Decarbonising Buildings Part 1: Building Materials
The journey towards decarbonising buildings is complex and multifaceted, particularly when it comes to the materials used in construction. Currently, there is a significant emphasis on reducing 'upfront carbon'—the emissions associated with the production and transportation of building materials. However, it's crucial to recognise that not all materials with high initial carbon footprints are inherently detrimental if used in the right context. Our focus here is on the most commonly used building materials, understanding their impact, what to avoid, and how to optimise their usage for sustainability.
In this discussion, we'll examine commonly used materials, their pros and cons in the context of carbon emissions. We'll explore strategies to avoid high-carbon choices where possible and how to make the most of materials where their use is advantageous. The goal is not to eliminate certain materials entirely but to use them more thoughtfully and efficiently, aligning with the broader objective of reducing the carbon footprint of our buildings.
Concrete
Concrete is a valuable material in the construction industry, particularly for its thermal mass properties, which are ideally harnessed when the concrete is insulated externally. This approach is beneficial across various climates, as it ensures that the thermal mass is effectively used to stabilise indoor temperatures, thus reducing the reliance on heating and cooling systems. By insulating externally, thermal bridges are minimised, which enhances the building's energy efficiency and often negates the need for vapour barriers, as the insulation layer can manage moisture transfer effectively.
Considering the diverse climatic regions worldwide, the consensus is that external insulation of concrete is a sound practice. In cold climates, it prevents heat loss; in temperate climates, it helps to regulate indoor temperatures year-round; in hot and dry climates, it wards off excessive heat during the day and supports night-time cooling.
The optimal utilisation of concrete to enhance energy efficiency while minimising its upfront carbon footprint involves strategic placement within the building's core as a structural element, rather than in the external envelope. By concentrating the use of concrete in the core, where it can provide both strength and thermal mass, we can limit the quantity required and thus reduce the associated carbon emissions from its production. This core can then be thermally activated, either for heat storage or for cooling purposes, depending on the climate and design of the building.
Steel
Steel is an indispensable material in the construction industry, valued for its structural integrity and flexibility. Unlike concrete, steel does not have significant thermal mass properties, but it is extremely strong and has a high strength-to-weight ratio, making it ideal for framing and supporting tall buildings. Insulating steel externally is essential since it is highly conductive and can lead to significant thermal bridging if left exposed.
Similar to concrete the practice of external insulation is preferred for steel structures. In cold and hot climates alike, it prevents thermal bridging and heat loss.
For steel, the reduction of its upfront carbon footprint is less about leveraging thermal mass and more about optimising its use as a structural framework. By utilising steel primarily where its strength is indispensable and combining it with other lower carbon materials, we can minimise the amount of steel needed.
Timber
Timber stands out in the construction industry not only for its aesthetic and natural appeal but also for its environmental benefits, particularly when sourced from sustainably managed forests. Unlike concrete, or steel, timber inherently provides insulation due to its low thermal conductivity. In timber framed buildings the insulation can be placed in between studs, or externally. For mass timber buildings external insulation is placed exterior, similar to concrete.
Timber, along with wood fiber-based insulation, also brings a suite of thermally advantageous properties to the table. These materials possess an inherent heat capacity that enables them to absorb and store heat energy, a feature that is particularly valuable in temperate and cooler climates. This heat capacity contributes to thermal lag, meaning that the peak temperatures are delayed, which has the effect of evening out temperature fluctuations within a building. In summer, this can reduce the cooling load, as the warmest outside temperatures are not immediately transferred indoors. Conversely, in winter, the heat from the day can be released into the building during the cooler nights, lessening the heating requirements.
The hygroscopic nature of timber and wood fiber insulation is another key benefit. It can absorb and release moisture, which allows it to manage indoor humidity levels effectively, contributing to a comfortable indoor climate and improved air quality. This ability to moderate moisture also makes wood and wood fiber-based insulation materials a good choice in regions with significant temperature swings between day and night or across seasons.
To maximise energy efficiency while minimising wood's carbon footprint, strategic use within a building's structure is key. By employing wood in the framework and leveraging its insulative properties, it's possible to limit the use of other materials with higher embodied carbon. Furthermore, wood can serve both structural and aesthetic purposes, potentially reducing the need for additional finishing materials. Wood also acts as a carbon sink, sequestering carbon dioxide throughout its lifecycle, which can offset emissions associated with the construction process.
