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Tertiary Venetian blinds: aluminium or wood adjustable slats

At a screen workstation exposed to the south, class 3 solar performance requires a g_tot factor below 0.15.

Class 3 of NF EN 14501 (g_tot < 0.15) cannot be achieved with a roller blind on a south-facing facade: only the venetian blind with adjustable slats modulates transmission from 0 to 90 % over 180°, compared with a fixed 15 to 40 % for a screen. This modulation range explains the preference for the venetian blind on renovated commercial floors with a south-facing facade. Combined with a BMS protocol (DALI, KNX, BACnet), the 24V motorisation reduces air conditioning consumption by 10 to 18 % on a south-west facade. Kytom sizes each installation according to orientation, the g coefficient of the existing glazing and the intended aesthetic register.

The NF EN 14501 standard assesses solar protection on two axes: thermal comfort (g_tot solar factor of the glazing+blind assembly) and visual comfort (glare, contact with the outside, quality of view, colour rendering). For a screen workstation on a south-facing facade, a minimum of class 3 is required, i.e. g_tot between 0.10 and 0.15. A 25 mm light-lacquered aluminium venetian blind, with slats oriented at 45° behind a 4/16/4 double glazing (g = 0.58), reaches a g_tot of 0.12 to 0.14, compliant with class 3 (calculation according to the NF EN 13363-1 method).

Adjustable slats modulate light transmission according to three reference positions:

0 to 5 % slats closed (blackout and privacy)
30 to 45 % slats at 45° (anti-glare with view retained)
70 to 90 % horizontal slats (maximum light input)

Over 8 to 10 m of floor depth, the 45° orientation preserves 60 % of natural light input and limits artificial lighting on a south-facing facade between 10am and 4pm, which our lux-meter readings on the La Défense floors confirm. The commercial buildings decree and its OPERAT scheme (ADEME, 2023 data) include automated solar management among the levers for energy savings towards the 2030 milestones.

Our reading differs from common practice on one precise point: the venetian blind is not solar protection, it is an NF EN 14501 class modulator. The industry doxa treats the blind as glare equipment and chooses a screen or venetian blind on aesthetic criteria. For a thermal engineer, the only criterion that matters is the position of the g_tot in the class 1 to 4 grid according to orientation and depth. On the facades we have instrumented in recent years, the 5 % screen reaches g_tot 0.18 to 0.22 (class 2), insufficient on pure south at a screen workstation: the adjustable venetian blind is the only type that covers class 1 to class 3 with a single piece of equipment.

When the venetian blind is not the right answer. Below 1.20 m of window height or on a pure north-facing facade, the motorised venetian blind is not justified: the return on investment exceeds 12 years. For arched windows or very high heights (> 3.20 m), prefer a screen or exterior protection. For residential-like use (management offices occupied occasionally), manual control remains more cost-effective than BMS motorisation: control integration is unnecessary and the wiring extra cost is not recovered.

What budget should be planned to equip an 800 m² commercial floor with office venetian blinds?

For an 800 m² floor equipped with 60 to 80 windows of 1.40 to 2.40 m, the budget is between €12,000 and €22,000 excl. VAT for manual 25 mm lacquered aluminium venetian blinds, i.e. €90 to €140/m² covered. The DALI motorised version connected to the BMS comes out between €28,000 and €42,000 excl. VAT, low-voltage wiring included. These ranges are market benchmarks that we then adjust to the type of windows, the chosen finish level and the building’s constraints.