Building Physics: Heat, Light, Sound and Sustainability: Open-Cycle Cooling

Open-Cycle Cooling: The Overlooked Alternative

In hot-dry climates, evaporative cooling can replace or supplement air conditioning:

• Direct evaporative cooling: Air passes through a wet pad, giving up heat to evaporate water. The air is cooled but humidified.
• Indirect evaporative cooling: A secondary air stream is evaporatively cooled and used to cool a heat exchanger, which then cools the primary air supply without adding moisture.

When outdoor air is 42°C and 10% RH, a direct evaporative cooler can deliver air at 24°C — with zero compressor energy. The only cost is the fan and water.

LIGHT — The Luminous Environment

Designing for How You Actually See

The Physics of Light — What Your Eyes Evolved to Detect

the practitioner was a lighting consultant called in to fix a brand-new office tower where workers complained of headaches, fatigue, and an inability to concentrate. The lighting met every code requirement. The problem wasn't the quantity of light. It was everything else about it.

Light is electromagnetic radiation in the narrow wavelength band from 380 nm (violet) to 780 nm (red) — a tiny sliver of the full electromagnetic spectrum that happens to activate our visual system.

Colour: The Quality of Light

The colour of light is determined by its spectral composition — which wavelengths are present and in what proportions:

Colour temperature rates the apparent colour of a light source against the colour of a heated black body:

The counter-intuitive naming: "Warm" colours have low colour temperatures, "cool" colours have high colour temperatures.

Surface Colours: The Munsell System

The Munsell system describes any surface colour with three attributes:

• Hue: The colour name (red, yellow, green, blue, purple, and intermediates)
• Value (V): Lightness, 0 (black) to 10 (white). Converts to reflectance: ρ = V × (V−1) / 100
• Chroma: Saturation or intensity of the colour

Photometry: Measuring Light for Human Vision

Four quantities define the lighting in any space:

Key relationships:

E = I / d² (Inverse square law — illuminance drops with the square of distance)

E = E_n × cos θ (Cosine law — illuminance drops with angle of incidence)

Some reference illuminance values:

Luminous efficacy — how efficiently a source converts power into visible light — measured in lm/W:

Vision — Designing for the Biological Eye

the practitioner's investigation revealed the real problem: the office had uniform 500 lux across every surface, with fluorescent tubes of 6,500 K colour temperature — cold, flat, and relentless. The space had the visual character of a hospital laboratory. No contrast, no warmth, no visual hierarchy. The workers' eyes were stressed not by too little light, but by light that contradicted every biological expectation.

How the Eye Works

The retina contains two types of receptors:

• Cones (~6.5 million): Concentrated in the fovea, sensitive to both quantity and colour of light, operate only in good lighting (photopic vision)
• Rods (~125 million): Spread across the retina, more sensitive but see only brightness, no colour (scotopic vision)

Adaptation:

• Pupil response: Nearly instantaneous, adjusting to average luminance
• Retinal adaptation to darkness: Up to 30 minutes (visual purple production)
• Retinal adaptation to brightness: About 3 minutes (visual purple removal)

This is why emerging from a dark cinema into daylight is uncomfortable for seconds, but entering a dark room takes minutes to adjust. Design implication: Transition zones between bright and dim spaces should provide gradual luminance changes.

Visual Performance Factors

Contrast sensitivity — In full daylight, you can distinguish a 1% luminance difference. In poor lighting, surfaces with 10% difference may appear identical. Contrast is defined as:

C = (L₁ − L₂) / L₂

Visual acuity — Sharpness of vision improves with illuminance, following a diminishing-returns curve. A small increase at low light levels produces dramatic improvement; the same increase at high levels produces barely any difference.

This means the first 200 lux on a desk produces far more visual benefit than increasing from 500 to 700 lux. Over-lighting is wasteful twice — in energy and in effectiveness.

Glare: The Enemy of Good Lighting

Discomfort glare reduces visual efficiency and causes annoyance. Disability glare actually impairs vision.

