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IS 3792:1978 is the Indian Standard (BIS) for guide for estimation of thermal performance of buildings by calculation. This standard provides guidelines and mathematical methods for estimating the thermal performance of building envelopes. It details the calculation procedures for overall thermal transmittance (U-value), thermal time lag, decrement factor, and solar heat gain to aid in energy-efficient architectural design and accurate HVAC load estimation.
Provides methods for calculating the thermal performance of buildings, aiding in energy efficiency design.
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! While this standard establishes foundational thermal physics calculations, modern projects should always cross-reference baseline requirements with the Energy Conservation Building Code (ECBC).
! When referencing Table 1 for thermal conductivity (k-values), keep in mind that modern manufactured insulation and aerated concrete blocks may perform significantly better than the conservative 1978 baseline values.
! Calculations for time lag and decrement factor are crucial for designing passively cooled buildings in tropical climates to utilize thermal mass effectively.
ISO 52016-1:2017ISO (International Organization for Standardization)
MediumCurrent
Energy performance of buildings — Energy needs for heating and cooling, internal temperatures and sensible and latent heat loads — Part 1: Calculation procedures
Provides modern, dynamic hourly calculation procedures for building thermal loads, representing the current state-of-the-art for the same goal as IS 3792.
ASHRAE Handbook—Fundamentals 2021ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers), USA
HighCurrent
ASHRAE Handbook—Fundamentals
Provides the foundational physical data and calculation methodologies for heat transfer in buildings, which underpins guides like IS 3792.
ISO 13790:2008ISO (International Organization for Standardization)
HighWithdrawn
Energy performance of buildings — Calculation of energy use for space heating and cooling
Offered simplified quasi-steady-state monthly calculation methods very similar in complexity and approach to the methods in the 1978 Indian standard.
CIBSE Guide A: 2015CIBSE (Chartered Institution of Building Services Engineers), UK
HighCurrent
CIBSE Guide A: Environmental Design
Serves as a comprehensive reference guide for the principles and data needed to calculate building thermal performance and energy loads.
Key Differences
≠IS 3792 uses a simplified, steady-state or quasi-steady-state calculation method (e.g., using daily averages). Modern standards like ISO 52016-1 mandate dynamic hourly simulations that account for thermal mass effects.
≠The Indian standard has minimal or simplified consideration of thermal bridges. Modern standards require detailed calculation of thermal bridges (per ISO 10211) as they are recognized as a major path for heat transfer.
≠IS 3792 relies on basic, tabulated monthly average climate data. International practice now uses standardized hourly weather data files (e.g., EPW, TMY) for much greater accuracy.
≠Modern standards provide a more comprehensive analysis, explicitly including latent loads, detailed internal gain profiles, air infiltration, and interaction with HVAC systems, whereas IS 3792 focuses primarily on heat transfer through the building envelope.
Key Similarities
≈All standards are fundamentally based on the same principles of heat transfer: conduction, convection, and radiation.
≈The concept of Thermal Transmittance (U-value) is a core parameter for evaluating the performance of building components (walls, roofs) in both IS 3792 and modern international standards.
≈Both the Indian guide and international standards recognize solar radiation as a primary driver of heat gain and include methodologies to calculate its impact through windows and on opaque surfaces.
≈The overall objective is identical: to calculate or estimate a building's thermal performance to inform design, predict comfort, and estimate energy loads for mechanical systems.
Parameter Comparison
Parameter
IS Value
International
Source
Calculation Time Step
Daily or monthly averages (quasi-steady-state).
Hourly time steps are standard for dynamic simulations.
ISO 52016-1:2017
Solar Gain (Glazing)
Calculated using a 'Shading Coefficient' (SC), an older metric.
Calculated using 'Solar Heat Gain Coefficient' (SHGC) or 'g-value', which is a more direct measure.
ASHRAE Handbook—Fundamentals
Thermal Mass Effect
Handled crudely, if at all, through decrement factors or ignored in steady-state calculations.
Explicitly modeled in dynamic simulations using the heat capacity of materials to calculate heat storage and release over time.
ISO 52016-1:2017
Opaque Surface Heat Gain
Calculated using the 'Sol-Air Temperature' concept, which combines air temperature and solar radiation.
Sol-Air temperature is still a valid concept, but dynamic simulations model absorbed solar radiation and convective/radiative exchange at the surface directly.
ASHRAE Handbook—Fundamentals
Climatic Data Format
Tabulated design day or monthly average values from the IS handbook.
Standardized hourly data files (e.g., TMY, EPW) for a full year.
ISO 15927-4:2005
Surface Heat Transfer Coefficient
Uses simplified, constant values for internal and external surfaces.
Standardized values based on surface orientation, emissivity, and wind exposure (as per ISO 6946).
ISO 6946:2017
⚠ Verify details from original standards before use
Key Values3
Quick Reference Values
Internal surface resistance (Rsi) for horizontal heat flow0.12 m²K/W (typical)
External surface resistance (Rso) for normal exposure0.05 m²K/W (typical)
Calculation of thermal resistance (R)Thickness / Thermal Conductivity (L/k)
R = L / k — Thermal resistance of a single homogenous layer
q = U * A * (To - Ti) — Steady state heat transmission
Tables & Referenced Sections
Key Tables
Table 1 - Thermal Conductivity of Common Building Materials
Table 2 - Surface Resistances for Different Emissivities and Exposures
Table 3 - Thermal Resistance of Unventilated Air Spaces
Table 4 - Solar Absorptivity of External Surfaces
Key Clauses
Clause 3 - Thermal Properties of Building Materials
Clause 4 - Calculation of Overall Thermal Transmittance (U-Value)
Clause 5 - Time Lag and Decrement Factor
Clause 6 - Solar Heat Gain
Frequently Asked Questions3
How is the overall thermal transmittance (U-value) calculated?+
It is calculated as the reciprocal of the total thermal resistance of the building element, which includes the sum of internal surface resistance, material layer resistances, air gap resistances, and external surface resistance.
What is thermal time lag?+
It is the phase difference in time between the peak outdoor temperature and the peak indoor temperature, influenced by the building envelope's thermal mass.
What is the decrement factor?+
It is the ratio of the amplitude of the inner surface temperature variation to the amplitude of the outer surface environmental temperature variation.