Follow the sun before choosing the glass

Canadian planning context. Technical examples from other regions are identified in the guide; approval and performance requirements must be checked for the actual building and jurisdiction. Original explanatory diagrams are not construction details.

01 /Orientation is a starting point, not a complete exposure map

Two south-facing rooms can behave differently because one has an overhang, the other faces a reflective courtyard, and a neighbouring building shades only one. Trees change with growth and leaf season. Hills, balconies and fences alter low-angle sun. An opening’s orientation should therefore be recorded with its obstruction profile, not treated as its whole climate specification.

Use true orientation and note the season and time of any observation. A photograph at noon in October cannot establish whether a room overheats at 6 p.m. in July. A sun-path study can help with proposed changes, while existing-room observations show how occupants actually experience the space. Record when blinds close, where sun patches fall and whether the room is occupied during those hours.

02 /Why south, east, west and north need different thought

In the northern hemisphere, the sun is generally lower in winter and higher in summer. South-facing glazing can use that difference: a designed horizontal projection may shade high summer sun while admitting lower winter sun. The geometry depends on latitude, window position and the dates and hours you want to control. A generic overhang depth is not suitable for every Canadian home.

East and west often receive sun at low angles, so a shallow horizontal canopy may leave much of the glass exposed. Vertical fins, adjustable exterior screens or other systems may be more appropriate. West exposure can coincide with warm outdoor conditions and late-day occupancy. North usually offers substantial diffuse daylight without the same direct midday solar exposure, but northern latitudes and summer mornings or evenings complicate simple rules.

The U.S. Department of Energy’s passive-solar guidance describes the interplay of orientation, collection, storage, distribution and control. Apply its northern-hemisphere principles thoughtfully; do not import a U.S. climate target or a fixed orientation prescription as Canadian code.

03 /Solar gain can be useful and unwanted in the same room

A cold January room may welcome a sunny afternoon, while that same glazing causes a June bedroom to overheat. Annual heating benefit can hide hot occupied hours. Internal heat from people, cooking and equipment can also make the room warm before much sun arrives. A highly insulated enclosure reduces losses, which can make unwanted gains harder to release without ventilation or cooling.

Select SHGC alongside glass area and shading. Lower SHGC reduces the fraction of incident sun entering as heat, but may alter daylight or winter benefits. A high-performing coating can preserve useful daylight with lower solar gain, while a dark tint may reduce both. The solar-gain guide explains those differences. Compare actual complete-window values for the proposed packages.

Do not specify high-gain glass solely because the city has a long heating season. Ask when the room can use that gain, what happens in heat waves and whether shades will actually be operated. An option that depends on occupants keeping blinds closed all summer has a different daylight and view outcome from the showroom demonstration.

04 /Exterior shades intercept sun before it enters

An exterior shade blocks or redirects part of the incident radiation before it passes through the glazing. Interior blinds still help with glare and can influence heat flow, but some admitted solar energy is already inside the enclosure. An exterior system therefore deserves consideration where solar gain is a central problem, especially on exposed large glass.

Exterior options include fixed overhangs, fins, shutters, screens and adjustable louvers. Their performance depends on geometry, material, openness, distance from the glass and operating state. They also face wind, rain, snow, ice and maintenance. An exterior shade should have suitable anchorage, drainage and access, without damaging the wall’s water-control layers.

05 /Shade geometry should be drawn

A shading sketch should show the projection, its vertical position relative to the window, the glass height and width, side extensions and the intended sun angles. The location of the glazing within a deep reveal matters. A canopy that shades the top half at noon may leave the lower half exposed, and an adjacent opening may receive sun from the side.

For a new build or substantial alteration, have the designer test the geometry against relevant seasonal and daily periods, including occupied hours. For a retrofit, check structural attachment, property boundaries, heritage or strata requirements and any local approval. A shade can affect the external appearance and façade loads; treat it as a building element rather than an accessory with no consequences.

Keep access and maintenance practical. A motorised screen that cannot be serviced without dismantling a façade is an ownership concern. A fixed shade that prevents safe glazing replacement or blocks required egress needs revision. The plan should make those trade-offs visible before a purchase.

06 /Night cooling has conditions

Cool outdoor air can remove heat when it is actually cooler and suitable to bring indoors. If outside is hotter, humid, smoky or otherwise unsuitable, opening windows may worsen the indoor condition. Wind and temperature difference determine natural airflow; a large opening does not produce a fixed ventilation rate on every night.

Natural cooling also depends on security, noise, insects, child safety and whether occupants can safely operate openings. A design that assumes every window will remain wide open overnight is not robust for every household. In wildfire smoke, Health Canada advises limiting outdoor smoke entry while prioritising cooling during coincident extreme heat. A practical home needs a plan for the days when natural ventilation is unavailable.

Mechanical cooling and planned ventilation can be part of the solution. Better glass and shading can reduce loads, but they are not a complete heat-safety system. Keep local public-health guidance and the building’s cooling arrangements separate from a simplified window calculator.

07 /Thermal mass helps only within a complete strategy

Materials can absorb heat and release it later. That can moderate some temperature swings when there is a suitable means to release stored heat. It can also keep a room warm overnight after a hot day. The location of mass, actual sun exposure and night conditions matter; a heavy floor is not an automatic overheating remedy.

Avoid making window decisions from a single phrase such as passive solar home. A successful strategy coordinates glazing area, SHGC, thermal performance, shading, storage, air movement and controls. Furniture and rugs may alter exposure to the intended storage surface. Occupancy may differ from the designer’s assumptions. An hourly model helps reveal those interactions where they are consequential.

08 /A retrofit can improve the room without enlarging the glass

Before proposing larger windows for a dark or uncomfortable room, examine the existing clear glass area, frame width, tint, obstruction and shading. A better-balanced glass package or improved daylight distribution may solve the problem within the same opening. Enlarging adds structural, water-control, privacy, heat-loss and solar-load consequences.

An insert can reduce clear aperture by retaining old framing; a full-frame replacement may preserve more usable view at the same rough opening. Either choice needs an appropriate wall connection. Ask for clear-glass dimensions on the proposed schedule and compare them with the current window. Match the replacement decision with daylight and sightlines rather than treating every opening as an isolated product.

09 /Model the important uncertainty

An energy model or overheating assessment is only as useful as its assumptions. Include actual orientation, glass area, rating data, shading, occupancy, internal gains, ventilation and cooling. Ask which weather data and future or extreme-weather scenarios were considered, and whether the result represents a whole building or the affected room.

The NRC’s overheating work identifies windows, shading, insulation and building mass among design considerations. It does not support a universal claim that one glazing package prevents overheating. A good design review tests plausible variations: shades left open, a smoke event preventing ventilation, a changed office schedule or cooling unavailable. That is more useful than a single best-case result.

10 /Turn the room survey into a specification

List glass packages by elevation where appropriate, with whole-window U-factor, SHGC and VT, then add the shade geometry and expected operating states. Name what is fixed, what is adjustable and who maintains controls. Ensure the required safety, structure, egress and wall drainage remain satisfied.

• Opening schedule with orientation, clear glass and room use.
• Exact rated glass packages and any approved variants.
• Shade drawings, attachments, weather limits and access.
• Daylight and privacy review in typical occupied positions.
• Cooling and ventilation plan, including unsuitable outdoor-air days.
• Documented modelling assumptions and product substitutions.

This approach makes the window a part of a comfortable room. It also creates a useful record if the room behaves differently after replacement: the supplier can compare actual glass, shade operation and conditions with the design rather than attributing every hot day to a bad window.

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