Code of practice for daylighting of educational buildings

IS 7942: Scope & Key Specifications

• Scope (Clause 2.0): Defines terms related to daylighting design in buildings, focusing on natural light through windows and openings.

• Reference: Based on "Design for Daylighting" by Central Building Research Institute, Roorkee.

• Finish Reflection Factors (Clause 7.3 Notes):

  • Finish A: Ceiling white (0.7–0.8), walls off-white (0.45–0.55), floor grey (0.3)
  • Finish B: Ceiling off-white, walls off-white, floor grey
  • Finish C: Ceiling off-white, walls dark (0.25–0.3), floor grey

• Correction Factors Table (Clause 7.3, Table 3): Adjusts daylight factor based on:

  • Distance of point from window (d)
  • Side of one square in grid (a)
  • Floor area categories (10–25 m², 25–50 m², 50–100 m²)
  • Finish types (A1, B2, C3)

(Values vary with finish and floor area; see full table in IS 7942 Clause 7.3)

Summary

IS 7942 guides daylight design using reflection factors and correction tables to optimize window placement and interior finishes for natural lighting.

flowchart LR
    A[Window] --> B[Distance from window (d)]
    B --> C[Correction Factor Table]
    C --> D[Adjust Daylight Factor]
    D --> E[Interior Finish Reflection Factor]
    E --> F[Final Daylight Design]

IS 7942: Definitions and Units - Key Points

Clause 2.0: Definitions

• Defines terms used in the standard related to daylighting and window design.
• Units conform to SI system (lux for illuminance, meters for length).

Key Tables for Window Design (Clause 7.3 & 7.2.5.2)

Correction Factors Table (Excerpt)

Calculation Steps for Daylight Contribution (Clause 1.5)

1. Unobstructed Window Portion: Use Lux-grid I & II, average the results.
2. Obstructed Portion: Count dots/crosses in lux-grid II, multiply by 1.8 (if D ≤ 0.5H).
3. Combine: Add both contributions for total daylight.

For 0.5H < D ≤ 1.5H, use lux-grid II for both, reduce obstructed portion by 50%, then sum.

Units Summary

Key Specifications from IS 7942 for Visual Tasks in Education

1. Illumination Levels (Clause 4.2, Table 1)

2. Working Plane (Clause 2.15)

• Height: 0.85 m (typical), varies 0.60 to 0.90 m depending on age/task.
• Illumination is specified and measured on this plane.

3. Important Notes (Clause 5.1)

• Younger children require higher illumination due to developing vision.
• Illumination levels are a guide; factors like occupancy and uniform reflectance may reduce effective lighting.

Summary Formula for Daylight Factor (DF)

[ \text{DF} = \frac{\text{Illuminance inside}}{\text{Illuminance outside}} \times 100% ]

flowchart LR
    A[Visual Task] --> B[Recommended Illumination (lux)]
    B --> C[Measured on Working Plane @ 0.85m]
    C --> D[Adjust for Age & Task Complexity]
    D --> E[Special Lighting for Defective Vision]

Use these values as a baseline for designing school lighting to ensure adequate visual comfort and performance.

IS 7942: Standards of Illumination (Illuminance) for Educational Buildings

Key Definitions:

• Lux (lx): Illuminance produced by 1 lumen/m² (Clause 2.12).
• Daylight Factor: Ratio (%) of indoor illuminance to outdoor illuminance under overcast sky.

Recommended Illumination Levels (Table 1, Clause 4.2.1):

Important Notes:

• Illumination levels apply to working planes, walls, and chalkboards.
• Visibility depends on task size, contrast, and detail (Clause 3.2).
• Suggested values do not account for occupancy or uniform reflectance reductions; these can initially be neglected.

Formula for Illuminance:

[ \text{Illuminance (lx)} = \frac{\text{Luminous Flux (lumens)}}{\text{Area (m}^2)} ]

Summary:

• Design lighting to meet minimum lux values per task.
• Maintain daylight factor within recommended ranges for natural lighting adequacy.
• Provide special lighting for visually impaired students.

flowchart TD
    A[Visual Task] --> B[Required Illuminance (lux)]
    B --> C[Daylight Factor %]
    C --> D[Design Lighting System]
    D --> E[Ensure Visibility & Comfort]

This ensures compliance with IS 7942 for effective educational lighting.

IS 7942: Daylighting Principles & Window Design Key Points

1. Fenestration Percentage & Daylight Factor (DF):

• Window area as % of floor area determines daylight factor at center/rear of room (Fig. 3 & 4).
• Four window types considered:
  • a) Open aperture,
  • b) Glazed with 3 mm glass,
  • c) Wooden window frame,
  • d) Metallic window frame (least light cut-off).

2. Window Sill Height:

• Optimal sill height: 75 to 105 cm for balanced vertical & horizontal illumination.

3. Reflection Factors Assumed:

• Walls: 45-50%
• Ceiling: 70-75%
• Floor: 25-30%

4. Design Recommendations:

• Split window area into 2-3 parts for uniform daylight distribution.
• Provide louvers/overhangs to avoid direct sunlight glare.
• Clean glazing regularly to maintain light transmission.

