My current house is a single story built over a crawlspace and all the HVAC registers are in the floor. In all of the larger rooms, at least one (and usually more) register was placed near an exterior wall and almost always near a window. Looking throughout the house, it seems intentional. I've noticed this before in other houses, and I've talked a few friends and the informal poll tells me that for houses with floor registers built in different states built decades apart, it appears the registers were intentionally placed near exterior windows. In my case, given the layout of the crawl space, it seems like it would have been easy to place them them in other locations.

My amateur intuition tells me that for both heating and cooling, having registers near a window is going to lead to a lot of waste. Is this layout actually common? If so, why?

  • You are correct that it leads to some waste because the closer the vents are to windows, the higher the temperature difference across the windows. Higher temperature differences cause faster heat transfer. However, the energy you lose because of this is probably much less than the amount lost in the ducts themselves through air leaks and thin (if any) insulation. Apr 16, 2013 at 21:30
  • Windows condense moisture, heat evaporates it and takes it away. Window rot prevention. Mar 18, 2014 at 4:38

3 Answers 3


Yes, placing registers (or other heat sources such as radiators) near exterior windows and doors is the usual practice. This is done in order to combat cold drafts and ensure a more even temperature throughout the room. Here's a Q&A on the subject from Ask This Old House:

Window glass is the coldest part of a wall. When warm room air hits it, the air cools, and cool air sinks. The movement of cool air creates floor drafts that most people find uncomfortable. The placement of forced-air heat registers or baseboard heating units under the windows counteracts this process by sending up warm air to mix with the cool. The end result is that the room feels more comfortable.

  • 5
    It's also convenient for room usage. you are unlikely to want to put furniture underneath a window - so having a vent/radiator there doesn't inconvenience anyone.
    – mgb
    Jun 25, 2012 at 0:58
  • 2
    It also helps dry out a major water source, rot and mold haven. Mar 18, 2014 at 4:40

This DOE reference indicates, with better double glazed windows and better insulation, that ducts should not be run in outside walls anymore.

HVAC Ducts Shall Not Be Run within Exterior Walls

In the past, it was common practice to run ducts inside a wall cavity of an exterior frame wall. It is sometimes done today. However, we have learned that this practice will create an energy penalty and can cause durability issues within the wall cavity ... In older homes with poorly insulated walls and single-pane windows, supply registers are often located at or on exterior walls to condition the cold walls and cold air that would leak in at the windows. In new homes with better air sealing and insulation and higher performance double-pane windows, there is less heat transfer through exterior walls and less air leakage in and around windows. Exterior supply air throws are no longer necessary to maintain comfort; shorter duct runs with interior throws are preferred for improved energy efficiency and better HVAC performance.


Of course, this would not rule out floor registers below windows.

  • 1
    Nice find! I had not given that any thought.
    – wallyk
    Mar 26, 2014 at 1:12

Originally, homes throughout northeast, north and mid west that were new enough to have central heat, used gravity furnaces located in the basement. With no blower to move the air, the common practice was to place vents next to exterior walls and windows to counter drafts and temperature differences. Once fan forced heat was available, the old gravity furnace were removed and existing duct was left in place rather than relocating the registers, which some times was impractical or expensive. Research on fan flow and air distribution was scarce and contractors were focused on the next new step in indoor air quality called air conditioning. Todays research demonstrates that high sidewall and ceiling registers that move forced air from interior walls and ceilings to exterior walls and windows is more efficient.

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