Causes:

• Luminance ratio (L_max / L_min) within the field of vision greater than about 10–15
• Direct view of bright light sources
• Reflections of light sources in glossy surfaces ("veiling reflections")
• Bright windows behind a task (computer screens are the worst offenders)

Contrast grading is the primary defense:

• Visual task luminance = 100%
• Immediate surroundings ≥ 50%
• Rest of the visual field ≥ 20%

Daylight Design — Harnessing the Best Light Source in the Universe

the practitioner's recommendation was radical: reduce the electric lighting by 60% and redesign the window layout. "The sun delivers 100,000 lux for free," he told the building owner. "Your building is rejecting 99% of it and then spending a fortune to replace it with inferior artificial light."

Sky Conditions

The available daylight depends on sky type:

• CIE standard overcast sky: Luminance is three times brighter at zenith than at the horizon. The illuminance on the ground depends on the sun's altitude behind the clouds: E ≈ 200 × ALT (lux)
• Clear sky: Up to 100,000 lux direct sunlight, 40,000–50,000 lux diffuse
• Intermediate sky: E ≈ 500 × ALT (lux)

The Daylight Factor

The Daylight Factor (DF) is the ratio of indoor illuminance to outdoor horizontal illuminance under an overcast sky, expressed as a percentage:

DF = (E_indoor / E_outdoor) × 100%

It has three components:

• Sky Component (SC): Direct light from the visible sky
• Externally Reflected Component (ERC): Light reflected from external surfaces
• Internally Reflected Component (IRC): Light reflected from internal surfaces

Design targets:

Planning for Daylight: Rules of Thumb

• Room depth should not exceed 2× the window head height for adequate daylight penetration
• Window area of 20–25% of floor area typically provides adequate daylight in side-lit rooms
• Light shelves can redirect daylight deeper into spaces
• Roof lights are 3× more effective than side windows for the same area (they see more sky)
• Toplighting should provide 2–5% of floor area for workspaces

Electric Lighting — The Artificial Sun

Lamp Types and Their Characteristics

The relationship between colour temperature and illuminance level is crucial:

At high illuminances (500+ lux), people prefer higher colour temperatures (4,000–5,000 K — "daylight white"). At low illuminances (< 200 lux), lower colour temperatures (2,700–3,000 K — "warm white") are preferred. Getting this relationship wrong creates spaces that feel clinical, sterile, or uncomfortable, even though the illuminance level is "correct."

This was exactly the practitioner's diagnosis. The office had cold light (6,500 K) at moderate levels (500 lux) — a combination that falls in the "disliked" zone on the Kruithof curve.

Lighting Design Method: The Lumen Method

The total luminous flux required to achieve a target illuminance:

Φ = (E × A) / (UF × MF)

Where:

• E = target illuminance (lux)
• A = room area (m²)
• UF = utilization factor (fraction of lamp output reaching the work plane — depends on room proportions, surface reflectances, and luminaire type)
• MF = maintenance factor (accounts for lamp ageing and dirt accumulation — typically 0.6–0.8)

Number of luminaires = Φ / (flux per luminaire)

SOUND — The Sonic Environment

The Invisible Architecture of What You Hear

The Physics of Sound — Vibrations That Shape Experience

the practitioner was an acoustic consultant hired to save a new concert hall that had been called "acoustically dead" by every critic who heard it. The architect had created a visually stunning space. But sound doesn't care about aesthetics — it obeys physics.

Sound is mechanical vibration transmitted through a medium. In buildings, we deal with airborne sound (vibrations in air) and structureborne sound (vibrations in the building fabric).

The Fundamental Properties

The fundamental relationship: v = f × λ

At 340 m/s: a 100 Hz bass note has a wavelength of 3.4 m; a 4,000 Hz treble note has a wavelength of 0.085 m (85 mm). This wavelength difference is why bass sounds bend around obstacles (diffraction) while treble sounds cast sharp acoustic shadows.

Sound Power: A Sense of Scale

A full symphony orchestra produces just one-hundredth of a watt — less than a dying flashlight bulb. The human ear is extraordinarily sensitive.

