Silos Sustainability
Eco-Conscious Design: Sustainable Features of the Silo Conversion
“The most sustainable building is the one you don’t build,” so repurposing two redundant grain silos was a great start.
We worked with local designer Giles Cadle who took a ‘fabric first’ approach to the design. This means he concentrated on insulation, thermal bridges, air tightness, window placement and shading. Getting this right means that there will be less demand for heating, cooling and ventilation, and that these elements could be correctly sized.
Features
Insulation
Overheating
The existing metal shell of the Silos conducts heat very well (gets very hot and very cold) so it was important to introduce an air gap between the metal wall and the insulation layer.
The potential of the interior spaces to overheat was a concern. To mitigate this the air gap was maintained at 100 mm all round, and vented below (through grills in the access door) and above via a cowl at the apex of the roof. In summer the ‘Stack effect’ and convection will draw cooler air in through the grills at the bottom of the Silos, up through the ventilated cavity before exiting at the top. By keeping the bottom half of the Silos clear of structure the lower space is able to act as a plenum, providing a large volume of air, rather than additional structure that would retain heat, for the same reason the conical roof space was also left as a void. The result of this construction approach is that the entire inhabited space is surrounded by moving air. This passage of air helps to prevent overheating.
The interior walls are made from a number of curved timber frame panels. Insulation was placed in a continuous layer on the outside of these, facing on to the ventilated cavity, and also within the thickness of the timber frame. The outer layer of insulation needed to be rigid enough to take fixings for attachment, yet flexible enough to take the curve of the walls – for this reason we choose to use fairly stiff mineral wool sheets (100 mm thick) – these are also inherently very fire resistant, and a better sustainable choice to oil based insulation materials. Within the thickness of the timber frame we had more choice of insulation material as the sheathing boards to both sides are fire proof. Here I choose to use Pavatextil, which is made from 100% recycled cotton, mainly denim jeans and a little velvet. It is a soft insulation material that can be cut to size and pushed into all voids within the 160 mm thick timber frame. It has good insulating capacity against thermal losses in winter and heat transmittance in the summer, it is also has excellent acoustic properties and is great at dealing with any air born moisture.
Thermal Bridges
Thermal bridges occur where materials which are better conductors of heat are allowed to form a ‘bridge’ between the inner and outer face of a construction. This has been minimised by careful design achieved by the continuous outer insulation layer of mineral wool, and the use of thermally broken fixings between the silo wall and interior wooden construction.
Air Tightness
To prevent unintended heat loss through any small holes in the exterior walls a rigorous approach to air tightness was adopted throughout the construction. An air tight membrane was included and all junctions at windows and other smaller openings carefully sealed. A test of the air tightness was undertaken and gave an excellent result of 1.34 m3(h.m2) @ 50Pa.
Ventilation
A really good level of air tightness meant there is little uncontrolled ventilation into the building, as cold air drafts are eliminated. Ventilation is still required though, so a simple MVHR (mechanical ventilation with heat recovery) system was installed. Fresh air from the exterior is drawn into the MVHR unit and heated via a heat exchange by the extracted interior air, before being ducted into the interior space. This is a very efficient system set up to match the specifics of the silo spaces.
It is of course still possible for occupants to choose to additionally ventilate the building by opening a window.
Windows
The placement of the windows was carefully considered. From a thermal approach to achieve just the right amount of solar gain and also with regard to both distant views and not overlooking the other Silo.
Deep set window reveals limit the light spill to outside and control against excessive solar gain. Additional shading was added above south facing windows, the position and size determined by a study of the suns path.
Triple glazed windows were selected to give the best insulation values.
Heating
There was sufficient capacity within the existing GSHP (ground source) array to provide the heating and hot water for the Silos. A new heavily insulated district heating loop was installed between the Silos and the GSHP at the Eco Centre. Because the Silos are quite small volumes and are not in constant occupation it was decided to install a heating system that could respond rapidly, so radiators were chosen. So that the radiators could be accurately sized the heat loss from each Silo was carefully calculated based on their size, wall construction, glazing, orientation and location.
Ecology
During the planning application a number of studies were required to make sure that the Silo conversion would not impact on existing biodiversity. As the Silos were sealed units there were no problems with birds or bats using them for roosting. However, the Silos are immediately adjacent to a designated ‘ancient wood’ and the council ecologist raised concerns about light spill from windows having an adverse effect on ‘commuting bats.’ To mitigate for this windows orientated towards the ancient wood were adjusted and the deep reveals added, and a limiting interior lighting scheme adopted. This was done in collaboration with a lighting consultant who modelled the light spill towards the wood. Note there are no directly mounted ceiling lights, and all pendants have deep shades to limit any spill.
Interior Materials
Wherever possible the Silos are constructed in timber, including the internal finishes. Timber from a managed source is considered to be carbon negative as Co2 is sequestered in the wood as the tree grows. This carbon remains stored within the timber for its lifetime, and importantly the forest is also replanted.