Introduction: The Carbon Question in Retrofit Projects
In discussions about sustainable construction, the focus often falls on operational energy—how much electricity a building consumes and how efficiently it can be heated or cooled. This is important, of course, but embodied carbon is another equally pressing issue. Unlike operational emissions, which accumulate over time, embodied carbon is released immediately during building materials’ production, transport, and installation. In other words, these emissions immediately impact climate change, making material choices a crucial consideration in any retrofit or new construction project.
At Unagru Architecture Urbanism, we have spent years researching how to reduce the carbon footprint of buildings, particularly in retrofits. One clear conclusion is that electrification is a major step forward, particularly when powered by renewable energy. However, once heating and energy use are optimised, we must focus on the materials used in the construction process.
Our Approach to Reducing Embodied Carbon
Our strategy for minimising embodied carbon follows a clear hierarchy. First, we prioritise intelligent design to preserve and adapt what is already there—because the greenest building material is the one that doesn’t need to be produced at all. Second, we focus on reusing, recycling, and upcycling materials, making the most of what is already available to reduce waste and avoid the emissions of new production. Now, we are expanding our research into materials that are more difficult to recycle but significantly impact a building’s overall carbon footprint.
This brings us to screed—a seemingly minor component of a floor, but one with a surprisingly large carbon footprint. The industry standard is cement-based screed, but limecrete is a viable alternative. How do these two options compare in terms of cost and carbon emissions? And does using a low-carbon material like limecrete make more sense than relying on carbon offsetting?
Understanding Screed and Its Role in a Building
Screed is a layer of material applied over a reinforced concrete floor slab to create a smooth and even surface for the final floor finish, whether it be timber, tiles, or carpet. It also plays an important role in thermal mass, meaning it helps to regulate temperature by absorbing and releasing heat slowly. This is particularly useful when underfloor heating is installed, as a good screed will help distribute heat efficiently throughout a room.
Most screeds used today are made from cement, sand, and water, forming a dense, strong, and durable layer. However, cement production is one of the largest single contributors to global carbon emissions, responsible for around 8% of total CO₂ emissions worldwide. This makes any effort to replace cement-based materials with lower-carbon alternatives a priority for sustainable construction.
Limecrete, a mixture of hydraulic lime, aggregate, and water, is one such alternative. Historically, lime-based materials were used widely before cement became dominant in the 19th and 20th centuries. Unlike cement, lime allows buildings to “breathe” by regulating moisture levels, reducing the risk of trapped damp and condensation. More importantly for sustainability, limecrete has a significantly lower carbon footprint than cement screed.
Cement Screed vs. Limecrete: A Comparison
A Brief History of Lime and Cement in Construction
The use of lime in construction dates back thousands of years. The Romans, renowned for their engineering, used lime-based mortars and concretes in their vast network of roads, aqueducts, and buildings. One of the most famous examples of lime-based construction is the Pantheon in Rome, completed in 126 AD, which features the world’s largest unreinforced concrete dome—still standing today. The secret to its longevity lies in the volcanic ash and lime mix, which gave it both strength and self-healing properties when exposed to moisture.
Throughout the Middle Ages, lime remained the dominant binding material for construction across Europe. Traditional timber-framed buildings in England, France, and Germany used lime plaster and limecrete floors, valued for their breathability and flexibility. In Venice, builders developed a version of hydraulic lime—pozzolanic lime—that could set underwater, enabling the construction of structures that could withstand the city’s unique lagoon environment.
Cement, on the other hand, is a much younger material in comparison. While rudimentary forms of hydraulic cement date back to Roman times, modern Portland cement was only patented in 1824 by Joseph Aspdin, a British bricklayer from Leeds. Aspdin discovered that burning limestone and clay together created a material that, when ground into a fine powder and mixed with water, set much faster and harder than traditional lime mortars. By the late 19th and early 20th centuries, cement had largely replaced lime in construction due to its superior compressive strength and quick setting time.
This transition was accelerated by the Industrial Revolution, when the demand for rapid, large-scale construction made cement the preferred choice. The development of reinforced concrete in the late 19th century—pioneered by François Hennebique and later perfected by August Perret and Le Corbusier—further cemented (pun intended) the material’s place as the default choice for modern buildings.