The Decibel Scale: Compressing the Incomprehensible

The ear responds logarithmically to stimulus intensity (Fechner's law). The decibel (dB) scale compresses the enormous range of human hearing:

Sound Intensity Level: L = 10 × log(I / I₀)

Sound Pressure Level: L = 20 × log(p / p₀)

Reference values (threshold of audibility):

• I₀ = 10⁻¹² W/m²
• p₀ = 20 × 10⁻⁶ Pa

Critical: Decibels don't add linearly.

Two sources of 90 dB and 80 dB produce:

• NOT 170 dB
• But rather 90.4 dB (intensities add, then convert back to dB)

Rule of thumb: Doubling intensity adds 3 dB. Doubling sound pressure adds 6 dB. A 10 dB increase sounds approximately twice as loud.

Frequency Weighting: The A-Weighted Scale

The ear is not equally sensitive to all frequencies — most sensitive around 4,000 Hz, much less sensitive to low frequencies. The A-weighting filter approximates this frequency response. Sound levels measured with A-weighting are reported as dBA — the most commonly used single-figure descriptor of environmental sound.

Hearing — The Human Sound Receiver

The ear is a remarkable transducer:

• Outer ear (auditory tube): Collects and funnels sound to the eardrum
• Middle ear (ossicles): Three tiny bones (hammer, anvil, stirrup) amplify vibration 20×
• Inner ear (cochlea): 25,000 hair cells selectively respond to different frequencies, converting vibration to nerve impulses

The audible range: 20 Hz to 16,000 Hz (reducing with age — a 60-year-old may lose 70 dB sensitivity at 16 kHz, but only 10 dB at 1 kHz).

Hearing damage:

• Continuous exposure above 85 dBA causes gradual permanent damage
• Brief exposure above 120 dBA causes pain
• Above 140 dBA: immediate permanent damage possible

For building designers: The primary concerns are protecting occupants from harmful noise levels and providing acoustic conditions appropriate for the intended activities.

Noise Control — Defending Against Unwanted Sound

the practitioner's first job, years before the concert hall, was soundproofing apartments above a nightclub. "That project taught me," she said, "that noise control is a war, and every gap in your armour is a total defeat."

Noise Criteria

Background noise levels appropriate for different spaces:

Sound Transmission Loss (TL)

When sound hits a wall or partition, some is reflected, some is absorbed, and some is transmitted through. The Transmission Loss (TL) measures how much is stopped:

TL = 10 × log(1/τ) where τ is the transmission coefficient

The Mass Law: For a single-leaf partition, TL increases approximately 6 dB for every doubling of mass per unit area, and approximately 6 dB for every doubling of frequency.

The Weak Link Principle: Sound transmission is governed by the weakest element. A wall with STC 50 that has a gap of 1% of its area drops to effectively STC 30. Doors, windows, service penetrations, and gaps are where noise control succeeds or fails.

Flanking Transmission

Sound doesn't only go through walls — it goes around them:

• Through the floor slab connecting two rooms
• Through the ceiling plenum (above a dropped ceiling)
• Through ductwork connecting spaces
• Through structural elements that bypass the partition

The lesson: A partition is only as good as the weakest flanking path. You must address every route sound can take between spaces.

Impact Sound

Footsteps, dropped objects, and machinery create vibrations that travel directly through the building structure. Impact sound is controlled by:

• Soft surface coverings (carpet, resilient flooring) — absorb the initial impact
• Floating floors — a structural slab with a resilient layer supporting the finish floor
• Isolated ceilings — hung on resilient hangers below the structural slab

Room Acoustics — Shaping Sound Inside Spaces

Back to the practitioner's concert hall. The problem wasn't noise from outside — it was what happened to sound inside. The room was killing the music.

Reverberation: The Persistence of Sound

When a sound source stops, the sound doesn't disappear instantly — it bounces off surfaces, gradually losing energy with each reflection. Reverberation time (RT₆₀) is the time for sound to decay by 60 dB after the source stops.

