Embodied Carbon Emissions of Housebuilding - External Walls
Changing the way we build external walls in the UK could save 4.5 million tonnes of CO2 every year!
The External Walls of a standard construction UK new build house (2bed-79m2) has a carbon footprint of 10tonnes. If Hempcrete is used for these same walls then 5 tonnes of atmospheric carbon will have been sequestered. Representing an immediate carbon saving of approx. 15tonnes for every average new build home.
Governments are focusing on reducing the operational emissions of buildings, which is important, but if we forget to also consider the embodied carbon of the building materials themselves, then we are actually accelerating carbon emissions now in order to reduce them in 10 - 15 - 30 - 40 years?
The 2021 IPCC report suggests that the world can release another 400billion tonnes of atmospheric CO2 before we pass the 1.5degrees celsius of global warming (considered a point of no return). Given the worlds current emissions, that gives us a finite timeframe of 8 years! (1)
Net zero dates do not matter; what matters is the total cumulative emissions by the net zero date.(1)
Hempcrete and other photosynthetic materials have a considerable advantage over conventional materials in terms of embodied carbon emissions. (2)
Building all annual UK new build homes (80m2 each x 300,000) with Hempcrete walls instead of current conventional walls has the potential to save 4.5 million tonnes of CO emissions per year. Also 0.6% of the UK annual emissions (774 mt CO, 2019 (3))
This carbon saving estimate is based on the walls (not including windows or fixtures and fittings) of a 2 bed 4 person house - 79m2 (illustrated in the diagram below), multiplied by the estimated number of new builds each year (300,000).
Cradle to gate Carbon Footprint of building materials for different wall types per square metre. Each wall build up has a U value of 0.18W/m2K as per new build standards for Walls in Part L of Building Regulations. The consistency in U value assumes that each wall type will offer the same reduction in operational carbon emissions for the lifetime of the building.
How does Hempcrete store Carbon?
What is Hempcrete?
Hemp is a fast growing annual crop, its woody core (arguably a by product of other industries) is mixed with lime to make hempcrete.
How Plants sequester Carbon.
Every Plant absorbs CO2 in photosynthesis. It is estimated that around 50% of a plants dried biomass is Carbon, because Carbon atoms have an atomic weight of 12 this converts to approximately 1.8kg of CO2 removed from the atmosphere for every kg of dry biomass. (There are generally carbon emissions associated with farming, harvesting, processing and transport that also need to be considered.)
Hemp arguably absorbs more CO2 per acre than most forests, this is because of close planting and the plants ability to grow to 4m within one growing season. Hemp is an annual crop which means that if this biomass is left in the field it will rot and release most of this captured CO2 (some CO2 from photosynthesis is stored in soil networks). In comparison most forests would continue to absorb more CO2 if they were left to grow. Harvesting annual crops is likely a lower impact route to carbon storage than harvesting timber, this does not mean that it is not beneficial to plant trees, or that sustainably sourced timber is not a good building material, it is just that the yearly availability of sustainable timber is currently fairly finite as most timber species need at least a 30 year growth period.
Building Lime and its relation to Carbon
Generally: Limestone (CaCO3) is heated to make building lime (CaO), once it has been mixed with water it reabsorbs (or calcifies) the lost CO2 as it dries, returning to its hardened state of CaCO3. - (This is a simplified explanation)
(Although building limes reabsorb the lost CO2 as they set/ cure it is not a closed loop as there are normally carbon emissions associated with the heating process as well as extraction, transport etc.)
Once the hemp shiv has been mixed with lime and placed in the fabric of a building its chances of decomposing are significantly reduced. The lime gives the hempcrete good fire resistance and antibacterial properties which further reduce the likelihood of the sequestered CO2 being released.
Hempcrete has multiple other benefits over conventional construction:
-Exceptional operational performance - demonstrated by numerous post occupancy evaluations.
-Insulation to prevent heat loss and gain combined with the radiant heat potential and storage of thermal mass.
-The breathable and humidity buffering nature of hempcrete construction reduces damp and moulds and increases occupant comfort.
-Hempcrete has excellent fire resistance.
