Which of the following increase the ventilation rate produced by the stack effect?

A greater vertical distance between the inlet and the outlet

A greater temperature difference between indoor and outdoor air

A larger free area of the openings

A greater vertical distance between the inlet and the outlet

A greater temperature difference between indoor and outdoor air

A larger free area of the openings

A higher wind speed outside the building

A direct evaporative cooler of effectiveness 0.75 receives air at 40 °C dry-bulb and 24 °C wet-bulb. The supply air temperature is:

28 °C, since the dry-bulb falls by 0.75 of the wet-bulb depression of 16 K

24 °C, since the air always leaves at its wet-bulb temperature

28 °C, since the dry-bulb falls by 0.75 of the wet-bulb depression of 16 K

32 °C, since the dry-bulb falls by only half of the wet-bulb depression of 16 K

36 °C, since the dry-bulb falls by only a quarter of the wet-bulb depression of 16 K

Which description fits a Trombe wall?

A glazed south-facing mass wall with an air gap that absorbs sun and passes heat indoors after a delay

A tall shaft with openings at the top that catches the wind and carries cooled air down into the rooms below

A roof pond under movable insulation that absorbs the sun by day and radiates heat to the sky at night

A perforated stone screen over a window that cuts the glare and heat while letting air pass through

Why is a heavy, high-mass envelope a poor choice in the warm-humid zone?

Because the diurnal range is small and the nights stay warm, so heat stored by day cannot be shed and mass keeps re-radiating it indoors

Because high humidity makes masonry conduct heat faster than it would in dry air, which removes the benefit of thickness

Because mass cannot absorb any heat when relative humidity is high, so it has no effect on the indoor temperature

Because heavy walls are always weaker against heavy rainfall and therefore need much larger sections than light walls, which adds cost, weight and construction time

Because the diurnal range is small and the nights stay warm, so heat stored by day cannot be shed and mass keeps re-radiating it indoors

A design team halves the operational energy of an office building through an efficient envelope, daylighting and renewables. Which statement about the building's whole-life energy is correct?

Embodied energy now forms a larger share of whole-life energy, so the choice of materials matters more than before

Embodied energy falls by the same fraction, because embodied energy is proportional to operational energy in every building

The embodied share is unchanged, because both embodied and operational energy are fixed by the floor area alone

Embodied energy now forms a larger share of whole-life energy, so the choice of materials matters more than before

Embodied energy becomes irrelevant, because life-cycle assessment counts only the energy used during occupation

Which statements about embodied energy are correct?

It includes the energy of extracting, manufacturing and transporting materials as well as constructing the building

A material with high embodied energy per kilogram may still be the lower-energy choice per functional unit if much less of it is needed

Locally sourced materials reduce the transport component of embodied energy

It includes the energy of extracting, manufacturing and transporting materials as well as constructing the building

It is incurred only once at construction, so maintenance and replacement add none to it

A material with high embodied energy per kilogram may still be the lower-energy choice per functional unit if much less of it is needed

Locally sourced materials reduce the transport component of embodied energy

Under the whole-building performance route of an energy conservation building code, compliance is shown by:

Simulating the proposed design and showing that its annual energy use is no more than that of a reference building of the same geometry, climate and use

Meeting a stated U-value, SHGC, lighting power density and equipment efficiency for every wall, roof, window and fixture one by one, without simulating the whole building

Metering the first year of occupation and comparing the result with the average EPI of all buildings in the country

Showing that the installed renewable capacity of the building is at least equal to its total connected electrical load

Simulating the proposed design and showing that its annual energy use is no more than that of a reference building of the same geometry, climate and use

A simulated office predicted 30% less annual energy than its meter later shows. Which could explain the gap?

Occupancy hours and plug loads in use exceed the schedules assumed in the model

The model used a typical meteorological year, but the metered year was hotter than typical

The HVAC controls were commissioned poorly, so equipment runs longer than designed

Occupancy hours and plug loads in use exceed the schedules assumed in the model

The model used a typical meteorological year, but the metered year was hotter than typical

The HVAC controls were commissioned poorly, so equipment runs longer than designed

The simulation engine ignores solar radiation and therefore understates every cooling load

In planning the energy supply of a new building, the sound order of decisions is:

Reduce demand through form, envelope and daylight; improve the efficiency of the systems; then meet the remaining load from on-site renewables

Install the renewable generation first because it is the most visible and marketable measure, then reduce the load until the system can cover all of it

Size the chiller and boiler for the full unmodified load, then add insulation later to cut running costs

Choose efficient appliances first, then decide the envelope, and fix the orientation of the building last

The wind speed at a rooftop turbine site rises from 4 m/s to 6 m/s. Other things equal, the available wind power changes by a factor of:

3.375, since power is proportional to the cube of the speed

2.25, since power is proportional to the square of the speed

3.375, since power is proportional to the cube of the speed

1.5, since power is proportional to the speed itself

5.06, since power is proportional to the fourth power of the speed

Which statement about rating systems is correct?

GRIHA was developed by TERI and adopted by the Ministry of New and Renewable Energy as India's national rating system

GRIHA was developed by the US Green Building Council and later adapted for use in the Indian climate and building market

The BEE star label for buildings rates water use and materials as well as the energy performance of the building

GRIHA was developed by TERI and adopted by the Ministry of New and Renewable Energy as India's national rating system

LEED is the mandatory national code that every new commercial building in India must meet before occupation

Which of the following are examples of kinetic architecture?

