What is the best way to install ductwork in a Toronto home with a slab-on-grade foundation?

Slab-on-grade homes present one of the most challenging ductwork scenarios because there is no basement or crawlspace beneath the main floor to run ducts — every duct run must go through interior walls, above ceilings, in soffits, or in the attic. While less common than basement homes in the GTA, slab-on-grade construction is found in certain mid-century Toronto neighbourhoods, some Scarborough bungalows, many commercial-to-residential conversions, and newer townhome developments where the ground floor sits directly on a concrete slab.

The most common approach for slab-on-grade homes in the GTA is a high-delivery system with ducts in the attic or ceiling space. Supply ducts run through the attic from a centrally located furnace or air handler, dropping down through interior walls or soffits to ceiling-mounted or high-wall supply registers in each room. Return air is typically collected through low-wall or floor-level return grilles that connect to ducts running through interior wall cavities back to the air handler. This layout works well for cooling because cold air naturally falls from ceiling-level registers, but it is less ideal for heating because warm air delivered at ceiling height tends to stay high. Ceiling fans and proper register selection with adjustable louvers that direct airflow downward help mitigate this issue during heating season.

Attic duct installations in GTA homes require careful insulation and sealing. Toronto's climate means attic temperatures swing from over 50 degrees Celsius in summer to minus 20 in winter. Every supply duct in the attic must be insulated to a minimum of R-8 with a continuous, sealed vapour barrier to prevent condensation. All joints must be sealed with mastic and UL 181 foil tape before insulation is applied. Poorly insulated attic ducts in a GTA home can lose 25-40 percent of their heating or cooling energy before the air reaches living spaces — a massive energy waste that shows up directly on your utility bills.

An alternative approach gaining popularity in GTA renovations is a mini-duct high-velocity system such as SpacePak or Unico. These systems use small-diameter flexible ducts (typically 2-inch round) that can be snaked through walls, ceilings, and tight spaces without the bulky soffits required by conventional ductwork. The small supply outlets are nearly invisible in ceilings or walls. High-velocity systems are significantly more expensive than conventional ductwork — typically $10,000-$20,000 for a complete installation in a GTA home — but they solve the space constraints of slab-on-grade homes elegantly.

For homeowners considering in-slab ductwork (ducts embedded in the concrete slab), this approach is generally not recommended for GTA homes. While some mid-century homes were built with in-slab ducts, Toronto's clay soil and high water table cause moisture infiltration, rust, and mould problems in below-slab duct systems. Existing in-slab ducts that are deteriorated should be abandoned and replaced with an above-slab system rather than repaired.

A complete duct system for a slab-on-grade GTA home typically costs $5,000-$15,000 for conventional ductwork depending on home size and complexity, with the higher end reflecting the additional labour and materials needed for attic routing, soffits, and ceiling drops. This is a project that absolutely requires professional design using Manual J load calculations and Manual D duct sizing — the constrained routing options mean every duct run must be carefully planned to deliver adequate airflow without excessive static pressure or noise.