Window-to-Wall Ratio

How is window-to-wall ratio defined and measured?

Window-to-wall ratio (WWR) is glazed area divided by total facade area. It is quoted in two incompatible ways. Per-facade WWR measures glazing on one orientation (south, north, etc.) and reveals each orientation's contribution to the energy balance. Whole-building WWR averages across all facades, hiding performance differences. A building with 50% south glazing and 10% north glazing has roughly 30% whole-envelope WWR, masking the fact that south performs well while north drains heat. The distinction is critical because the right amount of glazing on a south wall differs completely from the right amount on a north wall. The denominator includes both glass and frame area in energy modeling, because the frame is structural enclosure and contributes to thermal performance.

Why does orientation matter more than a single target percentage?

A 40% south-facing window wall and a 40% north-facing wall perform utterly differently in heating and cooling energy, daylight quality, glare risk, and summer comfort. This is because the optimization problem involves four trade-offs that pull in different directions, and their relative importance flips depending on orientation: winter heat loss, winter solar gain, summer overheating, and daylight. On a south facade at 48–49°N (Slovakia's latitude), winter solar gains are substantial and desirable. On a north facade, the sun never provides useful gains, and larger glazing is pure winter heat loss. On east and west facades, afternoon sun creates summer overheating risk that fixed shading cannot control. This is why orientation-specific design is essential and why a single rule of thumb applied to all facades will fail.

How does winter solar gain change the winter heat loss equation?

Window glazing always loses heat to the exterior in winter because the interior is warmer than outside air. This is characterized by the window's U-value (whole-window Uw), which describes transmission losses in W/(m2.K). Typical triple-glazed windows lose at 0.7–1.0 W/(m2.K), compared to 5.5–6.0 W/(m2.K) for single glazing. A rule-of-thumb conclusion would be to minimize glazing. But on a south facade, this misses the benefit of solar gains.

The solar heat gain coefficient (g-value) ranges from 0.3 (reflective selective coatings) to 0.8 (clear glass). A selective coating with g-value 0.60 is standard for passive-house design because it admits strong winter gains while remaining transparent. The net thermal performance of a south window over the heating season is thus transmission loss (negative) plus solar gain (positive). A well-specified triple-glazed south window with Uw = 0.95 W/(m2.K), g-value 0.60, and good frame detail can be close to thermally neutral or even net positive in annual terms. This is exactly why passive-house design puts substantial glazing on the south and minimizes it on other orientations. North glazing has no compensating solar gain and is pure transmission loss in winter and summer.

Why is summer overheating the real constraint in a well-insulated house?

In older houses, winter heating dominates, so minimizing window area seems logical. But in a modern well-insulated passive-house-standard house, heating demand is already dramatically reduced. Summer overheating becomes the binding constraint. The risk is acute because high-performance envelopes often lack natural thermal inertia (timber-frame construction stores little heat), so excess solar gain has nowhere to be absorbed and temperatures spike quickly.

Summer sun angles are high (above 60° in July at 49°N), so horizontal shading on south facades blocks most direct sun. But on east and west facades, the sun is still low in morning and evening, so horizontal overhangs fail. West-facing windows receive direct low-angle afternoon sun at peak heat hours (14:00–18:00), exactly when interior comfort is most at risk and when the sun defeats a standard overhang. Unshaded west glazing causes the most common summer overheating complaints. This is why 40% west-facing WWR without external shading will overheat; 20% WWR with dynamic shading devices (motorized louvers) is far more robust.

What role do g-value and shading play in making higher WWR possible?

Selective coatings (g-value 0.50–0.60) reduce summer gains while retaining winter benefit; clear glass (g-value 0.70+) admits more solar heat and restricts possible WWR. A south facade might tolerate 45% WWR with selective glazing and a fixed overhang, but only 30% with clear glass. Frame fraction also matters: a window with 15–30% frame area has higher transmission loss per unit glazed area than a large window with the same total glass. One large window often outperforms two smaller windows with the same total glazed area, purely because of frame-to-glass ratio.

Fixed external shading works on south and east facades because summer sun is high enough to be intercepted. West-facing glazing requires dynamic shading (motorized louvers, retractable awnings) because afternoon sun is still low. Interior shading (roller blinds, curtains) absorbs heat inside and is far less effective at preventing summer overheating. Building codes now require external shading on facades at risk.

How should window-to-wall ratio be specified and verified?

Energy modeling and building codes require specifying WWR separately by orientation, not as a whole-building average. When a designer quotes a target WWR without orientation, ask for the per-orientation breakdown. Verification requires the full window specification: centre-pane U-value, frame U-value, whole-window U-value (Uw), g-value, and shading strategy. A specification stating only 40% WWR without U-value or g-value is incomplete and cannot be evaluated for feasibility. The most robust specification pairs each facade's WWR with its glazing spec (U-value, g-value) and shading strategy, because the three together determine whether the design will deliver comfort and energy efficiency. Specifying 40% WWR everywhere is a shortcut that usually fails.