1) What is Superficie Aeroilluminante (SAI)?
Superficie Aeroilluminante literally means “aero‑illuminating surface” and is widely used in Italian building practice to describe the effective net area of openings that provide both natural light (illuminazione) and natural air exchange (aerazione). In most municipal rules, SAI sets the minimum window area for habitable rooms, measured as a ratio against the room’s floor area or volume.
While national and regional regulations differ, the intent is consistent: size openings so that a room receives adequate daylight and can be aired without relying exclusively on mechanical systems.
2) Common code rules & ratios
Local ordinances (Regolamenti d’Igiene) typically impose ratios such as:
- WFR (Aw/Af) ≥ 1/8 or ≥ 1/10 — window area at least 12.5% or 10% of floor area.
- Operable fraction ≥ 1/2 — at least 50% of the window must open for air exchange.
- Minimum room height — e.g., 2.70 m for living spaces, affecting daylight penetration.
- Cross‑ventilation — bedrooms or living rooms often require a second opening or equivalent vent path.
3) Formulas & definitions
Let the room floor area be Af (m²). Let the total net glazed area be Aw (m²). The basic SAI condition is typically:
Aw / Af ≥ k ⟹ Aw ≥ k · Af
where k is the prescribed ratio (e.g., 0.125 or 0.10). When skylights are present, some jurisdictions allow a fraction α of their area to contribute (e.g., Aw = Aw_windows + α · A_skylights). External obstructions reduce effectiveness; introduce a shading factor s (0–1):
Aw_eff = s · (Aw_windows + α · A_skylights)
Ventilation capacity uses the effective opening area Aop (sum of free areas of operable components). For cross‑flow between two openings A1 and A2, the instantaneous flow is limited by the smaller section and the pressure difference Δp due to wind or buoyancy.
4) Worked examples
Example A — Living room 4.5 m × 4.0 m
Af = 18.0 m². Code requires WFR ≥ 1/8 (12.5%). Required Aw = 0.125 × 18.0 = 2.25 m². Choose a window 1.50 m × 1.60 m (net) ⇒ Aw = 2.40 m² → OK. Ensure at least 50% can open (Aop ≥ 1.20 m²).
Example B — Bedroom 3.6 m × 3.0 m with shading
Af = 10.8 m². Required Aw = 0.10 × 10.8 = 1.08 m². An external balcony shades 20% (s = 0.8). Provide Aw_raw = 1.35 m² → Aw_eff = 1.08 m² → OK.
Example C — Room with skylight credit
Af = 14.0 m². Side window Aww = 1.0 m². Skylight area Ask = 0.8 m² with α = 0.6 ⇒ Aw = 1.0 + 0.6 × 0.8 = 1.48 m². If k = 0.10, requirement = 1.40 m² → Pass.
5) Ventilation checks
For comfort and hygiene, openings must permit sufficient air changes. A simple approach is to verify that the operable free area Aop is at least a fixed fraction of Af (e.g., 1/20). For cross‑flow, place openings at different pressure zones. Stack effect can be used via high and low vents with a vertical separation H.
A practical target in mild climates is to provide Aop ≥ 4–5% of Af combined across operable windows, and ensure obstruction‑free flow paths.
6) Daylight checks
SAI is a coarse compliance metric. For quality, supplement with Daylight Factor (DF) or Spatial Daylight Autonomy (sDA). For quick vetting:
- Keep window heads close to the ceiling to deepen daylight penetration.
- Use light‑colored interior finishes (reflectance ≥ 0.5 for walls, ≥ 0.7 for ceilings).
- Control glare with external shading (overhangs, louvers) rather than dark tints.
7) Design details that impact SAI
- Frame and mullions: SAI often counts net glazed area; thick frames reduce Aw.
- Recess depth: Deep reveals increase shading; apply factor s.
- Balconies and adjacent buildings: Use vertical sky component or a simple obstruction factor where codes allow.
- Safety & privacy: Obscure glass does not reduce Aw, but view and sDA may suffer.
- Acoustics: In noisy streets, consider dual openings (quiet court + street) to preserve ventilation without acoustic penalty.
8) Frequently Asked Questions
Is SAI the same as daylight factor?
No. SAI is an area‑based compliance metric. Daylight Factor (DF) measures horizontal illuminance relative to outdoor overcast sky. Use both: SAI for code, DF/sDA for quality.
Do tinted or low‑E glasses reduce SAI?
SAI counts area, not transmittance. However heavy tints can make the space look dim; prefer clear low‑E with external shading.
How do I count corner windows?
Sum the net glazed areas of the two faces. If a post occupies the corner, subtract its projected area.
What about bathrooms and kitchens?
Non‑habitable rooms may have different ratios or allow mechanical ventilation alternatives. Always check your local text.