Introduction
The thermal performance of a building’s fabric plays an important role in reducing heat loss, which in turn lowers space heating requirements, reduces carbon emissions, and enhances the thermal comfort of occupants. Achieving good thermal performance, can not only lower heating demands but also create spaces that are more sustainable and resilient.
Designing a building’s fabric requires careful consideration of multiple factors from material selection and construction techniques to addressing issues like thermal bridging. Understanding these is key to creating energy-efficient and environmentally responsible buildings.
In this post, we will explore these factors in detail and examine what needs to be done to achieve excellent thermal performance.
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Heat Transfer
First, we will look at how heat can be transferred.
Heat transfer is often described as the exchange of thermal energy between physical systems (in our case the envelope of the building), depending on the temperature and pressure. The transferred thermal energy is the ‘heat.’ In terms of direction, heat transfer always occurs from a warm area to a colder one. It seeks to form a balance of energies across the system.
It is a crucial factor in buildings as it can help determine the overall efficiency of the building.
Heat transfer in buildings can occur through three main processes:
Conduction
Conduction transfers heat by molecular collision and is the most common form of heat transfer that occurs via physical contact. It happens without the movement of the objects involved. An example of conduction would be placing metal into an open flame. Without any movement, even the parts of the metal that are not in contact with the flame, will eventually become hot.
Convection
Convection is the transfer of heat through a material by the body movement of particles. This occurs with fluids and gases but not in solids. An example of heat transfer through convection can be explained using a space heater. The heater heats the air surrounding it, this air slowly increases in temperature, expands, and rises to the top of the room. This upward movement of the now heated air forces the cooler air that existed at the top, to move down so that it too can be heated. This creates a convection current.
Radiation
Radiation is the transfer of heat energy through electromagnetic waves. Unlike conduction and convection, radiation does not require a medium (like air or water) to transfer heat and it can occur in a vacuum. The sun is a good example of heat radiation that transfers heat across the solar system.
K-values, R-values, and U-values
In order to measure the effectiveness of the materials we specify and the suitability of a building assembly, we need to consider a number of values. These include K-values, R-values and U-values which represent the thermal properties of building materials and structures.
K-Value
The K-value, also called the thermal conductivity or lambda (λ)-value, measures how easily heat can travel through a material by conduction. It is expressed in units of W/mK (watts per meter per Kelvin).
The lower the K-value, the better the insulation property of a material, as it resists the transfer of heat more effectively.
In most cases, the manufacturers of the materials you are specifying will provide the thermal conductivity value of those materials. If not, you can refer to the table we have provided.
The R-value, or thermal resistance, measures the ability of a material to resist heat flow. It is simply the thickness of a material divided by its K-value and has units of m²K/W (square meters Kelvin per watt).
The thickness and conductivity of a material contribute to the overall thermal resistance of a construction. In a building assembly, the thermal resistance for each material is measured to get the overall thermal resistance of the composite.
The higher the thermal resistance, the better the insulator. For instance, rock wool insulation has a high thermal resistance therefore it is a good insulator, while concrete has a low thermal resistance, making it a bad insulator.
The U-value, or thermal transmittance is the measure of the overall rate of heat transfer through a particular section of the construction, like a wall, roof, or window.
It is calculated as the reciprocal of the total R-value for the assembly. The unit for the U-value is W/m²K (watts per square meter per Kelvin).
The lower the U-value the better thermal performance as it means less heat flows through the structure. This means the material is better at keeping heat inside.
You can read more about U-values and look at some supporting documentation and calculations by going to our U-value guide:
A Quick and Easy Guide to U-Values
In the UK, U-values are a critical metric for energy efficiency compliance, with regulations specifying maximum permissible U-values for different building components to ensure minimal heat transfer. The Approved Document Part L: Conservation of Fuel and Power (Volume 1: Dwellings) outlines the current requirements for U-values in notional dwellings (new builds).
As of the latest update, the recommended U-values to aim for in a new dwelling are shown below:
Table adapted from Approved Document Part L: Conservation of Fuel and Power (Volume 1: Dwellings).
If the building you are designing is not a dwelling please refer to the Building Regulations Part L: Conservation and Power (Volume 2: Buildings other than dwellings). Also, refer to Building Regulations Part L: Conservation and Power (Volume 1: Dwellings) for extension and retrofit U-value requirements.
For the most up-to-date information and detailed requirements, please refer the UK government’s official website for the Approved Documents:
Conservation of fuel and power: Approved Document L – GOV.UK
Insulation
Specifying the correct insulation material for your project will have a large impact on energy performance, along with providing environmental benefits.
A well-insulated building will not only reduce the heating requirements in the winter, but also help to keep the building cool in summer. This is as long as ventilation and solar gain are also well controlled. For larger buildings it is often the case that more energy is used in summer cooling rather than winter heating.
Well-designed insulation also reduces the risk of surface condensation because the internal surfaces are kept at room temperature which is above the dew point.
Common insulation materials include fiberglass, mineral wool, cellulose, and foam boards. Each of these has unique properties and applications.
