Guide for heat insulation of non-industrial buildings

IS 3792: Scope - Key Formulas, Tables & Specifications

Scope:
IS 3792 guides heat insulation for non-industrial buildings, focusing on thermal performance of walls, roofs, shading devices, and surface finishes across climatic zones.

Important Tables & Data

1. Correction Factors for Thermal Performance Index (TPI)

2. Thermal Performance of Walls

3. Thermal Performance of Sloped Roofs

4. Thermal Performance of Shading Devices

IS 3792 - Definitions of Terms and Symbols (Clause 2.1)

IS 3792 – Temperature-Cum-Humidity Zones in India

Zones Classification (Clause 3.1)

Key Notes:

• Altitude plays a role in classification.
• Zones guide thermal insulation design and building material selection.
• Refer Appendix B for detailed town listings per zone.
• Shade Factor values (Clause 3.1.3) relate to heat gain control (see Table 9 in the code).

Example: Defining Hot and Arid Zone

• ( T_{max} \geq 38^\circ C )
• RH ≤ 40%
• Altitude ≤ 500 m

flowchart TD
    A[India] --> B{Temperature-Cum-Humidity Zones}
    B --> C[Hot and Arid]
    B --> D[Hot and Humid]
    B --> E[Warm and Humid]
    B --> F[Cold]
    C --> |Tmax ≥ 38°C, RH ≤ 40%, Alt ≤ 500m| G[Agra, Jaipur, Bikaner]
    D --> |Tmax ≥ 26°C, RH ≥ 70%, Alt ≤ 100m| H[Ahmedabad,

IS 3792: General Recommendations for Heat Insulation

Key Thermal Performance Standards (Table 2)

• U-value: Overall thermal transmittance (lower is better insulation)
• TPI: Thermal Performance Index (higher means better thermal comfort)
• Thermal Time Constant (T): Time lag in hours for heat transfer
• Thermal Damping (D): Reduction in temperature amplitude (%)

Relationship between Thermal Time Constant & Thermal Damping

• Refer to Fig. 1 in IS 3792 for limiting curves showing the relationship between T (hours) and D (%).

Important Formulas

• Thermal Transmittance (U-value):

[ U = \frac{1}{R_{total}} = \frac{1}{R_{si} + \sum \frac{d_i}{k_i} + R_{so}} ]

Where:

• (R_{si}), (R_{so}) = inside and outside surface resistances

• (d_i) = thickness of layer i

• (k_i) = thermal conductivity of layer i

• Thermal Performance Index (TPI):
  A function of U-value, thermal mass, and shading; refer IS 3792 for detailed calculation.

Thermal Properties of Materials (Appendix C)

IS 3792: Methods of Heat Insulation - Key Formulas, Tables & Specs

1. Thermal Conductivity (k) & Specific Heat Capacity (c)

• Thermal conductivity (k) in W/(m·K) and specific heat capacity (kJ/(kg·K)) are key for insulation design.
• Refer to Appendix C for typical values at 50°C:

2. Thermal Performance Index (TPI) and Overall Thermal Transmittance (U)

• From Table 2 (Clause 4.1) for roofs and walls:

• U-value measures heat transfer; lower is better insulation.
• TPI indicates thermal performance relative to heat gain.
• Thermal Time Constant (T) and Damping (D) relate to heat delay and reduction.

3. Basic Formula for Overall Heat Transfer Coefficient (U)

[ U = \frac{1}{R_{total}} = \frac{1}{R_i + \sum \

IS 3792: Thermal Conductivity Values of Materials

Key Points from Appendix C (Clause 6.1)

Notes:

• Thermal conductivity values are typical and approximate.
• Values determined by Guarded Hot Plate Method and ASTM Heat Flow Method.
• Always verify with manufacturer or testing labs.

Important Formula

Overall Thermal Transmittance (U-value):

[ U = \frac{1}{R_{total}} = \frac{1}{\sum \frac{d_i}{k_i} + R_{surface}} ]

• (d_i) = thickness of layer i (m)
• (k_i) = thermal conductivity of layer i (W/mK)
• (R_{surface}) = surface resistance (m²K/W)

Thermal Performance Parameters (Clause 8.1)

IS 3792 Key Points on Thermal Transmittance (U) and Thermal Time Constant (Q/U):

1. Definitions:

• Thermal Transmittance (U): Heat flow rate per unit area per °C difference, W/(m²·K).
• Thermal Time Constant (Q/U): Time (hours) for the wall to respond to temperature changes, ratio of heat capacity to U.
• Damping (D): Percentage reduction of temperature amplitude inside.
• Thermal Performance Index (TPI): Overall thermal efficiency percentage.