Windows
Glass and various joinery materials like aluminium, thermally broken aluminium, steel, timber, and uPVC, play essential roles in shaping a building's thermal efficiency and, by extension, its operational carbon footprint. Glass, pivotal for daylighting, can, if improperly specified, become a thermal liability. However, advancements in glazing technologies—such as double or triple glazing, inert gas fills, and low-emissivity coatings—have transformed glass into an asset for energy conservation, enabling passive solar heating in cold climates and reducing unwanted heat gain in warm climates.
Aluminium joinery, with its inherent strength and longevity, can be a poor insulator. Yet, with the advent of thermally broken aluminium, which incorporates a non-conductive barrier to reduce heat transfer, the material's impact on operational carbon is significantly mitigated, aligning better with energy-efficient building standards.
Steel joinery, similar to aluminium in its conductive properties, necessitates thermal break technologies to curb its thermal transfer. While steel's robustness allows for expansive glass that can harness solar gain, it can also lead to increased operational carbon without proper thermal breaks and glazing.
Timber joinery stands out for its natural insulation properties and lower carbon footprint. It offers improved thermal performance, contributing to reduced operational carbon through minimised heating and cooling demands as well as typically being less prone to surface condensation than its metal counterparts.
uPVC joinery has risen in favour due to its excellent insulation capabilities and minimal maintenance. Being a non-conductor, uPVC significantly reduces heat loss or gain, thus positively influencing a building's operational carbon as well as typically being less prone to surface condensation.
Metals like aluminium and steel have high embodied carbon, their durability and recyclability can somewhat offset this impact over their lifecycle. Timber, with its carbon-sequestering ability and lower embodied energy, often represents a more sustainable choice, provided it comes from responsibly managed forests. uPVC, although not as environmentally benign in production, offers longevity and energy efficiency that can contribute to lower operational carbon. The choice between these can vary significantly between building designs and is ideally verified using energy modelling.
Insulation
Insulation materials are at the heart of a building’s energy conservation strategy, directly affecting the operational carbon footprint through their influence on heating and cooling demands. The variety of insulation materials available—PIR (polyisocyanurate), PU (polyurethane), mineral wool, glass wool, wood fiber, sheep wool, polyester, and stone wool—to name the most common - each bring distinct properties and environmental impacts.
PIR and PU foams are known for their high insulation values per mm thickness, which makes them particularly effective in reducing heat transfer. However, they are petroleum-based products, which means their production is associated with higher embodied carbon. Yet, their effectiveness can significantly lower a building’s operational carbon over time. These products are best used in applications without thermal bridges as the high insulation value increases thermal bridging effects. We typically don’t recommend using these products in between timber framing either as their moisture behaviour can be problematic, especially with foil coatings. These are ideally used in SIPS (structurally insulated panels) with a metal, or OSB skin.
Mineral wool and glass wool are widely used insulation materials that offer good thermal performance and sound absorption. They are made from abundant or recycled materials, which lowers their embodied carbon. Their performance can lead to reduced energy usage for heating and cooling, thus diminishing operational carbon.
Wood fiber insulation is derived from a renewable resource and has the added benefit of sequestering carbon. It provides not only thermal insulation but also moisture regulation within the building envelope, contributing to a reduction in operational carbon.
Sheep wool is a natural fiber that not only insulates but also can absorb and release moisture, which helps to manage indoor humidity levels. It’s a sustainable option with a low embodied carbon footprint, and it contributes to a building’s thermal efficiency.
Polyester insulation, often made from recycled PET bottles, offers a middle ground with moderate insulation performance and a better environmental profile than foams. Its use can lead to a decrease in operational carbon, particularly in climates where extreme thermal resistance is not required.
Stone wool, similar to mineral wool, is made from natural rock and offers high thermal resistance and fire retardancy. While its production is energy-intensive, its durability and thermal properties can offset this initial carbon cost by reducing the energy needed for climate control in the building. Stone wool insulation is becoming a popular exterior insulation methodology for high rise buildings because of its fire resistant properties.
All insulation is good insulation – as long as you consider its part in the moisture management of the building fabric we think you should not focus on the embodied carbon as much as improving operational carbon and using the right product for your building fabric!