Building Form
Designing a building to have less and smaller surfaces exposed to the external environment will result in lower heat loss and therefore less energy being used for heating. This is because less external surface means less opportunity for heat to escape. The heat exchange between the interior and exterior of a building is directly proportional to the surface area and the difference in temperature between the inside and outside.
Buildings with compact shapes are able to retain more heat, reducing the need for artificial heating systems and lowering overall energy consumption because they have less surface area relative to their volume. This concept is sometimes referred to as the surface-to-volume ratio or in Passivhaus design, form factor.
Another thing to consider is the complexity of design. If there are intricate shapes, projections, or irregular contours the building form will most likely have more thermal bridges. These areas can allow heat to escape or enter the building more easily, which can undermine the building’s thermal insulation.
In contrast, a simpler building form is less prone to thermal bridges. This is because it is easier to design continuous insulation around the structure, reducing heat loss. Additionally, a more straightforward design can streamline the construction process, resulting in cost savings and fewer potential errors during the installation of insulation materials.
Thermal bridging and heat loss
When we consider U-values, we assess the building assembly as a whole. It should however also include adjustments for thermal bridges.
A thermal bridge also known as a cold bridge is an area where there is missing or reduced insulation, which creates a higher U-value in that area when compared to the surrounding insulated parts. This leads to heat loss, which can cause unwanted heat transfer and cold spots in the building fabric. These cold areas not only compromise the building’s thermal performance but also increase the potential for the build-up of condensation and mould growth. The presence of thermal bridges can also have a negative impact on user comfort and health.
While the impact of thermal bridging is less noticeable in poorly insulated buildings, it becomes significant in well-insulated buildings, where even small areas of poor insulation can cause substantial heat loss.
Additionally, in colder climates like the UK, more heat will be lost through areas with thermal bridges because of the significant temperature difference between the interior and exterior of buildings.
Research suggests that, on average, about 25% of internal heat loss in a dwelling occurs due to thermal bridges.
The main types of thermal bridges:
- Repeating thermal bridges
These are usually evenly distributed through the thermal envelope. An example being timber frame studs, or steel wall ties that create consistent thermal bridges along the structure. - Non repeating thermal bridges
These usually occur at specific locations, like around openings (windows, doors) or where materials with different thermal conductivity meet. - Geometrical thermal bridges
These are related to the shape of the building envelope usually where planes intercept. An example of this type of thermal bridge could be a junction or corner. - Point thermal bridges
These are single, isolated gaps in the building envelope. Examples where heat loss can occur are areas around fasteners, bolts, or fixings.
Areas with Poorly Installed or Missing Insulation
Junctions
Wall to foundation
Wall to floor
Wall to wall (corner)
Wall to ceiling
Wall to roof
Wall to balcony
Openings
Window frames and sills
Door frames and sills
Timber Members
Studs
Joists
Steel Members
Lintels
Beams
Columns
Concrete Elements
Walls
Floors
Foundations
Reinforced Concrete Members
Beams
Columns
Slabs
Masonry
Mortar Joints
Fasteners
Metal ties
Screws
Nails
Service Components
Ducts
Pipes
Cables
Recommendations for reducing thermal bridging and heat loss
Thermal bridging can lead to significant energy loss, so addressing this issue is very important for ensuring good thermal performance. In order to reduce thermal bridging and the associated heat loss, we suggest:
- Having the thermal insulation and airtightness barrier should form a continuous line
- Avoiding gaps or breaks in the thermal insulation or air barrier
- Paying extra attention to the air tightness membrane at junctions
- Referring to best practice examples (BRE, ECDs, robust details, Recognised Construction Details)
- Isolating thermal bridges with insulation
- Changing the geometry of the thermal bridge
- Using high quality insulation materials and ensuring they are installed correctly
- Implementing thermal breaks in structural elements
- Performing regular thermal imaging
Minimising Thermal Bridging
The detail below demonstrates a foundation system of good practice for insulation placement and minimising thermal bridging.
You can check out this detail and other examples of good practice on our associate website, the Detail Library:
DL368 – Insulated Foundation Junction with Brick Faced Timber Frame Wall
Here are some recommendations for some hemp related further reading for you to indulge in:
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Conclusion
In summary, the thermal performance of a building is crucial to its overall functionality, as it impacts not only the comfort of its occupants but also the wider environment by influencing energy consumption.
By carefully considering factors such as heat transfer, insulation, building form, and thermal bridging, we can design structures that not only minimise heat loss but also reduce carbon emissions and improve resilience to environmental changes.
We hope this post sheds some light on this key design factor and inspires you to apply these principles in your own projects.
You can read all of the architecture and environment related posts here:
Architecture and the Environment
Thank you for reading!
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Author
Written by Emma Walshaw, Architectural Technologist and founder of First In Architecture and Detail Library. Emma has written a number of books about construction and architectural detailing.
Thanks for the insightful articles you’ve been posting they’re very helpful
Thank you :) We are very happy to hear the articles are helpful to you Cosam.
This is marvelous the article is straight to the point