Building Services Engineering Systems Handbook: Thermal Conductivity of Common Building Materials

Thermal Conductivity of Common Building Materials

Surface and Air Space Resistances

Heat transfer at surfaces and across air cavities adds additional resistance to heat flow:

Inside Surface Resistance (Rsi):

Outside Surface Resistance (Rso):

Exposure Categories:

• Sheltered: Up to 3rd floor of buildings in city centres
• Normal: Most suburban and rural buildings; 4th to 8th floors in city centres
• Severe: Coastal or hill sites; above 5th floor suburban; above 9th floor city centre

Air Space Resistances:

Calculating U-Values: Step-by-Step Method

The total thermal resistance of a composite element is the sum of all individual resistances:

R_total = Rso + R₁ + R₂ + R₃ + ... + Rn + Ra (if air cavity) + Rsi

U = 1 / R_total   [W/m²K]

Worked Example: Cavity Wall U-Value

Consider a cavity wall with:

• 102.5mm outer brick leaf (λ = 0.84)
• 50mm air cavity
• 100mm lightweight concrete block (λ = 0.19)
• 13mm lightweight plaster (λ = 0.16)
• Normal exposure

Rso = 0.06 m²K/W
R_brick = 0.1025/0.84 = 0.122 m²K/W
R_cavity = 0.18 m²K/W
R_block = 0.100/0.19 = 0.526 m²K/W
R_plaster = 0.013/0.16 = 0.081 m²K/W
Rsi = 0.12 m²K/W

R_total = 0.06 + 0.122 + 0.18 + 0.526 + 0.081 + 0.12 = 1.089 m²K/W

U = 1/1.089 = 0.918 W/m²K

Adding 50mm of polyurethane insulation (λ = 0.025) to the cavity:

R_insulation = 0.050/0.025 = 2.000 m²K/W
R_total = 1.089 + 2.000 = 3.089 m²K/W (cavity air resistance replaced by insulation)
Actually: R_total = 0.06 + 0.122 + 2.000 + 0.526 + 0.081 + 0.12 = 2.909 m²K/W
U = 1/2.909 = 0.344 W/m²K

That single layer of insulation reduced the U-value by 63%. This is where the money is.

Building Heat Loss Calculation

Total building heat loss has two components:

1. Fabric Heat Loss (through the building envelope):

Q_fabric = Σ(U × A × ΔT)   [Watts]

Where:

• U = thermal transmittance of each element (W/m²K)
• A = area of each element (m²)
• ΔT = temperature difference between inside and outside (K)

2. Ventilation Heat Loss (through air movement):

Q_ventilation = Cv × ΔT   [Watts]

Where:
Cv = (N × V) / 3   [W/K]

Or:
Cv = ρ × SHC × (volume flow rate)
Cv = 1.205 × 1.012 × (N × V / 3600)

• N = air changes per hour
• V = room volume (m³)
• ρ = air density (1.205 kg/m³ at 20°C)
• SHC = specific heat capacity of air (1.012 kJ/kgK)

Total Heat Loss:

Q_total = Q_fabric + Q_ventilation = Σ(UA) × ΔT + Cv × ΔT = [Σ(UA) + Cv] × ΔT

Boiler Power Sizing

The total boiler power must account for:

Total Boiler Power = Building Heat Loss + Hot Water Heat Load + Intermittent Use Margin

P_boiler = Q_total × F₁ × F₂ + P_hotwater

Where:

• F₁ = intermittent heating factor (typically 1.15 to 1.25 for buildings heated for less than 24 hours)
• F₂ = margin factor for future capacity (typically 1.10)
• P_hotwater = hot water storage recovery power requirement

Hot water storage power:

P_hw = (m × SHC × ΔT) / (recovery time in seconds)   [kW]

Where:

• m = mass of stored hot water (kg)
• SHC = specific heat capacity of water (4.186 kJ/kgK)
• ΔT = temperature rise required (typically 50°C for cold supply to 60°C storage)

Your Takeaway: A building's heat loss calculation is the single most consequential number in the entire design process. Oversize the boiler, and you waste capital and energy for decades. Undersize it, and occupants freeze on the coldest days of the year. Get it right, and everything downstream — radiators, pipes, pumps, controls — all fall into their optimal positions.