A facade of shading units that open and close under sensor and control logic as the sun moves

A house that rotates on a central column to follow the sun

A facade of shading units that open and close under sensor and control logic as the sun moves

A fixed concrete brise-soleil sized for the summer sun

A house that rotates on a central column to follow the sun

Which roof finish performs best as a cool roof in a hot climate?

White elastomeric coating, which has both high solar reflectance and high thermal emittance

Polished bare aluminium sheet, which has high solar reflectance but low thermal emittance, so it sheds absorbed heat poorly

Dark grey bituminous membrane, which has high thermal emittance but low solar reflectance

Bare concrete slab with no finish, since its high thermal mass keeps the roof cool all day

White elastomeric coating, which has both high solar reflectance and high thermal emittance

Passive Design, Building Energy and Sustainable Technologies

This is the second of two chapters on Part B1, Section B1.2 of the Architecture and Planning (AR) paper, “Climatology and Environmentally Responsive Design”. It answers the remaining headings: “Passive design principles and concepts; Building energy audit and management; Embodied and operational energy; Building energy performance simulation and evaluation; Alternate and renewable sources of energy for buildings; Building Performance Rating systems; Kinetic architecture; Sustainable building technologies”. It teaches the envelope and U-value, thermal mass and time lag, wind-driven and stack ventilation, courtyards, wind towers and evaporative cooling, passive heating with the Trombe wall, daylighting, embodied against operational energy, the energy audit, the energy performance index, the energy conservation code and simulation tools, renewable sources from PV to biomass, the rating systems, kinetic architecture and the sustainable technologies. The numericals are the U-value and heat flow of a composite wall, stack-driven air changes, the energy performance index, simple payback and PV yield.

1. Passive design: the envelope, U-value, insulation and thermal mass

Passive design uses form, orientation, envelope and natural flows of air, sun and water to deliver comfort with little or no mechanical energy. The order of decisions is fixed: read the climate; set the form and orientation (compactness, east-west axis, breeze); design the envelope; then openings and shading; and only after that, if comfort is still short, add active systems sized for a smaller load.

Steady-state heat flow through an envelope element is Q = U × A × ΔT (W). Each layer of thickness L and conductivity k has resistance R = L / k (m²K/W); resistances in series add, R_total = R_si + ΣL/k + R_air-gap + R_so, with R_si and R_so the inside and outside surface-film resistances (their values depend on the standard, so a problem states them), and U = 1 / R_total (W/m²K). U-values do not add; resistances do.

Thermal conductivity k, order of magnitude (W/m·K)
Material	k, approximately
Still air	0.026
Mineral wool, expanded polystyrene	0.03 to 0.04
Autoclaved aerated concrete block	0.1 to 0.2
Burnt-clay brick masonry, cement plaster	0.7 to 0.9
Dense concrete	1.4 to 1.8

Worked example. A wall of 12 mm plaster (k = 0.72), 230 mm brick (k = 0.81) and 12 mm plaster, with R_si = 0.12 and R_so = 0.06 m²K/W: R_total = 0.12 + 0.0167 + 0.2840 + 0.0167 + 0.06 = 0.497 m²K/W, so U = 2.01 W/m²K. Adding 50 mm of EPS (k = 0.04) adds 0.05 / 0.04 = 1.25, so R_total = 1.747 and U = 0.57, a cut in heat flow of 72%. For 20 m² and ΔT = 15 K the flow falls from 603 W to 172 W.

Thermal mass (heat capacity ρ c V) absorbs heat and releases it later, so a massive element delays and damps the outdoor temperature wave: the time lag is the hours between the outdoor and indoor peak, and the decrement factor (indoor amplitude over outdoor amplitude) is below 1. Thick masonry gives a long lag and a small decrement. It pays where the diurnal range is large (hot-dry, and cold-sunny for storing sun), when the afternoon peak is delivered into a night that can be flushed cool. It fails in the warm-humid zone, where nights stay warm and the stored heat never leaves. Insulation belongs on the outside of heavy walls and above the roof slab, with a reflective finish; the roof, which takes the most sun, is the first element to treat. A reflective foil only works against an air gap, and a cool roof needs high solar reflectance and high thermal emittance.

⚠️ A bright metal roof is not a cool roof

Polished metal reflects sunshine but emits long-wave heat poorly, so it runs hotter than a white coating of equal reflectance. A true cool roof is both reflective and emissive. Also, brick thickness gives mass (delay), not insulation: doubling a brick wall's thickness doubles only the brick layer's resistance (from about 0.28 to 0.57 m²K/W for 230 mm of brick), which is still small next to a few centimetres of insulation.

2. Natural ventilation and passive cooling

Wind-driven cross-ventilation needs an inlet on the windward side and an outlet on the leeward, low-pressure side. The flow is Q = C_v × A × V (m³/s), with A the inlet area, V the wind speed and the effectiveness C_v about 0.5 to 0.6 for wind square to the opening and 0.25 to 0.35 for oblique wind. Place the inlet at occupant level, make the outlet at least as large as the inlet, avoid partitions in the path, and note that wind at about 45° to a wall can distribute air better within the room than wind square on. The rate is stated as air changes per hour, ACH = 3600 Q / V_room.

