SANS 10400-XA is the deemed-to-satisfy energy rule under the National Building Regulations, and SANS 204 is the performance framework behind it. Fenestration is where most projects first fall outside the prescriptive route, because glass and aluminium are the weakest thermal link in an envelope.
Two routes: prescriptive or rational design
SANS 10400-XA gives you a prescriptive route and a performance route. The prescriptive route limits fenestration area to a percentage of the net floor area, typically 15%, unless a rational design or performance calculation is performed by a competent person. Cross the limit and you do not lose compliance, you change the process: the design must be justified by calculation rather than by the table.
Why glass drives the whole calculation
Fenestration affects a building twice: it loses heat by conduction through the glass and the frame, and it gains or loses heat through solar radiation. U-value answers the first question and SHGC answers the second, and a specification that fixes only one of them will still fail a rational design. Standard single 4 mm clear float in a standard aluminium frame is the worst of both: a U-value of approximately 5.8 W/m²·K and uncontrolled solar gain.
Climatic zones and what each one asks of the glazing
| Climatic zone | Where it applies | What it demands of the glazing |
|---|---|---|
| Zone 1 — Cold interior | Highveld: Johannesburg, Pretoria, the Free State and the eastern interior | Favour solar gain in winter but control summer overheating: Low-E glass or a DGU, with SHGC tuned to let winter sun in |
| Zone 2 — Temperate coastal | KwaZulu-Natal coast: Durban, Umhlanga, Richards Bay | High humidity and salt exposure: corrosion-resistant framing, controlled solar gain, and laminated panes chosen for storm and impact performance |
| Zone 4 — Mediterranean | Western Cape: Cape Town, the Winelands and the Garden Route | Strong winter wind and driving rain: the wind-load and water-penetration class of the system matters as much as the glass, with SHGC kept low against summer sun |
Climatic zones per SANS 204, as referenced by SANS 10400-XA.
The thermal specification ladder
| Assembly | U-value | When to specify it |
|---|---|---|
| 4 mm clear float in standard aluminium | ≈ 5.8 W/m²·K | Internal partitions only. Poor for an external envelope. |
| Toughened single glazing in thermally broken aluminium | ≈ 4.5–5.0 W/m²·K | Renovations where the frame is the dominant heat path and the budget is tight. |
| Single Low-E glazing | ≈ 3.5–4.0 W/m²·K | Existing single-glazed openings that cannot take a double-glazed rebate. |
| Double glazed unit, clear | ≈ 2.7–2.9 W/m²·K | The practical entry point to a compliant envelope. |
| Double glazed unit with Low-E and argon | < 2.8 W/m²·K, down to ≈ 1.6 W/m²·K | New residential and commercial envelopes chasing XA compliance without a rational design. |
The same opening in Umhlanga and in Johannesburg
Take a 2.4 m x 1.2 m sliding door, roughly 2.88 m² of glass. On the KwaZulu-Natal coast it faces a temperate, humid climate with salt in the air and a real storm and impact brief: thermally broken aluminium, Low-E glazing or a DGU, and laminated or toughened laminated panes chosen for coastal exposure. On the Highveld the same opening faces a cold interior climate with strong winter sun: the priority shifts to keeping winter solar gain while controlling summer overheating, which is a Low-E specification with a different SHGC target, and the wind-load category is lower than the coastal elevation. Same size, same frame family, two different glass schedules.
Air infiltration classification (AAAMSA)
| Class | Air leakage | Where it is normally required |
|---|---|---|
| A1 | Lightest allowable leakage | Internal and fully sheltered openings |
| A2 | Moderate leakage | Standard external residential openings |
| A3 | Tightest of the three | High-rise, coastal and exposed elevations where wind-driven loss and water ingress drive the design |
What actually moves the numbers
- 01Thermally broken frames, which separate the inner and outer aluminium so the frame stops short-circuiting the glass.
- 02Low-E coatings, which reduce radiant heat transfer without blocking daylight.
- 03Double glazed units, which add a still air or gas layer and roughly halve the U-value of the assembly.
- 04Correct SHGC for the zone, which is the difference between a comfortable room and a room that needs air conditioning in October.
- 05Sealed and correctly drained frames, because an air leak defeats every performance figure the assembly was tested to.