Clear sky visible through glazing — the quality and direction of light entering a room depends on the window's orientation and the sky conditions at that latitude.
Solar geometry at Canadian latitudes
The sun's elevation at solar noon on the winter solstice (December 21) varies across Canada's populated regions. At Toronto (43.7°N) it reaches approximately 23°; at Calgary (51.1°N) around 15.5°; at Edmonton (53.5°N) near 13°. This low angle means that direct sunlight penetrates deeply into a room through south-facing windows in December, whereas in June the sun is high enough that even a modest overhang blocks direct beam radiation from the same window.
This seasonal asymmetry is the foundation of passive solar design at northern latitudes: deep winter sun penetration provides heat and light when both are most needed, while summer shading prevents overheating. The geometry can be calculated precisely for any latitude, but a few general principles apply across most Canadian locations.
South-facing windows: aperture area and depth of penetration
The useful depth of daylight penetration through a side window is approximately 1.5 to 2 times the window's head height above the floor. A window with its top at 2.4 m above the floor (a standard ceiling height) distributes useful natural light to a depth of roughly 3.6 to 4.8 m from the window wall. Rooms deeper than this are dim in the middle ground regardless of how large the window is.
For south-facing glazing in heating-dominated climates, the recommended glazing area is typically in the range of 7% to 12% of the floor area of the room being served. Areas below this range make little contribution to daylighting or passive solar gain; areas well above this range can produce glare and local overheating unless carefully managed with glazing selection and thermal mass.
Header height and shelf daylighting
Raising the head of a window — even while keeping the total glazing area the same — pushes daylight further into the room and reduces the contrast between the bright window zone and the rest of the space. A window with its head at 2.7 m rather than 2.4 m increases daylight penetration by a meaningful amount in the back half of a 5 m-deep room.
A light shelf — a horizontal reflecting surface positioned partway up the window — is used in commercial buildings to bounce daylight from the upper portion of the window to the ceiling plane, from which it distributes downward. This technique is less common in residential buildings but is applicable in double-height spaces or rooms with high south-facing windows in libraries and home offices.
Orientation: east, west, and north
Windows on east and west facades receive direct morning and afternoon sun respectively, which is lower in elevation than midday sun. In winter this provides some useful gain; in summer the low eastern and western sun is difficult to shade with overhangs because it arrives at shallow angles.
For primarily heating-dominated locations — most of Canada's interior — east and west glazing areas are often reduced compared to south glazing to limit summer afternoon heat gain while preserving the ability to cool the building naturally overnight. The optimal east-to-south and west-to-south glazing ratios vary by specific climate and building program, but a common starting point for residential buildings in prairie climates is a south-to-other ratio of approximately 2:1.
North-facing windows receive no direct sun at northern latitudes. They provide diffuse sky light, which is visually soft and consistent in colour but contributes no passive solar heat. In cold climates, minimising north glazing area reduces conductive heat loss while having little effect on daylighting quality if south, east, and west windows are adequately sized.
Overhang depth calculation
The overhang depth required to shade a south-facing window at noon on the summer solstice can be estimated with the following relationship:
Overhang depth = Window head height above overhang attachment point ÷ tan(summer noon solar altitude)
At 51°N, the summer solstice solar noon altitude is approximately 62°. For a window 1.0 m tall with the overhang at its head, the calculated depth is approximately 0.53 m. This provides full shading at noon on the solstice while allowing full penetration during winter months when the sun is below 20° elevation.
Clerestory windows
Clerestory windows — windows placed high in a wall, typically above an adjacent lower roof or at the transition between two roof levels — introduce light into interior zones that cannot be served by conventional side windows. In a house with a central corridor, for example, clerestory windows on the south face of the upper portion of a split-level can daylight the corridor without affecting privacy in adjacent rooms.
Clerestory glazing at height also distributes light more evenly than low windows because the incoming beam strikes the ceiling and upper walls first, which diffuse it downward. This reduces the contrast between the window itself and the darker lower parts of the room.
Window performance and daylighting interaction
The visible light transmittance (VT) of a window is separate from its solar heat gain coefficient and its U-value. A window with a low U-value triple-glazed unit may also have a lower VT than a double-glazed alternative, depending on the low-e coating chemistry. In buildings where daylighting quality is a priority, VT should be specified alongside thermal properties rather than treated as secondary.
For most residential south-facing windows in Canada, a VT of 0.50 or above is a practical target. Units below 0.40 VT will appear noticeably darker and reduce the quality of natural light entering the room, particularly on overcast days when diffuse sky luminance is the primary light source.
Overcast sky conditions
Canada's northern cities experience a high proportion of overcast days between November and March. Under an overcast sky, there is no direct sun component; available light comes entirely from the diffuse luminance of the cloud layer. The overcast sky is brightest at the zenith and decreases toward the horizon — roughly three times brighter at the zenith than at the horizon in standard overcast sky models.
This means windows that look upward — clerestories, skylights, or windows in light wells — capture more of the available overcast light than windows that face the horizon. In buildings at high latitudes where overcast conditions dominate the daylit hours, this is a significant consideration that affects whether to prioritise vertical glazing or roof-plane glazing.
Internal layout and room geometry
Window placement decisions do not operate in isolation from the room plan. A large open-plan kitchen and living area can be served by a single bank of south-facing windows. A bedroom with the head wall on the north and the window on the south has a short depth-to-width ratio that benefits from the daylight penetration geometry described above. A narrow corridor with no exterior wall access requires either a borrowed light strategy — an interior window into an adjacent daylit room — or a tubular daylighting device from the roof.
Planning window locations in coordination with the room layout, rather than placing them according to elevation aesthetics alone, produces buildings where natural light is useful across a larger proportion of the floor area.