Stack effect is buoyancy: warm indoor air rises and leaves by a high opening while cooler air enters low, so it works when the wind is calm and the inside is warmer than the outside. For inlet and outlet of equal free area A: Q = C_d × A × √[2 g H (T_i − T_o) / T_i], with temperatures in kelvin and H the height from the centre of the lower opening to the neutral pressure plane (half the vertical separation for equal openings). Q grows with the opening height, the temperature difference and the area, and is independent of wind speed. A solar chimney raises the temperature difference. Worked example: C_d = 0.6, A = 0.4 m², openings 5 m apart so H = 2.5 m, T_i = 305 K, T_o = 293 K: the root is √(2 × 9.81 × 2.5 × 12 / 305) = 1.389 m/s, Q = 0.6 × 0.4 × 1.389 = 0.333 m³/s, and for a 160 m³ room ACH = 0.333 × 3600 / 160 = 7.5.

Wind tower (badgir, malqaf): a tall shaft with openings at the top that catches the wind and drives it down, or acts as a stack drawing air up and out; in hot-dry regions the air is cooled further by water or the shaded masonry before it enters.
Courtyard: in hot-dry and composite zones a deep, shaded courtyard fills with cool night air that persists into the morning and, with planting or water, evaporatively cools and draws air through the rooms; it also gives daylight. In the warm-humid zone the court is wide and open to the breeze. A perforated jaali screen cuts glare and heat while passing air.
Night ventilation flushes the structure with cool night air so that it absorbs heat the next afternoon; it needs a large diurnal range.
Evaporative cooling follows a constant wet-bulb line: the air cools toward its wet-bulb temperature. A direct cooler of effectiveness ε gives T_out = T_in − ε (T_in − T_wb); at 40 °C dry-bulb, 24 °C wet-bulb and ε = 0.75, T_out = 28 °C, at higher humidity. Passive forms are roof ponds and sprays, courtyard pools and wetted screens; an earth-air tunnel uses ground a few metres down, which stays near the annual mean air temperature.

🎯 Why stack ventilation is the night and calm-day tool

Wind-driven flow depends on weather you cannot promise, while buoyancy depends only on the indoor-outdoor temperature difference and the height you design in. That is why tall stairwells, atria, wind towers and solar chimneys are used to keep air moving on still, hot days, and why the effect weakens in the afternoon when the outside is nearly as warm as the inside.

3. Passive heating and daylighting

Passive solar heating collects, stores and distributes sun without machinery. Three arrangements are standard: direct gain (sun enters through sun-facing glazing and is stored in floor and wall mass); indirect gain, of which the Trombe wall is the type: a masonry wall behind sun-facing glass with an air gap, which absorbs the sun and passes heat inward by conduction (with a lag) and, through top and bottom vents, by thermocirculation; and isolated gain, a sunspace or attached greenhouse that is closed off from the rooms. All need glazing facing the sun (south in India), mass inside the insulation, night insulation of the glass, and an overhang sized to exclude the high summer sun while admitting the low winter sun.

Daylighting is measured by the daylight factor, DF = (E_in / E_out) × 100%, where E_in is the illuminance at a point indoors and E_out the simultaneous horizontal illuminance outdoors under an unobstructed overcast sky. It is the sum of the sky component, the externally reflected component and the internally reflected component. As guidance, an average DF of about 2% gives a space that looks daylit (though electric light is still often needed) and about 5% a well-daylit space. Example: 150 lux indoors under 6,000 lux outdoors is DF = 2.5%.

A higher window head sends light deeper; the usefully daylit depth is a small multiple of the head height (a rule of thumb is about two times).
A light shelf shades the lower window and bounces light onto the ceiling; north-facing glazing, clerestories and north-light roofs give steady, glare-free diffuse light; pale interior surfaces raise the internally reflected component.
Daylight and heat travel together: choose glass with high visible transmittance but low solar heat gain, avoid unshaded skylights in the tropics, and dim the electric lights when daylight is adequate, since lighting is also an internal heat gain.

⚠️ The daylight factor says nothing about orientation or the sun

DF is defined for the standard overcast sky, which is the same from every direction and at every hour, so a north room and a west room of equal geometry have the same DF although the west room suffers glare and heat. India's skies are often bright and clear, so a space judged only by DF can still overheat; newer annual metrics such as spatial daylight autonomy (the share of floor area meeting a target illuminance for a set share of hours) are needed beside it.

4. Embodied and operational energy; energy audit and management

Operational energy is what the building uses while it runs: cooling, heating, ventilation, lighting, appliances and hot water. Embodied energy is all the energy to extract, manufacture and transport materials, construct the building, maintain and replace its parts and finally demolish it. It has an initial part (built once), a recurring part (maintenance and replacement) and a demolition part. A life-cycle assessment counts both, with a stated boundary: cradle-to-gate, cradle-to-grave or cradle-to-cradle.

Over a typical 50-year life of a conventional, air-conditioned building, operational energy is the larger share. As operational energy is driven down (efficient design, renewables, net-zero buildings) the embodied share grows and can dominate, so material choice then matters as much as the HVAC.
Per kilogram the usual order is aluminium (highest), then steel, then glass and cement, then fired clay brick, then concrete, with local stone and earth the lowest. But per kilogram is not per building: concrete is low per kilogram yet used in such mass that it is often the largest embodied item, so compare materials per functional unit.
Reduce embodied energy by reusing and adapting existing buildings, using local and recycled materials (fly-ash bricks, recycled aggregate), lightweight efficient structure and long-lasting components, and by cutting transport distance.