Heating Systems — Choosing the Right Weapon Against Cold

Classification of Heat Emitters

Hot-Water Heating System Classifications

Pipe System Configurations

One-pipe system: Single pipe loop, each radiator connected in series. Later radiators receive cooler water. Simple but uneven heating distribution.

Two-pipe system: Separate flow and return pipes. Each radiator receives water at the same temperature. Better control but more pipework.

Three-pipe system: Two flow pipes (one for heating, one for cooling or domestic hot water) and one shared return. Used in buildings with simultaneous heating and cooling needs.

Four-pipe system: Separate flow and return for both heating and cooling. Full independent control of both systems. Most expensive but most versatile.

Microbore system: Small diameter (6-12mm) copper pipes from manifolds to individual radiators. Quick installation, minimal water content, rapid response.

Pipe Sizing for Hot Water Heating

The water flow rate required to deliver a specific heat output:

ṁ = Q / (SHC × ΔT)   [kg/s]

Where:
Q = heat to be delivered (kW)
SHC = specific heat capacity of water (4.186 kJ/kgK)
ΔT = temperature drop across the circuit (typically 10-20K)

The pump must overcome the pressure drop through the entire circuit:

Pump head = Σ(pressure drops through pipes, fittings, valves, heat emitters, boiler)

Pressure drop per metre of pipe depends on:
- Flow rate
- Pipe diameter
- Pipe material (roughness)
- Water temperature (viscosity)

Typical Pipe Sizing Parameters

Combustion: The Chemistry That Powers It All

When fossil fuels burn, the following reactions occur:

Carbon:      C + O₂ → CO₂ + Heat
Hydrogen:    2H₂ + O₂ → 2H₂O + Heat
Sulphur:     S + O₂ → SO₂ + Heat (undesirable)

Natural gas (methane):

CH₄ + 2O₂ → CO₂ + 2H₂O + Heat (38.7 MJ/m³)

Stoichiometric air requirement: The theoretical minimum air needed for complete combustion. In practice, 10-50% excess air is supplied to ensure complete combustion, depending on the fuel and burner type.

Flue Design Requirements

Building Energy Management Systems (BEMS)

For the practitioner's 12-storey building, manual control of heating was not an option. A BEMS provides:

• Optimum start control: Calculates the latest possible start time for heating to achieve comfort by occupancy time, based on outdoor temperature and building thermal mass
• Weather compensation: Adjusts heating water temperature based on outdoor conditions
• Zone control: Independent temperature control for different areas
• Monitoring and alarming: Continuous performance data and fault detection
• Remote access: Management from any location via network connection
• Energy monitoring: Tracking consumption against targets
• Trend logging: Historical data for analysis and optimization

Geothermal Heating: Energy From the Earth

the practitioner proposed a ground-source heat pump system for the building's base heating load. The principle:

Coefficient of Performance (COP) = Heat output / Electrical input

Typical COP for ground-source heat pumps: 3.0 - 5.0

This means that for every 1 kW of electricity consumed, the heat pump delivers 3-5 kW of heat by extracting energy from the ground.

Ventilation and Air Conditioning — Engineering the Air You Breathe

Ventilation Requirements: Why Fresh Air Is Not Optional

The human body produces carbon dioxide (CO₂), moisture, and biological effluents that must be continuously diluted and removed. Without adequate ventilation:

• CO₂ concentration rises above 0.1% (1000 ppm), causing drowsiness and headaches
• Moisture accumulates, causing condensation and mould growth
• Pollutants from building materials, furnishings, and cleaning products concentrate to harmful levels

Ventilation Rate Calculation from CO₂ Production

Q = n / (Cr - Cs) × 100   [l/s]

Where:
n = CO₂ production rate per person (l/s)
Cr = maximum permitted room concentration (typically 0.1% = 0.001)
Cs = supply air CO₂ concentration (typically 0.04% = 0.0004)

The Four Combinations of Ventilation

Air-Conditioning Systems: Complete Environmental Control

Air conditioning provides simultaneous control of:

1. Temperature (heating and cooling)
2. Humidity (humidification and dehumidification)
3. Air purity (filtration)
4. Air movement (distribution)