2. Calculation of Thermal Transmittance (U):

• Use thermal conductivity (k) values from Appendix C for materials.

• For multilayer walls:

  [ U = \frac{1}{\sum \frac{d_i}{k_i} + R_{surface}} ]

  where:

  • (d_i) = thickness of layer i (m)
  • (k_i) = thermal conductivity of layer i (W/m·K)
  • (R_{surface}) = surface resistances (internal + external)

3. Thermal Time Constant (Q/U):

• (Q) = heat capacity per unit area (kJ/m²·K), sum of ( \rho c d ) for each layer:

  [ Q = \sum \rho_i c_i d_i ]

  where:

  • (\rho_i) = density (kg/m³)
  • (c_i) = specific heat capacity (kJ/kg·K)
  • (d_i) = thickness (m)

• Time constant:

  [ \text{Thermal Time Constant} = \frac{Q}{U} ]

4. Reference Tables & Figures:

• Tables 3, 4, 5: Provide typical U, D, TPI, and Q/U values for common wall types.
• Figures 2, 3, 4: Illustrate thermal performance graphically.
• Appendix D: Detailed worked examples for calculations.

Summary Table (Example):

Key Thermal Performance Parameters (IS 3792 Clause 8.1)

Thermal Performance Standards (Table 2)

Example Thermal Performance of Walls (Table 3)

Correction Factors for Thermal Performance Index (TPI) (Table 6)

| Characteristic

IS 3792 - Values of Shade Factor

1. Shade Factor Correction Factors (Clause 8.1.1, Table 6)

2. Thermal Performance of Shading Devices (Table 7)

IS 3792 - General Principles of Heat Transfer: Key Formulas & Tables

Key Definitions & Formulas (Clause 2.1)

Thermal Resistance in Composite Walls

For multiple layers:

[ R_T = R_{si} + \sum \frac{L_i}{k_i} + R_{so} ]

• (R_{si}), (R_{so}): inside and outside surface resistances
• (L_i

IS 3792: Thermal Properties of Building & Insulating Materials

Key Thermal Properties (at 50°C) — Appendix C

Thermal conductivity values determined by Guarded Hot Plate and ASTM Heat Flow methods.

Important Formulas

• Overall Thermal Transmittance (U-value):

[ U = \frac{1}{\sum \frac{d_i}{k_i}} ]

where (d_i) = thickness of layer i (m), (k_i) = thermal conductivity of layer i (W/mK).

• Thermal Time Constant (T):

[ T = \frac{Q}{U} ]

where (Q) = thermal capacity per unit area (kJ/m²K), (U) = overall thermal transmittance (W/m²K).

• Thermal Damping (D) and Thermal Performance Index (TPI):
  Refer to Fig. 1 & Table 2 for limiting curves and max values per climatic zone.

Thermal Performance Standards (Excerpt from Table 2)

| Component | Zone | Max U (W/m²K) | Max TPI (%) | Min Thermal Time Constant (h) | Min Damping

IS 3792: Thermal Transmittance (U) & Thermal Time Constant (Q/U) Calculation

Key Formulas:

• Thermal Transmittance (U):
  [ U = \frac{1}{\sum R_i} = \frac{1}{R_1 + R_2 + \cdots + R_n} ]
  where (R_i = \frac{d_i}{k_i}) (thickness/thermal conductivity of each layer in m²K/W)

• Thermal Time Constant:
  [ \tau = \frac{Q}{U} ]
  where

  • (Q) = Thermal capacity per unit area (kJ/m²K)
  • (U) = Thermal transmittance (W/m²K)
  • (\tau) in hours

Definitions:

• Damping (D): Percentage reduction of temperature amplitude inside compared to outside.
• Thermal Performance Index (TPI): Percentage indicator of wall's thermal efficiency.

Typical Values (from Tables 3, 4 & 5):

Calculation Steps (Appendix D):

1. Calculate individual resistances (R_i = \frac{d_i}{k_i}).
2. Sum resistances to get total (R_{total}).
3. Compute (U = 1/R_{total}).
4. Determine thermal capacity (Q = \sum (\rho_i c_i d_i)) (density × specific heat × thickness).
5. Calculate time constant (\tau = Q/U).

flowchart TD
    A[Material Layers] --> B[Calculate