An energy audit is a systematic survey of how and where a building uses energy and what can be saved. The preliminary audit is a walk-through with bills and easy wins; the detailed (investment-grade) audit measures loads with power analysers, lux meters, data loggers, infrared thermography and blower-door tests, builds an energy balance by end use, benchmarks it against the EPI, and lists energy conservation measures (ECMs) with cost, saving and payback. The sequence is: collect data, walk through, measure, analyse, rank measures, implement, then monitor and verify the saving. No-cost and low-cost measures (housekeeping, set-points, switching off, power factor, tariff) come first, then retrofits (LEDs, variable-speed drives, efficient chillers, envelope, controls). In India the Energy Conservation Act, 2001 created the Bureau of Energy Efficiency (BEE), which promotes audits and the building code.

Energy management makes this continuous: an energy policy, a named energy manager, sub-metering by end use, monitoring and targeting against a baseline, a building management system, and maintenance and commissioning so the savings persist. The ranking tool is simple payback = investment / annual saving. Example: replacing 100 fittings of 72 W with 36 W LEDs, run 3,000 h a year at ₹8 per kWh, saves 100 × 0.036 × 3,000 = 10,800 kWh = ₹86,400 a year; at ₹1,800 per fitting (₹1,80,000) the payback is 2.08 years. It ignores the time value of money and the life of the measure, so large investments are also tested by discounted or life-cycle cost.

5. Energy performance evaluation: EPI, the energy conservation code and simulation

The energy performance index is the annual energy consumption divided by the built-up (or conditioned) area: EPI = annual kWh / area in m², in kWh/m²/year. It is a benchmark that lets a building be compared with others of its type and climate zone. Example: 45,000 kWh a month for 12 months in 4,500 m² gives 540,000 / 4,500 = 120 kWh/m²/year. The BEE star label for buildings rates a building from one to five stars by comparing its EPI with benchmarks, and a lower EPI earns more stars. Compare like with like: EPI depends on the hours of use and on whether the area is air-conditioned, and all fuels must be converted to the same unit.

The Energy Conservation Building Code (ECBC), issued by BEE under the Energy Conservation Act, 2001 (first launched in 2007 and revised in 2017; a later edition is styled the Energy Conservation and Sustainable Building Code and adds sustainability measures), sets minimum requirements for commercial buildings above a threshold of connected load or contract demand, which the revision lowered. It covers the envelope (wall and roof U-values, window U-value, SHGC, visible transmittance and window-to-wall ratio), HVAC efficiency, lighting power density (W/m²), electrical systems, hot water and pumping. A building may comply by the prescriptive route (every component meets its limit), by an envelope trade-off, or by the whole-building performance route: simulate the proposed design and show that its annual energy is no more than that of a reference (standard) building of the same geometry, climate and use. The 2017 revision added higher tiers, ECBC Plus and Super ECBC. For homes the counterpart is the ECO-Niwas Samhita, whose envelope measure is the residential envelope transmittance value (RETV).

Building energy performance simulation solves a heat balance for each zone at every hour or shorter time-step. The inputs are a weather file (a typical meteorological year, for example in EPW format), the geometry and orientation, construction properties (U-value, SHGC, thermal mass), internal gains and schedules (occupancy, lighting, equipment), infiltration, set-points and the HVAC system. The outputs are hourly loads, peak demand, annual energy by end use, the EPI and hours outside the comfort band. EnergyPlus (US Department of Energy) is the open calculation engine; DesignBuilder and OpenStudio are interfaces to it, and eQUEST and TRNSYS are other tools; Radiance handles daylight and CFD handles airflow. Typical runs are a design-day run to size equipment and an annual run to estimate energy, with a parametric series to compare options and a baseline-against-proposed pair for code compliance.

⚠️ A simulation is a prediction, and it compares better than it forecasts

Results are only as good as the assumptions: schedules, infiltration, set-points and the typical weather year are guesses, so measured use often differs from predicted use (the performance gap). Models are calibrated against metered bills with statistical indices such as NMBE and CV(RMSE). Because errors affect every option alike, the ranking of design options is far more reliable than the absolute kWh figure.

6. Alternate and renewable energy for buildings

Renewables come last in a hierarchy: first cut the demand by passive design, then raise the efficiency of what remains, and only then supply the remainder on site. A net-zero energy building generates over a year at least as much as it consumes.

Solar photovoltaic (PV) cells turn sunlight into direct current. Commercial crystalline-silicon modules convert roughly 15% to 22% of the sunlight; rated peak power (kWp) is quoted at standard test conditions of 1,000 W/m², 25 °C cell temperature and air mass 1.5. Output falls by roughly 0.3 to 0.5% for each °C above 25 °C, and with dust and shade. The annual yield is E = A × η × H × PR, with H the annual insolation on the module plane (kWh/m²/year) and PR the performance ratio covering temperature, soiling, wiring and inverter losses (typically 0.75 to 0.85). Example: 40 m² at η = 0.18, H = 1,800 kWh/m²/year and PR = 0.80 yields 40 × 0.18 × 1,800 × 0.80 = 10,368 kWh a year. For sizing, A = daily load / (H_day × η × PR): 18 kWh a day at 5 kWh/m²/day, 20% and 0.75 needs 24 m². Facing the equator with a tilt near the latitude maximises annual yield. Systems may be grid-connected with net metering, off-grid with batteries, or hybrid, and building-integrated PV (BIPV) makes the module part of the roof or facade, where it can also shade.