Psychrometric Processes

The psychrometric chart is the engineer's primary tool for air conditioning design. It plots the properties of moist air:

Key Air-Conditioning Processes

1. Sensible Heating (warming air without moisture change):

Q = ṁ × SHC × (t₂ - t₁)   [kW]
Where ṁ = mass flow rate of air (kg/s)

2. Sensible Cooling (cooling air without moisture change):

Q = ṁ × SHC × (t₁ - t₂)   [kW]

3. Humidification (adding moisture):

Steam injection: adds moisture at constant dry-bulb temperature
Water spray: adds moisture with evaporative cooling

4. Dehumidification (removing moisture):

Cool air below its dew point → water condenses out → reheat to desired temperature
Q_cooling = ṁ × (h₁ - h₂)   [kW]   (using enthalpies from psychrometric chart)

Types of Air-Conditioning Systems

Vapour Compression Refrigeration Cycle

The heart of every air-conditioning system is the refrigeration cycle:

1. COMPRESSOR: Low-pressure gas → High-pressure gas (work input)
2. CONDENSER: High-pressure gas → High-pressure liquid (heat rejected)
3. EXPANSION VALVE: High-pressure liquid → Low-pressure liquid/gas mix (pressure drop)
4. EVAPORATOR: Low-pressure mix → Low-pressure gas (heat absorbed from conditioned space)

Coefficient of Performance (COP):

COP = Cooling effect (kW) / Compressor power input (kW)

Typical values:
- Window/split units: COP 2.5-3.5
- Central chillers (reciprocating): COP 3.0-4.0
- Central chillers (screw): COP 4.0-5.5
- Central chillers (centrifugal): COP 5.0-7.0

Heat Gain Calculations for Cooling Load

Sick Building Syndrome (SBS)

the practitioner encountered a recurring concern from the property management consultants: would the building make people sick?

SBS symptoms include headaches, eye irritation, dry throat, fatigue, and difficulty concentrating. Causes include:

Chlorofluorocarbons (CFCs) and Refrigerant Management

Your Takeaway: If you are specifying or maintaining air-conditioning equipment, ensure refrigerant selection complies with current regulations AND anticipates future phase-downs. HFC refrigerants with high GWP values are facing increasing restrictions globally.

Hot- and Cold-Water Supplies — The Lifeblood of Every Building

Water Quality: The Starting Point

Before designing any water system, understand what comes out of the tap:

Cold Water System Types

Direct System (Mains Pressure):

• Water supplied directly from the main to all outlets
• Provides drinking water quality at all taps
• Subject to mains pressure variations
• Suitable for buildings up to 3-4 storeys (depending on mains pressure)

Indirect System (Storage Tank):

• Cold water storage tank at high level (roof or loft)
• Mains supplies the tank; gravity feeds outlets
• Only the kitchen tap is directly supplied from the main
• Provides reserve supply during mains interruption
• Reduces pressure variations

Pressure Boosted System (Tall Buildings):

• Required when building height exceeds mains pressure capability
• Options include: pump + pressure vessel, break tank + pump set, or pneumatic pressure system
• Zones may be created for different floor groups

Cold Water Storage Requirements

Hot Water Systems

Centralized System:

• Single boiler or heat source supplies all hot water
• Distribution through insulated pipework
• Primary circuit: boiler ↔︎ storage cylinder
• Secondary circuit: storage cylinder → outlets → return to cylinder
• Best for large buildings with concentrated demand

Decentralized System:

• Local heaters at point of use
• No distribution losses
• Instantaneous (gas multipoint or electric) or small storage units
• Best for scattered outlets with low simultaneous demand

Instantaneous vs. Storage:

Hot Water Heater Power Calculation

P = (m × SHC × ΔT) / t   [kW]

Where:
m = mass of water to be heated (kg)  [1 litre = 1 kg]
SHC = 4.186 kJ/kgK
ΔT = temperature rise (typically 50°C: from 10°C mains to 60°C storage)
t = recovery time (seconds)

Pipe Sizing: The Demand Unit Method

Rather than calculating exact simultaneous flow rates, engineers use demand units (DU) — a weighted probability system:

The total demand units are converted to a probable simultaneous flow rate using published charts, which account for the statistical likelihood that not all appliances operate at the same time.

