The Compass Brief: Specifying Windows Wall by Wall
Most window quotations are built around a single glazing specification applied to every opening in the house. It is an understandable simplification, but it treats a north-facing bathroom and a south-facing living room as though they perform the same job. In practice, each elevation of a UK home receives a different share of sun, wind and weather, and the glazing that suits one wall can be a poor fit for another.
A compass-led specification is not about buying four different window systems. It is about assigning the right glass, coating and ventilation strategy to each elevation, then keeping the frame family and sightlines consistent. The result is a home that is warmer in winter, less prone to summer overheating and easier to keep comfortable without relying on blinds and curtains to correct the glass.
Why a single specification rarely fits all
Solar gain is the obvious variable. A window that admits useful winter heat on a south elevation can become a liability in July, when the same glass contributes to overheating and glare. A north-facing window, by contrast, receives little direct sun but loses heat steadily through the coldest months, making thermal performance rather than solar control the priority.
There are secondary effects too. Ultraviolet and visible light fade fabrics, timber and artwork, and the risk is not evenly distributed around a building. Driving rain in the UK most often arrives from the south-west, so weather exposure and weather-tightness matter more on some walls than others. Low sun angles in winter can create glare on east and west elevations at times of day when rooms are in heaviest use.
South-facing windows: capture in winter, control in summer
South elevations are the best solar collectors in a UK home. In winter, when the sun is low, a reasonably high g-value allows useful free heat to enter. In summer, when the sun is high, a modest overhang or the angle of the glass can reduce some of that gain, but not reliably enough to prevent overheating in well-insulated rooms.
The specification question is therefore about balance. A solar-control coating with a moderate g-value can reduce summer gains while retaining some winter benefit. Where the room is heavily glazed, or where there is a large expanse of glass with little shading, it is worth asking for a calculated view of overheating risk rather than relying on a rule of thumb. For rooms with valuable furnishings, a laminated interlayer or a coating that reduces UV transmission will slow fading, although no glass eliminates it entirely.
West-facing windows: the evening heat problem
West elevations receive lower, stronger afternoon and evening sun. In summer this coincides with the warmest part of the day and with the hours when living rooms and bedrooms are most likely to be occupied. The result is often discomfort rather than daylight benefit: low-angle glare across a television or dining table, and solar gain arriving too late for the building fabric to release it overnight.
Solar-control glass earns its place here more often than on a south elevation. The aim is to take the edge off peak gains without turning the room grey on overcast days. Fading risk is also worth considering, particularly where a west-facing room holds upholstery, wooden floors or artwork that receive direct sun for several hours.
East-facing windows: morning glare and rapid swings
East elevations take the morning sun. In winter, that can be welcome; in summer, it can produce early glare in bedrooms and a rapid temperature rise before the day has warmed. The sun angle is low, so horizontal shading such as a blind or a projecting reveal can be effective, but the glass specification still matters.
A moderate solar-control coating can reduce morning glare without sacrificing too much daylight. Where the room is a bedroom, acoustic performance may be a higher priority than solar control, particularly on a busy road; the two requirements can usually be met in the same sealed unit, but they need to be stated together rather than added later.
North-facing windows: heat loss and diffuse light
North elevations receive little direct sun, so solar gain is rarely the issue. The priorities are thermal performance, condensation risk and daylight quality. A north-facing window is more likely to sit at a lower external temperature for longer, which makes the frame and spacer choice more important: warm-edge spacers and well-insulated frames reduce the risk of cold bridging and surface condensation.
Diffuse light is softer and more stable, which makes north-facing rooms attractive for studios and workspaces. The glass specification should preserve that quality rather than tint it unnecessarily. Where a north-facing room is also a bathroom or kitchen, ventilation becomes the dominant concern, and the window design should make purge ventilation straightforward.
A simple elevation schedule
A practical way to brief an installer is to set out a short schedule by elevation rather than a single line item.
| Elevation | Main risk | Specification focus |
|---|---|---|
| South | Summer overheating, glare, fading | Moderate g-value, solar control where glazing is extensive, UV reduction for furnishings |
| West | Late-day heat and low-angle glare | Solar-control coating, fading protection, shading strategy |
| East | Morning glare, rapid temperature change | Moderate solar control, acoustic performance if needed |
| North | Heat loss, condensation, dull light | Strong thermal performance, warm-edge spacers, clear glass |
The schedule does not need to be technical. It needs to record what each elevation is expected to do, so that the quotation can be checked against it.
Frames, ventilation and the details that cross elevations
Frame material and colour will usually stay consistent across the house for visual reasons, but performance details can vary. A north-facing wall may benefit from a warmer frame and a higher-performance sealed unit; a south-facing wall may need a different coating and a shading plan. Ventilation should be considered at the same time: trickle vents provide background ventilation, but they do not solve summer overheating. Where solar gain is high, an opening window that allows night purge ventilation is more useful than a larger vent.
Installation also varies by elevation. South and west walls often take the brunt of driving rain, so drainage paths, seals and sill details deserve attention. On exposed sites, it is reasonable to ask how the windows have been tested for weather-tightness and whether the installation method accounts for the prevailing wind.
Questions to put to your installer
- Which elevations have you specified for solar control, and what g-value have you assumed for each?
- Have you calculated summer overheating risk for the most glazed rooms, or is the specification based on standard practice?
- What UV reduction do the proposed units offer, and is that sufficient for the furnishings in each room?
- How will condensation risk be managed on north-facing windows, particularly in bathrooms and kitchens?
- Does the ventilation strategy rely on trickle vents alone, and how will summer purge ventilation work?
- Are the frame, spacer and sealed-unit specifications the same on every elevation, and if so, why?
- How have the windows been tested for weather-tightness on the most exposed elevations?
A window replacement is a long-term purchase, and the quotation is the point at which most of its performance is decided. Asking for an elevation-by-elevation brief is not an unusual request; it is a sign that the specification has been thought through rather than copied from a template.