Solar thermal collectors (flat-plate or evacuated-tube) heat water directly; a thermosiphon system needs no pump because heated water rises to the tank. The heat needed is Q = m c ΔT and the collector area is A = Q / (η H). Example: 150 L heated through 35 K needs 150 × 4.18 × 35 = 21,945 kJ = 6.1 kWh; at η = 0.5 and 5 kWh/m²/day, A = 6.1 / 2.5 = 2.44 m². Wind power is P = ½ ρ A V³ C_p, bounded by the Betz limit of 16/27 (59.3%): since power varies as V³, raising the wind speed from 4 to 6 m/s multiplies it by 3.375. Geothermal in buildings means the ground-source heat pump and the earth-air tunnel, which exploit the steady ground temperature a few metres down. Biomass in buildings is chiefly biogas, from anaerobic digestion of dung and food waste, together with briquettes and gasifiers.

Renewable sources at building scale
Source	What it supplies	Main limitation
Solar PV	Electricity	Needs unshaded area; intermittent without storage; output falls with heat and dust
Solar thermal	Hot water, low-grade heat	Needs backup in cloudy spells; low-temperature heat only
Wind	Electricity	Power varies as V³, so low and turbulent urban winds make small roof turbines poor performers
Geothermal (heat pump, earth-air tunnel)	Heating and cooling from the steady ground temperature	Needs ground area or boreholes and pumping energy
Biomass, biogas	Cooking fuel, heat, electricity	Needs steady feedstock and handling; combustion emissions

7. Building performance rating systems

A rating system is a voluntary, third-party-verified scorecard. It lists prerequisites that must be met and credits that earn points in categories such as site, water, energy, materials, indoor environmental quality and innovation, and the total fixes a certification level. Its purpose is to make sustainability measurable and comparable, but it rewards the design intent and the measures adopted, not the energy a building actually uses.

Rating systems, in principle
System	Origin and character
LEED	US Green Building Council; credits across location, sites, water, energy and atmosphere, materials, indoor quality and innovation; four levels from Certified to Platinum (in LEED v4 the thresholds are 40, 50, 60 and 80 points); energy and atmosphere carries the most points
IGBC	Indian Green Building Council, part of the Confederation of Indian Industry; tiered rating programmes adapted to Indian building types and conditions
GRIHA	Developed by TERI and adopted by the Ministry of New and Renewable Energy as the national system; points across criteria for site, energy, water, waste and indoor quality translate into one to five stars
BEE star rating	Energy only: the EPI of the building against a benchmark, one to five stars
BREEAM	The earliest of the group, from BRE in the United Kingdom, graded from Pass to Outstanding

⚠️ A code is mandatory floor, a rating is a voluntary ladder

ECBC is a code: where it applies, meeting it is the minimum. LEED, IGBC and GRIHA are voluntary ratings that go beyond the minimum, and the BEE star label is a performance label, not a code. Also remember that a certificate at design stage does not guarantee the measured performance: operation, occupancy and commissioning decide that, which is why post-occupancy tracking matters.

8. Kinetic architecture and sustainable building technologies

Kinetic architecture (the term was popularised by Zuk and Clark's book of 1970) covers buildings or building parts that physically move, in response to the sun, wind, weather or use. The case for it is climatic: conditions change by the hour and the season while a fixed element is a compromise tuned to one sun position, whereas a moving element can follow the sun. The kinds are deployable (retractable stadium roofs), rotating (a house turning on a column to follow the sun, as the Heliotrope at Freiburg does) and responsive facades, where sensors, actuators and a control logic open, close or tilt shading units: the Al Bahar Towers in Abu Dhabi use opening and closing screens, a modern mashrabiya, and the Institut du Monde Arabe in Paris (Jean Nouvel) put light-sensitive apertures in its south facade.

🎯 The price of moving parts

Moving systems add actuator energy, maintenance, failure risk and control complexity, and a stuck device can be worse than a fixed one. A kinetic scheme is justified only where the saving in cooling, heating or lighting, or the gain in function, clearly outweighs these costs; a well-sized fixed overhang with a good orientation is often the better engineering answer.

Cool and green roofs: reflective, emissive coatings or a planted layer cut heat gain and the urban heat island; a green roof also adds insulation and holds stormwater but needs structure, waterproofing and upkeep.
Water: rainwater harvesting and groundwater recharge, greywater recycling and on-site sewage treatment, low-flow fixtures.
Materials: fly-ash bricks, autoclaved aerated concrete, compressed stabilised earth blocks, bamboo and recycled construction waste, judged by embodied energy, durability and local availability.
Systems: high-performance (double, low-emissivity) glazing, heat recovery, daylight and occupancy controls, efficient HVAC, BIPV and earth-air tunnels.

The governing idea is to judge a technology by its whole life, not its label: its embodied and operating energy, water, durability and what it does to the occupants' comfort. The cheapest and most reliable savings are almost always the passive ones that come first in the design order.

