How does the thickness of a layer affect its U-value?

A thicker layer of the same material has a lower U-value, because heat must travel further through it. A single layer of double the thickness has roughly half the U-value.

How is the U-value of a wall made of several layers found?

The thermal resistance of each layer is found and the resistances are added together. The U-value is the reciprocal of the total, U = 1 ÷ ΣR.

A 0.10 m layer of mineral wool has a thermal conductivity of 0.040 W/mK. What is its thermal resistance?

The layer's thermal resistance is 2.5 m²K/W. The working is thickness ÷ conductivity = 0.10 ÷ 0.040 = 2.5 m²K/W.

CCEA A-Level Environmental Technology 3930

Fossil Fuels

Topic overview Coal, oil and gas in modern society Reserves, lifespan and accessibility Fossil fuel combustion and the IPCC evidence Carbon trading schemes Fossil fuel demand, emerging economies and fuel security Environmental impacts of fossil fuel use

Global Action on Energy Conservation and Climate Change

Topic overview Sustainable development and climate agreements Emissions and energy efficiency targets for 2020 Renewable energy and demand reduction in policy Nuclear power as an alternative

Macrogeneration

Topic overview Conservation of energy and the simple generator Thermal power stations and steam turbines Direct and indirect renewable electricity Electricity transmission through the National Grid Smart grids and interconnectors Combined heat and power

Microgeneration

Topic overview Microgeneration technologies Benefits of home and community microgeneration The heat pump cycle and coefficient of performance Air source and ground source heat pumps

Energy From Wind

Topic overview Horizontal and vertical axis wind turbines Kinetic energy, wind power and the Betz limit Power curves, rated output and energy losses Swept area, blade length and turbine mass Air density, hub height and turbine siting Power control, yawing and survival speed Choosing a location for a commercial wind farm

Energy From the Sun

Topic overview Solar radiation, tracking and concentrating solar power Flat plate and evacuated tube collectors Sizing a domestic solar thermal system Photovoltaic cells and semiconductor wafers Types of photovoltaic module compared Planning regulations and solar incentives Passive solar design in new and existing buildings

Energy From Biomass

Topic overview Defining biomass and its main categories Energy crops grown for biomass Advantages and disadvantages of biomass fuel Combustion, woodchip and fuel quality Gasification and syngas Biogas and commercial anaerobic digesters

Energy Storage

Topic overview Intermittency and reliability of renewable supply Why renewable energy needs storing Pumped hydro and compressed air energy storage Where energy storage is most beneficial

Plastics for the Future

Topic overview Crude oil as a fuel and a feedstock Refinery gases and the making of polymers Persistent plastic waste and the Great Pacific Garbage Patch Toxic gases from incinerating plastics Biodegradable, photodegradable and compostable plastics Testing photodegradable plastic against polythene Bio-derived polyethylene and a sustainable plastics future

Renewable Energy Investigations

Topic overview Desktop research on renewable energy technologies Choosing an investigation and its variables Apparatus and risk assessment for energy practicals Accuracy and reliability of energy measurements Presenting and analysing energy data Evaluating a method and site-based recommendations

Sustainable Development

Topic overview Population growth and demand for resources The I = PAT equation and the role of technology Ecological footprints, carbon footprints and One Planet Living

Waste Management

Topic overview Landfill dependence and the landfill tax Waste types, priority waste streams and EU directives The waste management hierarchy Recycling at a Materials Recovery Facility Landfill decomposition, methane and leachate Engineered landfill: dry tomb and bioreactor designs

Waste to Energy Technologies

Topic overview Embodied energy Incineration with energy recovery Digester design: solids content, residence time and staging Stages of anaerobic digestion and biogas CHP Composting and catering waste

Land Contamination and Remediation

Topic overview Contaminated land, risk and traditional remediation Bioremediation principles and bioreactor systems Micro-organisms that degrade named pollutants Bioremediation benefits and genetically engineered microbes Phytoremediation of metals and other elements Phytoextraction with Indian mustard and alpine pennycress Biohydrometallurgy and phytomining of ores

Hydrogen Fuel Cells

Topic overview Hydrogen as a fuel: properties and storage Hydrogen production: steam reforming, electrolysis and photocatalysis How a fuel cell works and the PEM cell Alkaline, phosphoric acid, molten carbonate and solid oxide fuel cells Fuel cell applications and the challenges of hydrogen