RT₆₀ = 0.161 × V / A

(Sabine formula)

Where:

• V = room volume (m³)
• A = total sound absorption in the room (m² sabins = Σ surface areas × absorption coefficients)

Ideal reverberation times depend on room use and volume:

the practitioner's concert hall had an RT₆₀ of 0.8 seconds — perfect for a lecture theatre, fatal for symphonic music. The hard surfaces the architect chose for their visual drama were absorbing almost no sound, but the room was too small for the reflections to build up properly. It was the worst combination: not reverberant enough for music, not dry enough for speech.

Absorption Coefficients

The absorption coefficient (α) of a surface measures the fraction of incident sound energy that is not reflected. It varies with frequency:

Design principle: Use absorptive materials (high α) to reduce reverberation and control noise. Use reflective materials (low α) to project sound and create liveliness. The art is in the balance.

Room Shape and Sound Distribution

The geometry of a room profoundly affects what listeners hear:

• Concave surfaces (domed ceilings, curved walls) focus sound, creating "hot spots" and "dead zones" — generally to be avoided
• Convex surfaces scatter sound, promoting even distribution — generally desirable
• Parallel walls create flutter echoes — can be broken by angling one wall 5–8°
• Large flat rear walls cause delayed reflections that interfere with the direct sound — should be treated with absorption

RESOURCES — Energy, Water, and the Survival Question

The Mathematics of Whether Civilization Continues

Energy — The Currency That Runs Everything

Twelve years into her career, the practitioner stood at a conference podium. "I spent the first years learning to design buildings that work," she told the audience. "Now I understand something harder: the energy that flows through every building I design is the same energy that's changing our climate. The building isn't just a thermal filter — it's an ethical statement."

Energy: One Unit to Rule Them All

Energy is the potential for doing work, measured in joules (J). Power is energy flow rate, measured in watts (W = J/s).

The practical energy unit: kilowatt-hour (kWh)

1 kWh = 3,600,000 J = 3.6 MJ

This is what appears on electricity bills worldwide. It's the energy consumed by a 1,000 W appliance running for one hour.

Conversion from legacy units:

The Power of Wind: A Physics Lesson

Wind power is proportional to velocity cubed:

P_wind = ½ × A × ρ × v³

Where A is the swept area, ρ is air density (≈1.2 kg/m³), and v is wind speed.

This means: doubling wind speed increases power 8×. A location with 8 m/s average wind has 8 times the wind energy of a 4 m/s location — not double.

The Energy Crisis: What the Numbers Say

The world consumes oil at the rate of roughly 71 million barrels per day. Total estimated reserves are approximately 960 billion barrels. The simple division (the "static index") gives about 37 years.

But it won't happen that way. As supplies diminish, prices rise, consumption falls, and alternatives become competitive. The real danger isn't running out — it's the climate damage from burning what we have.

Electricity: Humanity's Most Versatile Energy Form

Electricity generation from fossil fuels involves a triple conversion: chemical → thermal → mechanical → electrical. The overall efficiency is typically 30–35% — meaning 65–70% of the fuel's energy is wasted as heat.

Conversion efficiencies of various systems:

Cogeneration (Combined Heat and Power) captures the "waste" heat from electricity generation for useful heating, achieving 70–80% overall efficiency — more than double a conventional power station.

Renewable Energy — The Only Long-Term Answer

Solar Energy: Direct Applications

Solar thermal systems — Low-temperature applications:

Passive vs. Active distinction:

• Passive system: CoP > 50 (energy delivered / energy consumed by the system itself)
• Hybrid system: 20 < CoP < 50
• Active system: CoP < 20

Photovoltaic (PV) systems convert sunlight directly to electricity:

Wind Energy

Wind turbines convert kinetic energy to electricity. Key metrics:

• Capacity factor: Actual output / theoretical maximum (typically 25–40%)
• Cut-in speed: Minimum wind speed to start generating (typically 3–4 m/s)
• Rated speed: Wind speed at maximum output (typically 12–15 m/s)
• Cut-out speed: Maximum safe operating speed (typically 25 m/s)

Large modern turbines can reach 5–15 MW capacity with rotor diameters exceeding 150 m.

Hydroelectric Power

Potential energy of elevated water:

E_p = M × g × h

Where M is mass (kg), g is gravitational acceleration (9.81 m/s²), and h is height difference (m).

