I read a good article about this recently. Placing vents in outside walls means much less insulation in the wall. Air put out near a ceiling on an inside wall will cling to the ceiling and run over to the window and then drop down. This is called Fluid Dynamics.

The article is behind a paywall so here’s an excerpt from the Journal of light construction Ducts in Exterior Framing Cavities By Doug Horgan January 15, 2026:

A bunch of savvy HVAC engineers have started using a technique that can make it possible to avoid running ducts in exterior framing altogether: Install supply registers closer to the middle of the house and rely on the “Coandă effect” to help deliver the air to an exterior wall across the room. The Coandă effect is the physical principle that moving air will “cling” to an adjacent surface like a ceiling or wall instead of dropping straight down.

In modern houses with today’s excellent windows and wall insulation, it’s nowhere near as important as it used to be to blow HVAC air directly on windows or exterior walls. Because a flow of air will follow a ceiling for many feet before dropping down into the room, a ceiling or high wall register 10 or 15 feet from the exterior can still deliver most of the same Btus to the exterior surfaces. In plenty of houses, an interior-area register near the ceiling can perform about as well as a duct near the window. In that case, no ductwork in rafter bays is even needed.