By the mid-20th century, lime had almost disappeared from mainstream construction, relegated to heritage restoration and niche applications. However, in recent years, a renewed focus on sustainability, breathability, and carbon reduction has led architects, engineers, and builders to rediscover the benefits of lime-based materials.
The transition from lime to cement has had lasting consequences, some of which were only fully understood decades later. A striking example comes from the conservation of historic buildings. Many post-war renovations used cement-based mortars and plasters to repair old buildings originally constructed with lime. The result? Severe damp problems, as the non-breathable cement trapped moisture inside historic masonry, leading to accelerated decay. Conservation specialists now spend millions annually removing cement-based materials from historic buildings and replacing them with lime.
The Pantheon’s lime-based concrete has survived for nearly 2,000 years, while many mid-20th-century reinforced concrete buildings are already deteriorating due to corrosion of the steel reinforcement inside them. Scientists studying ancient Roman construction techniques are now looking at pozzolanic lime-based materials as a way to create more durable, sustainable concrete in the future.
Characteristics and Differences: Lime Screed vs. Cement Screed
With this history in mind, how do lime screed and cement screed compare in modern construction? While both serve the same basic function—creating a smooth, level surface for flooring—they differ significantly in composition, performance, and environmental impact.
1. Composition and Chemical Properties
• Cement screed is made from Portland cement, sand, water, and sometimes additives for faster curing or increased strength. The cement binds the sand particles together, creating a dense, rigid surface.
• Lime screed consists of hydraulic lime, aggregate (such as sand or crushed stone), and water. Unlike cement, which hardens by a chemical reaction with water (hydration), lime sets through a slow carbonation process, absorbing CO₂ from the air. This makes limecrete more breathable and flexible.
2. Strength and Durability
• Cement screed is extremely strong in compression, making it ideal for heavy loads and high-traffic areas. However, it is also brittle—once set, it does not accommodate movement well, leading to cracking if the substrate shifts or expands.
• Lime screed is more flexible, which means it can accommodate slight movements in a building without cracking. While its compressive strength is lower than cement’s, it is still sufficient for most residential and office applications.
3. Breathability and Moisture Control
• One of the biggest advantages of lime screed is its breathability. It allows moisture to pass through, preventing condensation and trapped damp. This is especially valuable in historic buildings and solid-wall constructions, where excessive moisture retention can cause structural damage.
• Cement screed, by contrast, is non-breathable. Once set, it forms a moisture barrier, which can lead to damp issues if not properly managed with membranes and ventilation.
4. Thermal Performance and Underfloor Heating
• Lime screed has better thermal mass properties, meaning it retains and gradually releases heat over time. This makes it an excellent companion to underfloor heating systems, as it helps maintain a stable indoor temperature.
• Cement screed heats up and cools down more quickly, which can make heating systems slightly less efficient in terms of maintaining a steady warmth.
5. Setting Time and Construction Logistics
• Cement screed sets quickly, often within 24–48 hours, and reaches full strength in a few weeks. This makes it ideal for fast-track construction projects where time is a priority.
• Lime screed takes much longer to cure—often several weeks or even months, depending on conditions. This slower process can be a drawback in projects with tight schedules but is beneficial in terms of reducing shrinkage and cracks over time.
6. Carbon Footprint and Environmental Impact
• Cement screed has a much higher embodied carbon footprint. Cement production is responsible for roughly 8% of global CO₂ emissions, making it one of the most polluting industries in the world.
• Lime screed produces far less CO₂, and because it reabsorbs carbon during the setting process (carbonation), it partially offsets its own emissions. On average, limecrete has 80–90% lower embodied carbon than cement screed.
A CASE STUDY COMPARISON
To assess whether limecrete is a viable alternative, we compared both options in a 100m² office retrofit requiring an 80mm-thick screed layer. We looked at two key factors: cost and carbon impact. At our office, we mistakenly didn't opt for limecrete, blinded by the fear of long setting times after a protracted completion process robbed us of two months on our construction programme. We now have time to review all our choices and learn for future projects.
1. Cost Comparison
Cement screed is the cheaper option, with an average cost of £15–£16 per square metre. For a 100m² project, this results in a total material cost of around £1,550.