1 m³ of water falling 100 m: E = 1000 × 9.81 × 100 = 981,000 J = 981 kJ ≈ 0.27 kWh

If that water flows at 1 m³/s, the power output is 981 kW (before efficiency losses).

Energy Use in Buildings — Where the Waste Lives

Buildings account for approximately 42% of all energy consumption globally. This breaks down into:

• Heating and cooling: 50–70% of building energy in most climates
• Lighting: 15–25%
• Hot water: 10–20%
• Equipment and appliances: 10–20%

Energy Rating Systems

Buildings worldwide are increasingly rated for energy performance:

CO₂ emission conversion factors for building energy:

Water, Wastes, and the Full Picture

Water in Buildings

Water consumption in buildings includes:

• Domestic/sanitary use: Toilets, showers, hand washing
• Mechanical systems: Cooling towers, boilers
• Irrigation and maintenance

Water conservation strategies:

• Low-flow fixtures and dual-flush toilets (can reduce consumption by 30–50%)
• Rainwater harvesting for non-potable uses
• Greywater recycling (shower/basin water reused for toilet flushing/irrigation)
• Waterless urinals

Waste Management in Building Design

Buildings generate approximately 25% of solid wastes and 20% of liquid wastes.

Design for waste reduction:

• Specify materials with recycled content
• Design for disassembly (bolted connections rather than adhesive)
• Minimize off-cuts through modular dimensioning
• Provide adequate space for waste sorting and recycling
• Compost organic waste on-site where possible

Sustainability — The Architect's Responsibility

the practitioner's career had taken her from that failed clinic to designing net-zero-energy buildings across three continents. But the final lesson was the hardest.

"Sustainability isn't a technology," she told a graduating class. "It's a commitment. Every building you design is a 50–100 year bet on the future. Make it count."

The Brundtland Definition

"Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs."

What Buildings Contribute to the Problem

Every one of these can be strongly influenced by architects and designers.

Embodied Energy: The Hidden Cost of Materials

The energy used to extract, manufacture, transport, and install building materials is the embodied energy (or Process Energy Requirement, PER):

The Operational/Capital energy ratio: In the 1970s, a building used as much energy in operation over 5 years as was embodied in its construction (O/C ratio = 5). Today, improved buildings show ratios of 30–50 — meaning better operations have shifted focus toward reducing embodied energy.

The Four Pillars of Sustainable Design

1. Site:

• Preserve undisturbed land where possible
• Use already-disturbed or derelict land
• Prevent soil erosion
• Promote higher density to reduce sprawl and transport

2. Energy:

• Maximize passive strategies first
• Right-size active systems
• Use renewable energy sources
• Target net-zero operational energy

3. Materials:

• Prefer low-embodied-energy materials
• Use renewable, plantation-sourced timber
• Specify recycled content
• Design for longevity, adaptability, and eventual disassembly

4. Wastes:

• Minimize construction waste through modular design
• Design for on-site water recycling
• Provide for waste sorting in operation
• Retain stormwater on-site

Building Materials Assessment

A comprehensive evaluation uses 14 criteria grouped across the building lifecycle:

Your Next Steps

If you're a beginner: Start with one thing — master the U-value calculation. Understand how heat flows through one wall. Then expand to the whole building.

If you're a practitioner: Audit your last project. How much of the heating/cooling load was caused by design decisions you could have changed? What would you do differently?

If you're making decisions about buildings: Ask your architect one question: "What is the building's annual energy consumption per square metre, and how does that compare to best practice?"

If they can't answer, you need a different architect.

Quick Reference: Essential Formulas

Complete Glossary of Key Terms

The building is not just a shelter. It is a selective filter — excluding the unwanted, admitting the desirable. Master heat, light, sound, and energy, and you don't just design buildings. You design the conditions for human life.

What question about your building's performance will you investigate first? Share in the comments below — or better yet, pick up a thermometer, a light meter, or a sound level app and start measuring. The physics is already happening. Your job is to make it work for the people inside.

Based on the principles presented in "Introduction to Architectural Science: The Basis of Sustainable Design" by Steven V. Szokolay — the foundational text bridging building physics with sustainable practice.