Why are heating vents always located on outside walls?

I find heating vents on outside walls in every house I’ve looked at that has forced air heating. This seems wasteful to me since that means the ducts must be running through outside walls, which means heat from the ducts would be escaping to the outside through the insulation. Wouldn’t it make more sense to locate the ducts in inside walls?

Answer from Green Energy Efficient Homes

While it’s true that in most houses you’ll find the heating vents on outside walls (or rather, coming out of the floor near an outside wall), this doesn’t mean that the actual ductwork is necessarily running through the outside walls. In many cases the ductwork comes up an inside wall cavity, then travels between joists to the heating register opening. The two main reasons for heating vents on outside walls are to reduce condensation on windows, which can prematurely deteriorate if water or ice builds up on the inside of the glass due to cold outdoor temperatures, and to prevent cold pockets from forming in the room near outside walls.

I’m grateful to Ryan from Anchorage, Alaska, for the following more detailed explanation – which he sent me to correct my original answer, in which I stated that locating heating vents on outside walls was merely a factor of the whim of builders or a question of poor design!

There are several reasons to put vents at exterior walls, or more specifically, at exterior windows. Any architect will tell you that they do this intentionally, even in brand new LEED certified buildings. When it is cold out, condensation builds up on windows if they do not have air moving past them. This moisture can prematurely destroy window sills and infiltrate the exterior wall, destroying its insulation and growing mold. Those of us in northern climates are also familiar with seeing ice on windows which don’t have proper ventilation. Other reasons stem from the creation of cold pockets. This creates chilly drafts, which people react to by turning the heat up. If the cold air near the window is heated, the whole room stays comfortable with less energy. You don’t need to heat the warm air, just the cold air. This all should be done in a way that maintains insulation between heated space and non-heated space. I’m sure that there probably are some retrofits that run ducts through exterior walls in ways they shouldn’t, but that’s a design problem, not a problem with the concept of venting at exterior walls.

It is indeed true that there are houses where the ducts run up outside walls – this is even the case in my own home. We have been undergoing some major remodeling lately – removing the center load bearing wall from the back portion of our house – and the builder routed three ducts that used to go up the center wall, up outside walls. While this is less desirable because those ducts are more exposed to the outside and therefore some of the hottest air in the house is close to the outdoors, the builder did try to put as thick a layer of insulation as possible between the ducts and the outside walls. Since we had decided to go with 6″ insulation (up from 4″ in the original ground floor), and since the ducts he used were 2″ deep, 14″ wide ducts instead of round ducts (designed to fit inside a stud hollow), we still managed to get 4″ of Roxul insulation behind two of the three ducts in question.