Key takeaways

Heat flow is Q = U A ΔT with U = 1 / R_total and resistances adding in series; insulation cuts the flow, mass delays and damps it, and mass works only where the diurnal range is large.
Cross-ventilation depends on wind (Q = C_v A V), stack ventilation on height and temperature difference (Q = C_d A √[2 g H ΔT / T_i]); courtyards, wind towers and evaporative cooling suit hot-dry air, and only air movement helps humid air.
The Trombe wall is indirect-gain heating through a glazed mass wall; the daylight factor is E_in / E_out × 100% under an overcast sky and ignores orientation and sun.
Embodied energy covers extraction to demolition and grows in share as operational energy falls; the audit runs from walk-through to measurement, ranked measures and verification, with simple payback = investment / annual saving.
EPI is kWh/m²/year, the benchmark behind the BEE star label; ECBC is the code, a minimum wherever it applies (prescriptive, trade-off, whole-building routes); simulation predicts and compares, and needs weather, schedules and calibration.
Reduce demand first, then add renewables (PV: E = A η H PR; solar thermal; wind ∝ V³; geothermal; biogas); LEED, IGBC, GRIHA and BEE ratings are voluntary scorecards; kinetic facades follow the sun but cost energy and upkeep.

Practice questions (20)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

A wall is built of 12 mm cement plaster (k = 0.72 W/m·K), 230 mm brick masonry (k = 0.81 W/m·K) and 12 mm cement plaster (k = 0.72 W/m·K). Take the inside and outside surface resistances as 0.12 and 0.06 m²K/W. Its U-value, in W/m²K correct to two decimal places, is ____.
Numerical answer — type the value.
Show answer
Answer: 2.01
R_total = R_si + 2 × (0.012 / 0.72) + 0.23 / 0.81 + R_so = 0.12 + 0.0333 + 0.2840 + 0.06 = 0.4973 m²K/W. U = 1 / 0.4973 = 2.011, so 2.01 W/m²K. Re-adding in another order, (0.12 + 0.06) + 0.2840 + 0.0333 = 0.4973, gives the same U.
A wall has U = 2.0 W/m²K. A 50 mm layer of insulation with k = 0.04 W/m·K is added to it. By about how much does the steady heat flow through the wall (same area and same temperature difference) fall?
1. About 29%
2. About 50%
3. About 71%
4. About 20%
Show answer
Answer: C — About 71%
R_old = 1 / 2.0 = 0.50 m²K/W. The insulation adds 0.05 / 0.04 = 1.25, so R_new = 1.75 and U_new = 0.571. The fall is (2.0 − 0.571) / 2.0 = 71%. The figure 29% is the fraction that remains, not the fall; 20% comes from misreading k as 0.4.
A room of volume 160 m³ has an inlet and an outlet of free area 0.4 m² each, with the height from the lower opening to the neutral pressure plane equal to 2.5 m. Indoor air is at 305 K and outdoor air at 293 K. Take C_d = 0.6, g = 9.81 m/s² and Q = C_d A √[2 g H (T_i − T_o) / T_i]. The air change rate by stack effect, in air changes per hour correct to one decimal place, is ____.
Numerical answer — type the value.
Show answer
Answer: 7.5
2 g H ΔT / T_i = 2 × 9.81 × 2.5 × 12 / 305 = 588.6 / 305 = 1.930, whose root is 1.389 m/s. Q = 0.6 × 0.4 × 1.389 = 0.3334 m³/s = 1,200 m³/h. ACH = 1,200 / 160 = 7.5. A common slip is to use the full 5 m separation for H, which would inflate ACH by √2.
Which of the following increase the ventilation rate produced by the stack effect?
1. A greater vertical distance between the inlet and the outlet
2. A greater temperature difference between indoor and outdoor air
3. A larger free area of the openings
4. A higher wind speed outside the building
Show answer
Answer: A — A greater vertical distance between the inlet and the outlet; B — A greater temperature difference between indoor and outdoor air; C — A larger free area of the openings
In Q = C_d A √[2 g H ΔT / T_i] the flow rises with the height H, the temperature difference and the opening area. Wind speed does not appear: stack flow is buoyancy-driven, and wind-driven flow is a separate mechanism that may add to it or oppose it.
A direct evaporative cooler of effectiveness 0.75 receives air at 40 °C dry-bulb and 24 °C wet-bulb. The supply air temperature is:
1. 24 °C, since the air always leaves at its wet-bulb temperature
2. 28 °C, since the dry-bulb falls by 0.75 of the wet-bulb depression of 16 K
3. 32 °C, since the dry-bulb falls by only half of the wet-bulb depression of 16 K
4. 36 °C, since the dry-bulb falls by only a quarter of the wet-bulb depression of 16 K
Show answer
Answer: B — 28 °C, since the dry-bulb falls by 0.75 of the wet-bulb depression of 16 K
T_out = T_in − ε (T_in − T_wb) = 40 − 0.75 × (40 − 24) = 40 − 12 = 28 °C. The wet-bulb temperature of 24 °C is the unreachable limit of effectiveness 1, so an actual cooler stops above it. The humidity of the air rises in the process, because it follows a constant wet-bulb line.
Which description fits a Trombe wall?
1. A glazed south-facing mass wall with an air gap that absorbs sun and passes heat indoors after a delay
2. A tall shaft with openings at the top that catches the wind and carries cooled air down into the rooms below
3. A roof pond under movable insulation that absorbs the sun by day and radiates heat to the sky at night
4. A perforated stone screen over a window that cuts the glare and heat while letting air pass through
Show answer
Answer: A — A glazed south-facing mass wall with an air gap that absorbs sun and passes heat indoors after a delay
The Trombe wall is the standard indirect-gain heating device: sun-facing glass, a gap, and a mass wall behind it that stores the heat and releases it indoors with a delay. The tall shaft is a wind tower, the roof pond is a separate passive heating and cooling system, and the perforated screen is a jaali.
Why is a heavy, high-mass envelope a poor choice in the warm-humid zone?
1. Because high humidity makes masonry conduct heat faster than it would in dry air, which removes the benefit of thickness
