ARCHIVED – Do supply air registers really need to be under windows? v15n3-15

Volume 15, Number 3, September 2010

Archived Content

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Conventional thinking for residential forced air systems is that supply air registers should be located under exterior windows. There were good reasons for this in the past (primarily to counteract the cold downdraft from the window) but new construction standards (well-insulated walls, better glazing and air tight wall/window interface) mean that there is now less downdraft.

The NRC Institute for Research in Construction (NRC-IRC) is conducting experiments in a new, well-constructed building to evaluate whether the supply registers can be moved away from the traditional location without impacting indoor air quality, predicted thermal comfort, or energy loads.

Experiments have been completed for the heating season. Testing took place at the Ventilation and Wall Research Facility, located on the NRC campus in Ottawa. It has R-20 walls and R-40 attic insulation, triple-glazed, double low-emissivity, argon-filled windows and moderately airtight construction. Heating season experiments were supplemented by computational fluid dynamics (CFD) simulations. The physical experiments showed that in both traditional and non-traditional placement:

  1. there are minimal changes in predicted thermal comfort (a measure of thermal discomfort – vertical air temperature difference much lower than 3°C), and
  2. contaminant-removal effectiveness (a measure of indoor air quality) values were in the same range. The mean values for contaminant removal effectiveness were 1.14 for a register location at a window and 1.10 for a location at an interior wall.

The CFD results agreed with the physical experiment results. This would indicate that, in terms of thermal comfort and indoor air quality, there is no significant benefit to positioning the supply air register under the windows in a well-insulated and constructed building during the heating season.

In the second phase of the project, the research team is examining supply register results during summer (cooling) conditions to determine whether they will have a similar performance to that of the heating season. The results will also be expanded using simulation.

Positioning the supply air register away from a window could have a large impact for new construction as duct lengths could be shortened (saving materials and construction time). This could allow greater architectural freedom as the forced air system would not need to be ducted to the building perimeter.

For more information on the Ventilation and Wall Research Facility, go to the website at For more information on the project, contact Iain Macdonald at 613-993-9676 or