Transport System Challenges

Topic overview Four key challenges for future transport New vehicle technologies and their infrastructure Bioethanol production and the heat energy of ethanol Biodiesel and the biofuel debate Strategies to reduce transport demand

Energy Performance of Buildings

Topic overview Energy conservation and residents' behaviour Insulation, air tightness and glazing U-values and the heat flow equation The zero carbon homes hierarchy BREEAM, the Code and upgrading existing homes

Emerging Technologies

Topic overview Tidal stream and tidal barrage power Wave energy converters Environmental impacts of wave and tidal energy Smart materials and smart systems Carbon capture and storage Geo-engineering, direct air capture and bio-photovoltaics

Sustainable Urban and Rural Communities

Topic overview Energy and buildings in sustainable urban developments Transport, waste and brownfield sites Water, drainage and urban green space Energy, water and waste for isolated rural dwellings Local food and communication technology in rural areas

The Code for Sustainable Homes

Topic overview Purpose, levels and assessment of the Code Energy, carbon dioxide and water categories Materials, surface water run-off and waste categories Pollution, health and wellbeing, management and ecology categories Measuring the performance of an existing building Performance profiles, improvements and evaluation

Energy Performance of Buildings

U-values and the heat flow equation

3 things to know
A2 1

Overview

The unit and meaning of U-value, Q = U × A × ΔT, conductivity, thickness and layers, thermal resistance, typical U-values for glazing and walls, worked window, wall, roof and whole-house heat loss calculations, and the equation's limits.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Give the unit of U-value and explain what it measures.

Key term

State the equation linking rate of heat flow, U-value, area and temperature difference.

Key term

Why do different construction materials have different U-values?

Questions from this subtopic

Each one with the model answer underneath.

1
Give the unit of U-value and explain what it measures.
Model answer
The unit of U-value is W/m²K. A U-value is the rate of heat flow through 1 m² of an element for each kelvin of temperature difference.
2
State the equation linking rate of heat flow, U-value, area and temperature difference.
Model answer
Q = U × A × ΔT, so heat flow equals the U-value multiplied by the area and the temperature difference. Q is in watts when U is in W/m²K, A in m² and ΔT in °C or K.
3
Why do different construction materials have different U-values?
Model answer
Materials conduct heat at different rates, measured by their thermal conductivity. Dense materials such as concrete and metal conduct well, while materials that trap air conduct poorly.
4
How does the thickness of a layer affect its U-value?
Model answer
A thicker layer of the same material has a lower U-value, because heat must travel further through it. A single layer of double the thickness has roughly half the U-value.
5
How is the U-value of a wall made of several layers found?
Model answer
The thermal resistance of each layer is found and the resistances are added together. The U-value is the reciprocal of the total, U = 1 ÷ ΣR.
6
A 0.10 m layer of mineral wool has a thermal conductivity of 0.040 W/mK. What is its thermal resistance?
Model answer
The layer's thermal resistance is 2.5 m²K/W. The working is thickness ÷ conductivity = 0.10 ÷ 0.040 = 2.5 m²K/W.

See the rest and revise

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.

1Closed book

2Open book

3Closed book

The 3-step method, run for you

Answer cold, learn it, then prove it stuck. Levo tracks the jump between your first attempt and your last.

Fossil Fuels

Global Action on Energy Conservation and Climate Change

Macrogeneration

Microgeneration

Cram a whole paper in one sitting

Sort every topic into what you know and what you do not, then drill only the weak ones.

Smart glass can switch from clear to .

tintedliquidbrittle

Blank Blitz makes facts automatic

Rapid timed gap-fills on the terms and numbers you need to recall without thinking.

AThe coil can be rotated faster.

BA stronger magnet can be used.

CThe coil can be given more turns of wire.

Build It gets the exam shape right

Assemble a full-mark answer from its marking points, in the order an examiner wants them.

Hazard means anything with the potential to cause harm.

TrueFalse

Test It catches misconceptions

Spot the fault in a statement. The traps are the mistakes students actually make.

721941163128510

Brain Breaks between sessions

Short focus games to reset attention, so the next set lands better than the last.

Every question in Energy Performance of Buildings, on a schedule.

Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.

Start revising, £5 a month See how the method works

Or £46 a year, about £3.83 a month. Cancel anytime.