2. Because mass cannot absorb any heat when relative humidity is high, so it has no effect on the indoor temperature
3. Because heavy walls are always weaker against heavy rainfall and therefore need much larger sections than light walls, which adds cost, weight and construction time
4. Because the diurnal range is small and the nights stay warm, so heat stored by day cannot be shed and mass keeps re-radiating it indoors
Show answer
Answer: D — Because the diurnal range is small and the nights stay warm, so heat stored by day cannot be shed and mass keeps re-radiating it indoors
Thermal mass helps only when there is a cool period in which it can discharge. In the warm-humid zone the night is nearly as warm and humid as the day, so mass stores heat and returns it indoors; a light, ventilated envelope that follows the outside conditions is better. Humidity does not change masonry conduction in the way the claim of faster conduction in humid air suggests.
At a work point indoors the illuminance is 150 lux, while the unobstructed horizontal illuminance outdoors under the same overcast sky is 6,000 lux. The daylight factor at the point is:
1. 0.25%
2. 2.5%
3. 25%
4. 40%
Show answer
Answer: B — 2.5%
DF = (E_in / E_out) × 100 = (150 / 6,000) × 100 = 2.5%. The value 40% results from inverting the ratio, and 0.25% or 25% from slips in the decimal place.
A design team halves the operational energy of an office building through an efficient envelope, daylighting and renewables. Which statement about the building's whole-life energy is correct?
1. Embodied energy falls by the same fraction, because embodied energy is proportional to operational energy in every building
2. The embodied share is unchanged, because both embodied and operational energy are fixed by the floor area alone
3. Embodied energy now forms a larger share of whole-life energy, so the choice of materials matters more than before
4. Embodied energy becomes irrelevant, because life-cycle assessment counts only the energy used during occupation
Show answer
Answer: C — Embodied energy now forms a larger share of whole-life energy, so the choice of materials matters more than before
Embodied energy is set by the materials and construction, not by how the building is later run. Halving the operational energy shrinks the denominator of whole-life energy, so the embodied portion becomes a larger share, and in net-zero buildings it can dominate. Life-cycle assessment counts both, which is why the claim that embodied energy is irrelevant is wrong.
Which statements about embodied energy are correct?
1. It includes the energy of extracting, manufacturing and transporting materials as well as constructing the building
2. It is incurred only once at construction, so maintenance and replacement add none to it
3. A material with high embodied energy per kilogram may still be the lower-energy choice per functional unit if much less of it is needed
4. Locally sourced materials reduce the transport component of embodied energy
Show answer
Answer: A — It includes the energy of extracting, manufacturing and transporting materials as well as constructing the building; C — A material with high embodied energy per kilogram may still be the lower-energy choice per functional unit if much less of it is needed; D — Locally sourced materials reduce the transport component of embodied energy
Embodied energy runs from extraction to demolition, so it is not a single initial amount: maintenance and replacement add a recurring part. Comparison per kilogram can mislead (a light, strong material may need far less mass per function), and shorter transport cuts the transport share.
A 4,500 m² building uses 45,000 kWh of electricity in every month of the year. Its energy performance index, in kWh/m²/year, is ____.
Numerical answer — type the value.
Show answer
Answer: 120
Annual consumption = 45,000 × 12 = 540,000 kWh. EPI = 540,000 / 4,500 = 120 kWh/m²/year. Check by the monthly figure: 45,000 / 4,500 = 10 kWh/m² a month, and 10 × 12 = 120.
A retrofit replaces 100 fittings of 72 W each with LED fittings of 36 W each. They run 3,000 hours a year, electricity costs ₹8 per kWh and the installed cost is ₹1,800 per new fitting. The simple payback period, in years correct to two decimal places, is ____.
Numerical answer — type the value.
Show answer
Answer: 2.08
Power saved = 100 × (72 − 36) W = 3.6 kW. Energy saved = 3.6 × 3,000 = 10,800 kWh a year, worth 10,800 × 8 = ₹86,400. Investment = 100 × 1,800 = ₹180,000. Payback = 180,000 / 86,400 = 2.083, so 2.08 years. Check per fitting: 0.036 × 3,000 × 8 = ₹864 saved a year against ₹1,800, again 2.083.
Under the whole-building performance route of an energy conservation building code, compliance is shown by:
1. Meeting a stated U-value, SHGC, lighting power density and equipment efficiency for every wall, roof, window and fixture one by one, without simulating the whole building
2. Metering the first year of occupation and comparing the result with the average EPI of all buildings in the country
3. Showing that the installed renewable capacity of the building is at least equal to its total connected electrical load
4. Simulating the proposed design and showing that its annual energy use is no more than that of a reference building of the same geometry, climate and use
Show answer
Answer: D — Simulating the proposed design and showing that its annual energy use is no more than that of a reference building of the same geometry, climate and use
The whole-building performance route compares simulated annual energy of the proposed building with a reference building. Meeting component limits one by one is the prescriptive route, which is the tempting confusion. Metered data and installed renewable capacity are not the compliance test.
A simulated office predicted 30% less annual energy than its meter later shows. Which could explain the gap?
