Last reviewed 16 Sept 2026 · Facts as of 16 Sept 2026 · 11 min read
Heat transfer in buildings
| Mode | Description | Building example |
|---|---|---|
| Conduction | Heat flow through a material from hot to cold side by molecular contact | Heat through walls and roofs |
| Convection | Heat carried by moving fluid (air) | Air movement at wall surfaces, ventilation, air leakage |
| Radiation | Heat transfer by electromagnetic waves without a medium | Solar radiation on roofs and windows; radiation across cavities |
FormulaConduction and thermal resistance
Fourier's law:
- = thermal conductivity (W/m·K) — lower means better insulation.
- Thermal resistance of a layer: (m²·K/W).
- Total resistance of a wall: (surface resistances inside, typically about 0.13, and outside, about 0.04 m²·K/W).
- Thermal transmittance (U-value): (W/m²·K) — lower U means less heat flow.
- Heat flow through an element:
Typical thermal conductivities (approximate)
| Material | (W/m·K) |
|---|---|
| Still air | About 0.025 |
| Polyurethane foam (PUF) | About 0.022–0.028 |
| Extruded polystyrene (XPS) | About 0.030–0.035 |
| Expanded polystyrene (EPS), mineral wool | About 0.033–0.045 |
| Cork board | About 0.04–0.05 |
| Timber | About 0.1–0.2 |
| Autoclaved aerated concrete (AAC) | About 0.12–0.20 |
| Burnt clay brick masonry | About 0.6–1.0 |
| Glass | About 1.0 |
| Dense concrete | About 1.4–1.8 |
| Steel | About 50 |
| Aluminium | About 200 |
Why insulators work: most insulating materials trap still air (or gas) in small cells or between fibres, reducing conduction and convection.
Thermal mass and time lag
Heavy materials (thick masonry, concrete, earth) store heat and delay and dampen temperature swings (time lag and decrement factor) — useful in hot-dry climates with large day–night variations. Insulation reduces heat flow; thermal mass shifts it in time; both are used in climate-responsive design.
Thermal insulating materials
| Material | Description | Uses / remarks |
|---|---|---|
| Mineral wool — glass wool, rock (stone) wool, slag wool | Fine fibres of glass or rock bonded into mats, rolls and boards | Roofs, walls, ceilings, HVAC ducts, pipes; non-combustible, also good sound absorbers |
| Expanded polystyrene (EPS) | White beaded foam boards | Roof and wall insulation, sandwich panel cores, geofoam; combustible (use flame-retardant grades, protect with covering) |
| Extruded polystyrene (XPS) | Closed-cell rigid boards with low water absorption | Inverted/under-tile roof insulation, below-slab and foundation insulation |
| Polyurethane / polyisocyanurate foam (PUF/PIR) | Rigid foams or sprayed foam with very low conductivity | Cold storages, sandwich panels, roof insulation, pipes |
| Cellular (foam) glass | Rigid closed-cell glass | Non-combustible, vapour-tight — flat roofs, industrial insulation |
| Expanded perlite and exfoliated vermiculite | Lightweight expanded minerals | Loose fill, insulating plasters and concretes, fire protection |
| Cork | Natural bark of cork oak — boards and granules | Floors, walls, acoustic and thermal uses |
| Wood-wool and wood-fibre boards | Wood fibres/shavings bonded with cement or resin | Roof decks, ceilings, acoustic panels |
| AAC blocks, cellular lightweight concrete, lightweight aggregate concrete | Porous masonry/concrete | Insulating walls and roof screeds |
| Reflective insulation (radiant barriers) | Aluminium foils/foil-faced bubble films | Reduce radiant heat — under roofs with an air gap |
| Aerogel blankets, vacuum insulated panels | Very low conductivity | Thin, high-performance insulation (costly) |
| Natural fibres | Coir, jute, straw, sheep wool, hemp | Low-embodied-energy insulation |
| Double/triple glazing, low-E glass | Gas-filled gaps and coatings | Windows and facades |
Desirable properties: low thermal conductivity, low density, low water absorption (moisture greatly increases conductivity), durability, fire resistance (or protection), vermin and fungus resistance, dimensional stability, ease of installation, low cost and low embodied energy.
