Running ducts outside the envelope is never a good idea, especially in cold climates like Toronto. This home had some creative duct work done to it:
Looking left of the hydro pole, you see a return trunk spanning the basement to the 3r floor.
The close up:
Running from the basement to the ceiling of the 3rd floor, seemed uninsulated. What were they thinking!
This job was cheap and cheerful!
Another house using a commercial roof-top unit and not only are the ducts running outside, they go into through the attic for distribution.
This roof-top furnace and AC will be taxed both summer and winter. Folks, it might be convenient but it’s not right to put the heart of a house on a roof.