1. Occupancy hours and plug loads in use exceed the schedules assumed in the model
2. The model used a typical meteorological year, but the metered year was hotter than typical
3. The HVAC controls were commissioned poorly, so equipment runs longer than designed
4. The simulation engine ignores solar radiation and therefore understates every cooling load
Show answer
Answer: A — Occupancy hours and plug loads in use exceed the schedules assumed in the model; B — The model used a typical meteorological year, but the metered year was hotter than typical; C — The HVAC controls were commissioned poorly, so equipment runs longer than designed
The performance gap comes from assumptions and operation: real schedules and plug loads, a metered year hotter than the typical weather year assumed, and poor commissioning all raise measured use above prediction. Energy simulation engines do model solar radiation, so that cause is false.
In planning the energy supply of a new building, the sound order of decisions is:
1. Reduce demand through form, envelope and daylight; improve the efficiency of the systems; then meet the remaining load from on-site renewables
2. Install the renewable generation first because it is the most visible and marketable measure, then reduce the load until the system can cover all of it
3. Size the chiller and boiler for the full unmodified load, then add insulation later to cut running costs
4. Choose efficient appliances first, then decide the envelope, and fix the orientation of the building last
Show answer
Answer: A — Reduce demand through form, envelope and daylight; improve the efficiency of the systems; then meet the remaining load from on-site renewables
The hierarchy is demand first, efficiency second, supply last: passive measures are cheapest and most durable, they shrink the plant and the renewable system that follow, and orientation, which is nearly free, must be fixed at the start. Sizing plant before cutting the load locks in oversized equipment.
Rooftop PV modules of area 40 m² have an efficiency of 18%. The annual insolation on the module plane is 1,800 kWh/m² and the performance ratio is 0.80. The annual electricity yield, in kWh, is ____.
Numerical answer — type the value.
Show answer
Answer: 10368
Incident energy = 40 × 1,800 = 72,000 kWh. Module output = 72,000 × 0.18 = 12,960 kWh. System output after losses = 12,960 × 0.80 = 10,368 kWh. Check: 40 × 0.18 × 0.80 = 5.76 m² of effective area, and 5.76 × 1,800 = 10,368.
The wind speed at a rooftop turbine site rises from 4 m/s to 6 m/s. Other things equal, the available wind power changes by a factor of:
1. 2.25, since power is proportional to the square of the speed
2. 3.375, since power is proportional to the cube of the speed
3. 1.5, since power is proportional to the speed itself
4. 5.06, since power is proportional to the fourth power of the speed
Show answer
Answer: B — 3.375, since power is proportional to the cube of the speed
The kinetic energy flux through the rotor is ½ ρ A V³, so power scales as V³: (6/4)³ = 3.375. The square-law answer 2.25 is the common slip, and the factor applies to available power before the Betz limit and turbine efficiency are applied.
Which statement about rating systems is correct?
1. GRIHA was developed by the US Green Building Council and later adapted for use in the Indian climate and building market
2. The BEE star label for buildings rates water use and materials as well as the energy performance of the building
3. GRIHA was developed by TERI and adopted by the Ministry of New and Renewable Energy as India's national rating system
4. LEED is the mandatory national code that every new commercial building in India must meet before occupation
Show answer
Answer: C — GRIHA was developed by TERI and adopted by the Ministry of New and Renewable Energy as India's national rating system
GRIHA was developed by TERI and adopted by MNRE as the national system, scored in one to five stars. The BEE star label is based on the energy performance index alone. LEED is a voluntary rating of the US Green Building Council, not a mandatory Indian code; the minimum energy requirements, where they apply, come from the code, ECBC.
Which of the following are examples of kinetic architecture?
1. A retractable stadium roof
2. A facade of shading units that open and close under sensor and control logic as the sun moves
3. A fixed concrete brise-soleil sized for the summer sun
4. A house that rotates on a central column to follow the sun
Show answer
Answer: A — A retractable stadium roof; B — A facade of shading units that open and close under sensor and control logic as the sun moves; D — A house that rotates on a central column to follow the sun
Kinetic architecture means building parts that physically move in response to conditions or use: a retractable roof, a responsive shading facade and a rotating house all qualify. A fixed brise-soleil is a static shading device, however well sized.
Which roof finish performs best as a cool roof in a hot climate?
1. Polished bare aluminium sheet, which has high solar reflectance but low thermal emittance, so it sheds absorbed heat poorly
2. Dark grey bituminous membrane, which has high thermal emittance but low solar reflectance
3. Bare concrete slab with no finish, since its high thermal mass keeps the roof cool all day
4. White elastomeric coating, which has both high solar reflectance and high thermal emittance
Show answer
Answer: D — White elastomeric coating, which has both high solar reflectance and high thermal emittance
A cool roof must reflect the sunshine and also emit the absorbed heat as long-wave radiation. Polished aluminium reflects well but emits poorly, so it runs hot; the dark membrane absorbs most of the sun. Bare concrete absorbs heat and stores it; mass delays heat but does not make a roof cool.