Can ductwork be installed in exterior walls in a Toronto home without freezing issues?

Installing ductwork in exterior walls is strongly discouraged in Toronto and the GTA — the risk of freezing, condensation, and massive energy loss is simply too high given our climate. While it is technically possible with extreme care, the practical challenges and ongoing risks make interior wall routing the far better choice in almost every situation.

The core problem is thermodynamics. Toronto winters regularly see temperatures of -15 to -25 degrees Celsius, and exterior walls are the boundary between heated indoor air and that extreme cold. A supply duct carrying warm air at 45 to 55 degrees Celsius through an exterior wall loses heat rapidly through the wall cavity to the outside, wasting energy and delivering lukewarm air to the room. In cooling season, the reverse happens — a cold supply duct at 12 to 15 degrees Celsius sits inside a warm, potentially humid wall cavity, and condensation forms on the duct surface. That moisture accumulates inside the wall where you cannot see it, promoting mould growth, rotting framing, and degrading insulation over months and years.

The freezing risk is real and specific to Toronto's climate. If the furnace cycles off during a cold snap — whether from a thermostat setback, a power outage, or an equipment failure — air inside the duct cools quickly. In an exterior wall with limited insulation between the duct and the outside sheathing, that duct can reach freezing temperatures within hours during a -20 degree night. Any residual moisture from condensation can freeze inside the duct, and repeated freeze-thaw cycles crack flex duct jackets and corrode metal duct surfaces. Toronto experiences over 50 freeze-thaw cycles per winter, which is punishing for any ductwork in a vulnerable location.

If you absolutely must run ductwork in an exterior wall — for example, in a renovation where there is no feasible interior routing option — there are ways to reduce the risk, though they do not eliminate it. The duct must be positioned on the warm side of the insulation, meaning the wall's full insulation depth sits between the duct and the exterior sheathing. The duct itself should be insulated with R-8 minimum duct wrap and sealed at every joint with mastic. The wall cavity must include a continuous vapour barrier on the warm side to prevent moisture migration. Even with these measures, you are creating a weak point in your building envelope that will underperform compared to properly routed interior ductwork.

The better approach is to route ducts through interior walls, floor joist cavities, or purpose-built soffits and bulkheads. In basement-fed systems common across the GTA, supply ducts typically run through the basement ceiling and up through interior wall cavities to registers on upper floors. For rooms on exterior walls where you want a register under a window — which is ideal for counteracting cold drafts — the branch duct should run through the floor joist space from an interior route, not down through the exterior wall.

If you are planning a renovation that involves ductwork routing, consult with an experienced HVAC contractor during the design phase. They can find interior routing solutions that avoid the exterior wall risks entirely. Get matched with a local ductwork professional through the Toronto Construction Network for a free assessment of your options.