing, it eliminates thermal bridging through the joists. For higher R-value requirements (the BC Energy Step Code and Part 9 of the BC Building Code specify minimum thermal performance for floors over unconditioned or semi-conditioned spaces), you can increase spray foam thickness or combine it with rigid mineral wool board on the underside of the joists, protected by a weather-resistant soffit.

What not to use is equally important. Fibreglass batt insulation is a poor choice in this application — it absorbs moisture readily, loses R-value when wet, and creates ideal conditions for mould growth inside the joist cavities. Open-cell spray foam is also problematic here because it is vapour-permeable and can absorb moisture in Metro Vancouver's humid conditions. Rigid EPS or XPS foam board between joists can work if perfectly air-sealed at every edge, but achieving that seal in a field-cut application is difficult and any gap allows moist air infiltration.

Vapour control matters here. In a deck-over-room assembly, the warm side is below (the room) and the cold side is above (the deck). You want to stop interior moisture from migrating upward into the structure. Closed-cell spray foam on the underside of the deck sheathing — applied from below, filling the joist bays — acts as both insulation and vapour barrier in one step, which is why it is the preferred approach among experienced Vancouver contractors.

Practically speaking, budget $8–$18 per square foot for closed-cell spray foam professionally applied, depending on thickness and access conditions. This is in addition to the waterproof membrane cost ($15–$30 per square foot). For a 200 square foot deck over a room, expect $1,600–$3,600 for the insulation component alone.

This work requires a professional — both the membrane installation and the spray foam application involve specialized equipment, and errors in either create expensive structural damage. The spray foam contractor should also be aware of the full assembly so the foam thickness and placement coordinates correctly with the membrane system above.

If you're planning a deck over a room and need help finding experienced contractors in Metro Vancouver, Vancouver Deck Contractors can match you with builders who understand this specific assembly. Browse the Vancouver Construction Network directory at vancouverconstructionnetwork.com/directory?trade=decks.