Pipe Pressure Loss Calculation

Available pressure = Static head ± Mains pressure - Fitting losses - Pipe friction losses

Static head: 1 metre of vertical water = 9.807 kPa (approximately 10 kPa)

Pipe friction loss is found from pipe sizing charts using:
- Flow rate (l/s)
- Pipe diameter (mm)
- Pipe material (copper, plastic, steel)

Equivalent length method for fittings:

Solar Hot Water Heating

For the practitioner's project, the architect specified rooftop solar thermal panels for the residential floors:

Flat-plate collector:

• Absorber plate (blackened copper) + glazing + insulation
• Typical efficiency: 30-50% of incident solar energy
• Best for domestic hot water pre-heating

Evacuated tube collector:

• Higher efficiency (40-70%) especially in cold or cloudy conditions
• More expensive per m² but better performance per m²
• Suitable for higher-temperature applications

Solar energy collected per year (simplified):
E = A × I × η × days   [kWh/year]

Where:
A = collector area (m²)
I = average daily solar radiation on collector plane (kWh/m²/day)
η = collector efficiency (decimal)
days = days of useful solar contribution per year

Soil and Waste Systems — What Goes Down Must Go Right

How Trap Seals Are Lost

The Physics of Waste Pipe Flow

Waste pipes do not flow full (under pressure) — they flow partially full, with air sharing the pipe. This creates a complex three-phase flow:

• Water phase: The waste fluid flowing along the bottom and walls
• Air phase: Air travelling with and around the water
• Plug phase: Complete blockage of the pipe cross-section by a slug of water (creates pressure surges)

Critical Design Parameters

Waste Pipe Sizing

Discharge Unit Pipe Sizing

For multiple appliances, the discharge unit (DU) method is used:

Stack diameter is selected from total DU capacity charts. For the most common scenario:

Testing Drainage Systems

Surface-Water Drainage — Taming the Rain Before It Tames You

Rainfall Flow Rate Calculation

Q = Area drained (m²) × Rainfall intensity (mm/h) × Impermeability factor

Standard design rainfall intensity: 75 mm/h for roofs; 50 mm/h for ground surfaces

Ground Impermeability Factors

Gutter Sizing

Gutter capacity depends on cross-sectional area, depth of flow, and gradient:

Flow capacity of a level gutter:
Q = 2.78 × 10⁻⁵ × Ao × (2g × D)^0.5   [litres/second]

Where:
Ao = cross-sectional area at outlet (mm²)
D = depth of water at outlet (mm)
g = gravitational acceleration (9.807 m/s²)

For gutters with a fall (slope), capacity increases by approximately 20-40% depending on gradient.

Gutter and Downpipe Sizing Table

Soakaway Pit Design

Where connection to a surface-water sewer is not available, rainwater can be disposed of via soakaway pits (infiltration systems):

Storage volume required = Peak rainfall volume - Infiltration during storm

V = Q × storm duration - (Percolation rate × pit surface area × storm duration)

Below-Ground Drainage — The Silent Infrastructure Nobody Sees

Design Principles

Below-ground drainage operates entirely by gravity (wherever possible), requiring:

• Even gradients maintained throughout
• Full accessibility for clearing blockages
• Protection from building loads and traffic
• Watertight joints resistant to root penetration
• Connection to the public sewer system

Access Requirements

Drain Pipe Sizing Using Discharge Units

External Loads on Buried Pipelines

Pipes must withstand:

• Dead load: Weight of backfill material above the pipe
• Live load: Traffic or other surface loading

Total load on pipe = Dead load (from backfill) + Live load (from surface traffic)

Factor of safety required: Minimum 1.25 (rigid pipes)

Sewage Lifting

When parts of the building are below the sewer invert level, pumped systems are required:

Condensation in Buildings — The Hidden Destroyer Inside Your Walls

The Physics of Condensation

Air contains water vapour as an invisible gas. The amount of moisture air can hold depends on its temperature — warmer air holds more moisture. When air cools to its dew-point temperature, it becomes saturated and water vapour condenses into liquid water on the nearest surface.