Methods of thermal insulation
Roofs (largest heat gain in single-storey buildings in hot climates)
- Over-deck insulation — insulation boards over the RCC slab protected by screed, tiles or waterproofing (with XPS for inverted roofs).
- Under-deck insulation — insulation fixed below the slab or as a false ceiling with an air gap.
- Reflective (cool) roofs — white or high-reflectance coatings and tiles reducing solar absorption.
- Roof shading — pergolas, solar panels, green roofs, broken china mosaic (traditional), water on roofs, lightweight insulating screeds.
Walls
- Cavity walls — air gap (with or without insulation) between two leaves.
- External insulation and finish systems (EIFS) — insulation boards fixed outside and plastered — reduce thermal bridges.
- Internal insulation — insulation with gypsum board lining (quicker to install).
- Insulating masonry — AAC blocks, hollow and perforated clay blocks, insulated concrete forms (ICF).
Windows and openings
Double glazing, low-E and solar-control glass, external shading (chajjas, louvers, fins — most effective in reducing solar gain), weather-stripping, appropriate window-to-wall ratio and orientation.
Floors
Insulation below ground-floor slabs or over basements; insulation of floors over unheated/open spaces.
Vapour barriers, condensation and thermal bridges
- Condensation occurs when warm moist air meets a surface (or layer within the wall) below its dew point — can cause mould and damage insulation.
- Vapour barriers/retarders (polyethylene sheets, foil facings) are placed on the warm (humid) side of insulation in cold climates (location must suit climate).
- Thermal bridges — areas of higher heat flow (RCC beams and columns in masonry walls, metal frames, lintels) — reduced by continuous insulation and thermal breaks.
Energy codes in India
- Energy Conservation Building Code (ECBC) — for commercial buildings (Bureau of Energy Efficiency; ECBC 2017 version), specifying envelope U-values, glazing properties, lighting and HVAC efficiency.
- Eco-Niwas Samhita (ECBC-Residential, 2018) — envelope standards for residential buildings (e.g. limits on residential envelope heat transmittance).
- These codes have been progressively expanded under amendments to the Energy Conservation Act; the latest notified versions should be consulted.
- Green building rating systems (GRIHA, IGBC) also reward insulation and passive design.
Sound in buildings
Basics
- Sound — pressure waves in air; frequency (Hz) — audible range about 20–20 000 Hz; speech mainly about 250–4000 Hz.
- Sound level in decibels (dB) — logarithmic scale (see Noise Pollution).
- Airborne sound — transmitted through air (speech, music, traffic noise) and through walls by vibration.
- Impact (structure-borne) sound — generated by direct impact on the structure (footsteps, dropped objects, machinery) and transmitted through floors and walls.
Sound absorption versus sound insulation
| Sound absorption | Sound insulation |
|---|---|
| Reduces reflected sound (echoes, reverberation) within a room | Reduces sound transmission between spaces or from outside |
| Uses soft, porous, light materials | Uses heavy, dense, airtight constructions |
| Improves acoustic quality (speech intelligibility, music) | Provides privacy and quiet |
Absorption coefficient and reverberation
- Sound absorption coefficient — fraction of incident sound energy absorbed by a surface (0 to 1); an open window absorbs completely ( = 1).
- Absorption of a surface (in sabins, m² units); total absorption .
- Noise reduction coefficient (NRC) — average of absorption coefficients at 250, 500, 1000 and 2000 Hz.
- Reverberation time (RT) — time for sound to decay by 60 dB after the source stops.
FormulaSabine's formula
in seconds; = room volume (m³); = total absorption (m² sabins).
- Too long RT → echoes and poor intelligibility; too short → "dead" room.
- Optimum RT depends on use and volume — shorter (roughly around 0.5–1 s) for lecture halls and speech, longer (roughly 1.5–2 s) for concert halls.
Common acoustic defects: echoes, excessive reverberation, sound foci (concave surfaces focusing sound), dead spots, insufficient loudness, external noise.
Sound insulation and mass law
- Transmission coefficient — fraction of incident sound transmitted through a partition.
- Transmission loss (sound reduction index): dB.
- Mass law — for single-leaf partitions, TL increases by about 6 dB for each doubling of mass per unit area (and for each doubling of frequency).
- Sound transmission class (STC) — single-number rating of airborne sound insulation.
- Weak points: gaps, cracks, doors, windows, ducts and services ("flanking" paths).