Simplified Formula for Fenestration Percentage (Approximate from figures):

Diagram: Window Placement & Daylight Factor Concept

flowchart LR
    A[Window on Longer Wall] --> B[Higher Daylight Factor at Centre]
    C[Window on Shorter Wall] --> D[Lower Daylight Factor at Centre]
    E[Split Window Area] --> F[Uniform Daylight Distribution]
    G[Sill Height 75-105 cm] --> H[Optimal Illumination]

Summary: Use IS 7942 Fig. 3 & 4 to size windows based on room area and desired daylight factor, maintain sill height 75-105 cm, consider frame type impact, and provide shading devices to optimize daylighting.

IS 7942 - General Instructions for Daylighting

Though IS 7942 references the CBRI publication and focuses on educational buildings, here are key general guidelines and formulas typically used in daylighting design:

Key Formulas:

• Daylight Factor (DF):
  [ DF = \frac{E_i}{E_o} \times 100% ]
  Where:

  • (E_i) = Illuminance inside the room (lux)
  • (E_o) = Exterior horizontal illuminance under overcast sky (lux)

• Sky Component (SC), Externally Reflected Component (ERC), Internally Reflected Component (IRC):
  [ DF = SC + ERC + IRC ]

Typical Specifications:

• Minimum Daylight Factor:

  • Classrooms: 1.5% - 3%
  • Libraries: 3% - 5%
  • Laboratories: 2% - 4%

• Window-to-Floor Area Ratio:
  Usually between 15% to 25% to ensure adequate daylight.

• Window Head Height:
  Should be high enough (typically 2.0 m or more) to allow daylight penetration.

General Instructions:

• Use north-facing windows in the northern hemisphere to reduce glare.
• Avoid direct sunlight causing glare; use shading devices if necessary.
• Use light-colored interior surfaces to maximize internal reflection.

flowchart LR
    A[Exterior Illuminance (Eo)] --> B[Window Opening]
    B --> C[Interior Illuminance (Ei)]
    C --> D[Calculate Daylight Factor (DF)]
    D --> E[Evaluate Daylighting Adequacy]

For detailed tables and design charts, refer to CBRI's "Design for Daylighting" or IS 1947 (Code of Practice for Daylighting).

IS 7942: Window Design & External Obstructions - Key Points

1. External Obstruction Influence (Clause 6.6.1 & 6.6.2)

• External obstructions (trees, buildings) significantly reduce interior daylight.
• Example: A 6 m tall tree 6 m away can reduce daylight factor by ~50%; at 15 m away, reduction ~10%.
• Obstruction impact depends on orientation: Rows parallel to window are more obstructive than perpendicular.

2. Window Design Parameters (Clause 2.75 & Fig.7,8)

• Ceiling height: 2.75 m
• Wall + louver thickness: 0.6 m (60 cm)
• Ground reflection factor: 0.25
• Louvers: Used to reduce direct sunlight incursion.
• No external obstruction assumed for base daylight factor calculations.

3. Projection of Obstruction on Lux-Grid (Fig. 8A & 8B)

• Obstruction is projected onto the lux-grid to estimate daylight reduction.
• Unobstructed window portion vs. projected obstruction area helps calculate effective daylight.

Simplified Daylight Factor (DF) Estimation (No Obstruction)

[ DF = \frac{T \times R \times W}{A} ]

Where:

• ( T ) = Transmittance of window
• ( R ) = Reflection factor (ground 0.25)
• ( W ) = Window area (m²)
• ( A ) = Room floor area (m²)

Summary Diagram: Impact of Obstruction on Window Daylight

graph LR
    W[Window]
    O[External Obstruction]
    D[Daylight Reduction]
    W -->|Unobstructed Portion| D
    O -->|Projected Obstruction| D

Note: For detailed daylight factor calculations, always consider local obstruction geometry and use lux-grid projection as per IS 7942 Fig. 7 & 8.

IS 7942: Window Design with External Obstruction — Key Points

Context from IS 7942 (Clause 7.2.6.5 & Appendix A)

• Appendix A provides a worked example to calculate total illumination inside a room due to two windows with external obstructions.
• Uses Lux-grid I and II methods to evaluate daylight reduction caused by obstructions.
• External obstructions are projected onto lux-grids to identify unobstructed window portions contributing to daylight.
• Fig. 7 & 8 illustrate typical projections of obstructions on lux-grids.

Important Specifications (Clause 2.75 & 6.6.2)

• Ceiling height: 2.75 m
• Wall + louver thickness: 0.60 m
• Ground reflection factor: 0.25
• Louvers used to reduce sunlight incursion.
• External obstructions complicate daylight calculations; each case must be assessed individually.

Key Formula for Daylight Factor (DF) with External Obstruction

[ DF = \frac{E_i}{E_o} \times 100 ]

Where:

• (E_i) = Illuminance inside the room (lux)
• (E_o) = Outdoor illuminance on a horizontal surface (lux)

Adjustments for obstruction:

• Calculate unobstructed window area from lux-grid projections.
• Use obstruction angle to reduce effective window area contributing to daylight.

Typical Calculation Steps:

1. Project obstruction on Lux-grid I & II to find shaded portions.
2. Calculate effective window area = Total window area - Projected obstruction area.
3. Calculate daylight factor using effective window area and correction factors (louvers, wall thickness, ground reflection).
4. Sum contributions from multiple windows.

Visualization of Obstruction Projection

flowchart LR
    A[Window] --> B[Lux-grid I]
    A --> C[Lux-grid II]
    B --> D[Projected Obstruction on Lux-grid I]
    C --> E[Projected Obstruction on Lux-grid II]
    D & E --> F[Calculate Unobstructed Window Area]
    F --> G[Determine Daylight Factor]

For detailed numeric example, refer to Appendix A of IS