Versatility:
Cast on Site
Spray Applied
Non structural Blocks
Prefabricated construction
Compatible with most external finishes when detailed correctly.
A couple more wall types analysed.
I’ve also had a look at the Embodied Carbon of wall types that are becoming popular with volume house builders across the UK. These are based on an insulated structural timber frame (often offsite prefabrication) with different types of cladding, mostly bricks.
Because of a preference in the UK for Brick faced buildings, I’ve also included a hempcrete option that includes external bricks, this actually brings the wall build up closer to Zero Carbon instead of Carbon Negative.
Clay Bricks have historically been a fairly sustainable option as they can last over a thousand years and have been traditionally reused at end of life. However the prevalence of the use of cement based mortars in place of lime has made them much more difficult to reuse, as well as reducing their lifespan - cracks through the entire building incase of any settlement, frost damage after water ingress. New bricks actually have a really high Carbon Footprint and we should probably be considering different options: lime render, wood cladding, stone cladding. Metal and composite cladding options also have insanely high carbon footprints as well as a shorter lifespan than bricks.
Obviously the Carbon Footprint of materials isn’t the only important thing that needs to be considered. Resource depletion, pollution (of water sources and ground), toxicity (both during production and once in your home), social implications of certain industries (modern slavery etc). These are all things that we need to consider when choosing our building materials responsibly.
(1) https://www.carbonindependent.org/54.html
(2) ‘Exploring the energy performance of Hemcrete in affordable housing and future implications for carbon reduction in the housing sector’ Cathie Eberlin and Ljubomir Jankovic
Calculations:
Carbon footprint values taken from ICE Database v.2019, except for Hempcrete (not included on ICE Database) taken from Limetechnolgy Datasheet 2011.
NB: Values for timber (not including sequestration) for all wall types.
Material densities taken from ICE Database v.2019 where available. Where unavailable taken from product data sheets online, Comparable products to ICE listings used.
1. Standard Cavity Wall Construction (prevalent across the UK) = 70.3 kgCO2/m2
Clay brick wall w/mortar (1:5, cement: sand) - 36.5 kgCO2/m2
Polyurethane Rigid Foam (110mm) - 4.56kgCO2/kg - Density = 3.58kg/m2 - 15.2508 kgCO2/m2
Medium Density Concrete Block wall w/ mortar (1:5, cement: sand) - 15.1 kgCO2/m2
Plasterboard - 0.38kgCO2/kg - Density = 800kg/m3 - Thickness = 10mm - 3.04 kgCO2/m2
Gypsum Plaster - 0.12kgCO2/kg - Density = 1120kg/m3 - Thickness = 3mm - 0.4032 kgCO2/m2
Total = 70.294kgCO2/m2
2 bed/ 4person House - 79m2 - Wall Area = 175m2 - Openings (assumed 20%) = 36m2
Total Constructed Wall Area for comparison = 140m2
140m2 X 70.294kgCO2/m2 = 9898.52kgCO2
Total Carbon footprint for external walls = 9.9tonnes CO
2. Hempcrete Wall (Structural Timber Frame) = -34.71 kgCO2/m2
Hempcrete - -110kgCO2/m3 - Thickness = 360mm - -39.6kgCO2/m2
Softwood Timber - 0.263kgCO2/kg (not incl. sequestration) - Density = approx. 600kg/m3 - 0.225m3/ m2 (150mm x 47mm at 600mm centres) - 3.5505kgCO2/m2
Lime Plaster - Thickness = 10mm - Ratio Lime:Sand = 1:3 -
Lime - 0.78kgCO2/kg - Density = approx. 640kg/m3 - Thickness = 2.5mm - 1.248kgCO2/m2
Sand - 0.00747kgCO2/kg - Density = approx. 1600kg/m3 - Thickness = 7.5mm - 0.089kgCO2/m2
- 1.33764kgCO2/m2
Total = -34.71kgCO2/m2
2 bed/ 4person House - 79m2 - Wall Area = 175m2 - Openings (assumed 20%) = 36m2
Total Constructed Wall Area for comparison = 140m2
140m2 X -34.71kgCO2/m2 = 4887.72kgCO
Total Carbon footprint for external walls = -4.9tonnes CO