Super energy-efficient building exceeds client's expectations
In 2007, the Wine Society decided it needed a fourth wine warehouse at its Stevenage site and appointed Vincent and Gorbing to lead its design team. With increasing energy costs a major concern, the Society wanted to avoid the additional burden that a new building would impose on its running costs so the need for an energy-efficient building became a primary feature of the design brief.
Early design meetings with The Wine Society discussed sustainable design and the need to retain a steady-state environment within the new warehouse to reduce energy consumption. Primary considerations were airtightness, thermal mass and high levels of insulation.
The practice became aware of Tradical Hemcrete at the concept stage, while researching alternatives to early suggestions of either in situ or precast concrete as the primary walling element. Hemcrete is manufactured from locally-grown hemp 'shiv' - the plant's woody core - which is mixed with a lime-based binder to form a composite material which is carbon negative and has high thermal inertia. This gives it excellent thermal and acoustic properties, but the suitability of the product for a large-scale commercial project was questioned.
Hemp spraying had been used extensively on domestic projects but not on a building the size of the Wine Society's warehouse project, where calculations had suggested that to accommodate the required volume of wine (about 3,500,000 bottles) within the floor space available, the building would have to be up to 21m high.
A visit to a recently-completed brewery warehouse, also using Tradical Hemcrete, confirmed the benefits of hemp in terms of thermal performance and carbon emissions, but the design team remained concerned that Hemcrete was used there at a domestic rather than commercial scale.
Nevertheless, the unique properties of Hemcrete - thermal mass, carbon negative, air permeability, etc - made it the first choice for the external envelope. During discussions with the main contractor, Vincent and Gorbing developed the idea of using the sprayed form of Hemcrete in pre-fabricated wooden cassettes, measuring 2.4m high, 3.6m wide and 300mm thick.
The cassettes were framed up using standard engineered joists, with the backing made from Samox board, which is both breathable and vapour-permeable like Hemcrete and forms the visible inside face of the external wall construction. The cassettes were then transported to a disused airfield in Suffolk where they were sprayed and cured before delivery to the site in Stevenage. Here they were stacked up and tied back to a structural steel frame for both support and stability. Between the walling cassettes and the external cladding (a composite metal panelling system) is a void, literally a 'breathing space', which provides natural ventilation for the Hemcrete.
Extensive discussions followed with the structural engineers to see if the Hemcrete cassette system could be adapted for the roof construction. Unfortunately, structural limitations and cost constraints precluded the use of this approach and a more conventional roof construction was adopted.
Another design innovation was the use of a Kalwall translucent walling system to the whole of the north elevation of the warehouse. Kalwall has good thermal insulation properties and allows a high level of daylight penetration, so its northerly aspect meant that there was little or no risk of solar heat gain throughout the year.
At the time of its construction, the internal racking system for the pallets holding the wine was, at 18m, the highest installed anywhere in the world. This in itself was an achievement, but the compact nature of the wine storage was to be a significant part of the energy saving that was a fundamental briefing requirement. Wine has a similar density to water, and a similar thermal mass; the presence of over 2,500m³ of wine would be an essential element in stabilising the internal temperature, a key factor not only in energy efficiency but also in the storage of wine in optimum condition.
The building was completed in the autumn of 2008 and over the following months the Wine Society's staff began stocking the building and familiarising themselves with its operating requirements. Once this initial period was complete, the project's services engineers, MLM Consulting Engineers, and the Wine Society monitored the internal and external temperatures for a full year, along with the building's consumption of both gas for heating and electricity for lighting.
The resulting energy costs were closely monitored against the projections of a thermal model that MLM had developed at the design stage to enable evaluations of alternate construction systems.
From an early stage in the monitoring process, it was clear that the building was demonstrating remarkable temperature stability with very little use of the M&E installations, notwithstanding significant daily external temperature variations and extended periods of sub-zero temperatures during the winter of 2009/2010.
The table in the first image on this page indicates the actual recorded end-of-year energy usage figures compared to the projected figures from the thermal model and the savings, both in terms of kWh/annum and in terms of CO2/annum.
These figures are some 65% better than those expected from the thermal model, in effect quantifying the dynamic component of the Hemcrete, and translating into a significant reduction in predicted annual running costs of:
Electricity at 7.3p/kWh = £13,854
Gas at 2.7p/kWh = £1,420
Taking the two CO2 savings, totalling 90.5 tonnes per annum, the carbon reduction over the projected 40-year life of the building will be over 3,600 tonnes of CO2.
About 730m3 of Hemcrete was used in this development. As it is a carbon negative material with an embodied carbon figure of 130kg CO2/m3 this equates to an additional contribution of 94.9 tonnes of CO2.
The existing warehouses on the site have been constructed to various different standards and have different levels and ages of equipment. However, it was also useful to make a comparison in running costs between the new and the old warehouses by comparing the energy usage in terms of kWh used per m3 of warehouse space. This exercise indicated that the new warehouse is operating around 70% more efficiently than the existing warehouses.
The new warehouse is undoubtedly a success story for the Wine Society. Its design team's innovative concept has been able to exceed the Society's dual 'green' aspirations for both a highly sustainable building and radical reductions in its energy costs, as well as demonstrating convincingly that sustainable design techniques are now as applicable to commercial high-bay warehousing as they are to smaller scale developments.
Furthermore, it is worth noting that the energy reductions arise solely from the design of the building and will be fully achievable for the whole life of the building without further interventions.
The Wine Society's environmental aspirations for its fourth warehouse were two-fold: the building should be highly sustainable, and it should achieve radical reductions in energy consumption compared to its predecessors.
Furthermore, wine is best stored between 13°C and 16°C and will only tolerate slow temperature variations if it is to be maintained in optimum condition.
Vincent and Gorbing's elegant design solution took account of all these factors. The external walls are constructed of Tradical Hemcrete, framed in innovative wooden cassettes, fixed to the structural steel frame. Hemcrete absorbs energy as external temperatures fluctuate. As they rise, Hemcrete dries out by absorbing energy; as the temperature falls, the reverse happens.
This process necessitated a 'breathing space' between the external metal panelling and the Hemcrete cassettes but ensured that internal temperature variations were substantially smaller than those externally.
Over 18 months, the Society and its engineers, MLM, monitored the building’s energy consumption in tandem with internal and external temperatures. The results were impressive: actual energy consumption was 65% less than predicted, with internal temperatures remaining stable within the optimum range.
The savings are dramatic: £15,250 less in annual energy costs than predicted and 90.4 tonnes of CO2 less per annum. These savings arise solely from the design of the building fabric itself which should not require replacement or alteration during the whole of its expected 40-year life.