Limecrete, by contrast, is more expensive, typically costing between 1.5 to 2 times more than cement screed. This means the total material cost rises to £2,325–£3,100 for the same 100m² area.
While limecrete does come at a premium, its benefits extend beyond cost alone. To determine whether the extra expense is justified, we must examine its environmental impact.
2. Carbon Impact Comparison
The key reason to consider limecrete over cement screed is its significantly lower embodied carbon. Cement-based screed has an estimated embodied carbon of 100 kg CO₂e per tonne, meaning a typical 100m² floor using 17.6 tonnes of cement screed will produce around 1,760 kg of CO₂ emissions.
Limecrete, on the other hand, has an embodied carbon of just 13.58 kg CO₂e per tonne, meaning a 14.4-tonne application would generate only 195.5 kg of CO₂ emissions.
The difference is stark: switching to limecrete reduces the project’s carbon footprint by over 1,500 kg of CO₂e, an almost 90% reduction in emissions from screed alone.
Carbon Offsetting vs. Low-Carbon Materials
When discussing embodied carbon, a common counterargument is that emissions can be offset rather than prevented. But what does this actually mean?
Carbon offsetting is a method of compensating for emissions by funding projects that reduce or absorb an equivalent amount of CO₂ elsewhere. These might include reforestation schemes, renewable energy projects, or carbon capture technologies.
Offsetting has become popular because it allows companies and individuals to “neutralise” their emissions without having to make substantial changes to their materials or processes. However, there are two major concerns with this approach:
1. Uncertainty in Carbon Offsets – Not all offset projects deliver the promised reductions. Some forests planted as carbon sinks are later destroyed by fire or logging, and some renewable energy projects would have happened anyway without the offset funding. This means that while the offset exists on paper, it does not always represent an actual reduction in atmospheric CO₂.
2. Timing of Emissions – Embodied carbon is released immediately, whereas offsets often take years or even decades to absorb the equivalent amount of CO₂. A tree planted today will take 40 years to capture the carbon emitted by cement production today. Meanwhile, climate change continues to accelerate.
If we compare the cost of using limecrete to the cost of offsetting, the numbers are revealed.
• The extra cost of limecrete is approximately £1 per kilogram of CO₂ saved.
• The cost of voluntary carbon offsets varies but typically ranges from £4 to £30 per tonne of CO₂e (or £0.004 to £0.03 per kilogram).
• At these prices, offsetting the 1,760 kg CO₂e emissions from cement screed would cost only £7–£52—far less than the additional £775–£1,550 required to use limecrete.
At first glance, this suggests that offsetting is far more economical than switching materials. However, given the growing scrutiny of offsetting schemes and the fact that offset prices are expected to rise dramatically as climate policies tighten, the long-term viability of this strategy is questionable.
More importantly, preventing emissions is always preferable to compensating for them later. Once carbon is released into the atmosphere, it contributes to climate change immediately. Using low-carbon materials like limecrete directly reduces emissions at the source, rather than relying on uncertain future offsetting mechanisms.
Is Limecrete Worth the Extra Cost?
Cement screed remains the cheaper option for projects with tight budget constraints. However, for those committed to reducing embodied carbon in a meaningful and verifiable way, limecrete offers a proven, immediate, and measurable reduction in emissions.
Additionally, limecrete provides other practical advantages beyond carbon reduction. It is more breathable, reducing the risk of trapped moisture and improving indoor air quality. It is also more flexible than cement, making it less prone to cracking over time. These properties may extend a building's lifespan in specific contexts, further contributing to sustainability.
Ultimately, the choice between cement screed and limecrete comes down to priorities. If reducing carbon emissions in construction is a serious goal, then switching to limecrete is a real and effective step forward—even if it comes at a higher initial cost.
Final Thoughts
The discussion of sustainability in construction often focuses on high-tech solutions, but sometimes, the best answer is simply using better materials. Swapping cement screed for limecrete is a small change that drastically reduces embodied carbon—something that is not always true for other sustainability strategies.
Choosing materials with lower embodied carbon is one of the most reliable, transparent, and effective ways forward for those looking to make a genuine impact in sustainable construction.
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