Guide for heat insulation of non-industrial buildings
1978 Edition

Overview of Scope

This section defines the scope of thermal insulation for non-industrial buildings, emphasizing optimization of walls, roofs, shading elements, and surface finishes according to India's diverse climate zones.

Essential Data Tables

1. Thermal Performance Correction Factors

   Element	Hot Dry	Hot Humid	Warm Humid
   Roof	1.00	0.95	0.92
   Wall (W)	1.00	0.85	0.75
   Wall Orientation (N)	0.45	0.38	0.34
   External Roof Finish (Dark)	1.00	0.95	0.92

2. Wall Thermal Characteristics

   Wall Composition	U-Value (W/m²K)	Thermal Time Constant (h)	Damping (%)	TPI (%)
   Thin plaster + moderate brick layer + plaster	3.00	7.72	58.9	164
   Thin plaster + thick brick layer + plaster	1.35	67.13	90.9	61

3. Sloped Roof Thermal Properties

   Roof Type	U-Value (W/m²K)	Thermal Time Constant (h)	Damping (%)	TPI
   Thin asbestos sheet	5.47	0.015	12	186
   Thatch with bamboo	1.69	18.0	20	102

4. Performance of Shading Elements

   Shading Type	U-Value (W/m²K)	Shade Factor
   Plain glass (3 mm)	5.23	1.00

Definitions of Key Terms and Symbols (Clause 2.1)

Classification of Temperature-Humidity Zones in India (Clause 3.1)

Important Notes:

• Altitude influences zone classification.
• Zones guide selection of insulation levels and materials.
• Refer Appendix B for detailed town listings.
• Shade factor values (Clause 3.1.3) relate to solar heat gain control.

Example: Hot and Arid Zone Criteria

• Max temperature ≥ 38°C
• Relative humidity ≤ 40%
• Altitude ≤ 500 m

Thermal Performance Benchmarks (Table 2)

• U-value indicates overall heat transmission; lower values represent better insulation.
• Thermal Performance Index (TPI) reflects effectiveness in reducing heat gain.
• Thermal Time Constant represents delay in heat passage.
• Thermal Damping measures the reduction in temperature fluctuations.

Relationship Between Time Constant and Damping

• Refer to Figure 1 in the standard for curves linking thermal time constant and damping.

Key Equations

• Thermal Transmittance (U):

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

Where:

• (R_{si}) and (R_{so}) are internal and external surface resistances respectively

• (d_i) is the thickness of the i-th layer

• (k_i) is the thermal conductivity of the i-th layer

• Thermal Performance Index (TPI): Function of U-value, thermal mass, and shading; see detailed calculation in the code.

Thermal Characteristics of Materials (Appendix C)

Thermal Insulation Practices (IS 3792)

1. Thermal Conductivity and Heat Capacity

• Thermal conductivity (k) and specific heat (c) are fundamental parameters for insulation design.
• Appendix C lists typical values at 50°C:

2. Thermal Performance Index (TPI) and U-Value Limits

• From Table 2 (Clause 4.1):

• Lower U-values correspond to better insulation.
• TPI, time constant, and damping quantify thermal comfort and heat flow delay.

3. Basic Formula for U-value

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

Where (R_i) and (R_o) are inside and outside surface resistances.

Thermal Conductivity Information (Appendix C, Clause 6.1)

Remarks:

• Values are average approximations.
• Determined using Guarded Hot Plate and ASTM heat flow methods.
• Confirm with material suppliers for precise data.

Formula for Overall Heat Transfer Coefficient (U-value):

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

Where (d_i) is thickness and (k_i) is conductivity of layer i; (R_{surface}) includes surface resistances.

Definitions and Computations for Thermal Transmittance and Time Constant

1. Thermal Transmittance (U): Heat flow rate per unit area and temperature difference, expressed in W/(m²·K).

2. Thermal Time Constant (Q/U): Duration for temperature changes to propagate through a wall, given by dividing heat capacity per unit area by U-value.

3. Thermal Damping (D): Percentage reduction in temperature fluctuation amplitude indoors.

Formulae:

• Calculate U-value for multilayer walls using:

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

where:

• (d_i) = thickness of layer i

• (k_i) = thermal conductivity of layer i

• (R_{surface}) = sum of internal and external surface resistances

• Compute heat capacity per unit area:

[ Q = \sum \rho_i c_i d_i ]

where:

• (\rho_i) = density of layer i

• (c_i) = specific heat capacity of layer i

• (d_i) = thickness of layer i

• Thermal time constant:

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

expressed in hours.

Reference Tables and Illustrations:

• Tables 3, 4, 5 provide typical U, D, TPI, and time constant values for common wall constructions.
• Figures 2, 3, 4 depict thermal performance trends.
• Appendix D includes detailed calculation examples.

Key Thermal Performance Metrics (Clause 8.1)

Thermal Performance Requirements (Excerpt from Table 2)

Sample Wall Thermal Performance (Table 3)

Correction Factors for Thermal Performance Index (Table 6)

Shade Factor Correction Coefficients (Clause 8.1.1, Table 6)

Thermal Characteristics of Shading Devices (Table 7)

Essential Definitions and Equations (Clause 2.1)

Thermal Resistance in Multi-Layer Walls

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

Where (R_{si}) and (R_{so}) are internal and external surface resistances respectively.

Thermal Attributes at 50°C (Appendix C)

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

Key Equations

• Overall Thermal Transmittance (U-value):

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

where (d_i) and (k_i) are thickness and thermal conductivity of each layer.

• Thermal Time Constant (T):

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

where (Q) is heat capacity per unit area and (U) is overall transmittance.

• Thermal Damping (D) and Thermal Performance Index (TPI): Refer to Figures and Tables for limits based on climate zones.

Thermal Performance Standards (Excerpt)

Calculation Guidelines for Thermal Transmittance (U) and Thermal Time Constant

• Thermal Transmittance (U):

[ U = \frac{1}{\sum R_i} = \frac{1}{R_1 + R_2 + ... + R_n} ]

where each (R_i = \frac{d_i}{k_i}) (thermal resistance of layer i).

• Thermal Time Constant (τ):

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

where:

• (Q) = heat capacity per unit area (kJ/m²K)

• (U) = thermal transmittance (W/m²K)

• Definitions:

  • Thermal Damping (D): Percentage reduction in indoor temperature variation.
  • Thermal Performance Index (TPI): Percentage measure of overall thermal efficiency.

Typical Values from Tables:

Stepwise Calculation Procedure:

1. Calculate resistances (R_i) for each material layer.
2. Sum resistances to find total (R_{total}).
3. Compute U-value as reciprocal of total resistance.
4. Determine heat capacity (Q) by summing density × specific heat × thickness for all layers.
5. Calculate thermal time constant as ratio of Q to U.

flowchart TD
    A[Material Layers] --> B[Calculate Resistances]
    B --> C[Sum Resistances]
    C --> D[Calculate U-value]
    D --> E[Calculate Heat Capacity Q]
    E --> F[Compute Thermal Time Constant τ = Q/U]