Andrew Grant: I want to start by asking Richard about the timber superstructure that is at the very heart of the building. What is cross-laminated timber and why is it such an exciting material in a “fabric first” development?
Ybbs Mill cross laminated timber (CLT)
Richard Todd: Cross-laminated timber or CLT is essentially giant, engineering scale plywood. A CLT panel comprises wall-sized sheets of glued wood and is about 13 centimetres (5 inches) thick. It has been prefabricated and cut ready for assembly on site, with each panel shaped to form a bedroom wall, a floor plate or a roof surface.
The panels have been perfectly prepared by the manufacturer allowing them to be fitted together quickly on site like a building-scale jigsaw. It means that the Water Court Villa has had its superstructure and roof installed with weatherproofing and bathroom pods in place in a scant six weeks! So, as well as being a sustainable product – with sequestered carbon rather than high amounts of embodied carbon – its use means that we have only a fifth of the number of materials deliveries that would be expected for a build of this scale if it were made from brick and block, concrete or steel.
Univ North Water Court West built-out CLT frame. Note the Bathroom pods beside Villa ready to be lifted into the sister Villa as it is constructed.
With CLT, there is less noise, less waste and less dust; all of which is of huge benefit for the well-being of Univ’s neighbours, including the elderly residents safeguarded at Fairfield Residential Home. In addition, the CLT panels are available in different grades which allows for a higher quality visual finish where it is to be exposed. By having exposed CLT walls, the need for a finishing layer, such as plasterboard, is removed which lowers the weight of the building and removes additional materials from site. Student areas will have certain walls as exposed CLT.
AIG: Can you clarify what you mean by “sequestered” carbon?
Univ North – prefab panel being lifted into place
RT: It is important to distinguish between “sequestered” carbon and “embodied” carbon in construction. As CLT uses timber, the carbon trapped by the trees from the atmosphere is held within the frame – it is sequestered – in contrast to the large amount of carbon-emitting energy required to make concrete or steel for a building. In the latter case, the carbon cost of making the concrete or steel is considered “embodied” in the building in the sense that the cost to the environment has been realised and has to be accounted for accordingly.
Ybbs mill – infeed to the coating line
The CLT manufacturing is impressive. We visited the Ybbs sawmill in Austria as part of the final assurance prior to fabrication. Raw timber logs enter the mill at one end and prefabricated ready-to-assemble CLT panels are issued as a finished product from the factory gate in a continuous production process. Even the wood dust from the cutting and planing is retained and recycled. Ybbs Mill is the largest fully integrated mill in the world.
As a fun fact, Water Court Villa West required 410 cubic metres of CLT (its scale is such that it is about the size of Univ’s Hall). The remarkable statistic from Stora-Enso, the company owning the mill, is that the forests of Austria require just 1.14 minutes to replace that timber volume on a warm summer’s day! It is a truly sustainable product.
AIG: That is indeed extraordinary. Does CLT have any other advantages?
Univ North – Water Court West CLT in second floor bedroom
RT: Yes. Accurate pre-fabrication means that the construction is both speedy and can be delivered with close precision. This means that each room is relatively airtight – no nasty draughts and chills in the winter – and is therefore extremely energy efficient. Layers of insulation complete the picture delivering buildings that have low energy bills with the development close to Passivhaus standard.
AIG: Building on your point, the University, on behalf of itself and many of the colleges, forward-purchases Univ’s electrical power from low carbon supply. This means that we can effectively zero carbon save for the inefficiencies in the national grid. Importantly, Univ has demanded no gas power to the new buildings or indeed to the renovated buildings (the Nursery and 115 Banbury Road). It is a development of which we can all be proud. Have there been any consequences of this move to drop gas for the new buildings?
Univ North’s substation to provide the energy required for the new buildings which use no gas
RT: There are a few unintended consequences that come to mind, and a surprise. The renovation of the Victorian villa at 115 Banbury Road, an important building in the new project, has demanded yet-to-be resolved thermal insulation and draft exclusion interventions to enable it to be satisfactorily heated by air source heat pumps.
Air source heating is not straightforwardly up to the demands of unmodified Victorian structures! We have also had to augment the heating and hot water supply with direct electrical heating to ensure all is good through the higher demand winter months.
Unsurprisingly, when one takes away the gas supply, the energy void has to be met with additional electrical power. This has meant a substantially upgraded electricity substation on site and considerable runs of mains voltage distribution. Underpinning all of this has been careful full lifecycle carbon costing.
Univ North – Woodstock Pavilion made from low embodied carbon “Eco” Blocks
One piece of analysis revealed a surprise. As we sought to resolve the optimal structural materials for the Woodstock Pavilion, we deliberated over whether it should be CLT, structural insulated panels or traditional blockwork. It was found that on a full cycle, carbon cost basis, traditional blockwork was slightly better than CLT for this building, which differs from the villas. After much debate, we decided to build Woodstock Pavilion from blockwork with timber floors and mineral fibre board cladding. We have also sourced “low carbon” blocks, with reduced embodied-carbon concrete, to press the advantage.
AIG: How have we relied on other high-tech modelling to guide choices to maximise efficiency and improve carbon cost?
Univ North – Water Court West undercroft showing the CLT-insulation build-up before the high-quality brick outer skin
RT: The College has procured a “Heat Decarbonisation Masterplan” which covers all the College’s premises. The model that the College commissioned for that study has been modified to address the question of how the new buildings at Univ North will cope with real students in occupation, coming in and out of their rooms, opening windows and leaving doors ajar.
Univ North Nursery roof showing the solar panels to augment the electrical supply, September 2024
Having interlocks on the windows, and heating controls that respond according to actual room occupancy, means that if an occupant opens the window, or leaves the room entirely, the heating will switch off quickly and efficiently. This creates huge advantages in avoiding wasted energy. For the Nursery, we have photovoltaic cells to augment the electrical power and a green roof (planted with vegetation) to absorb heat and limit solar gain on the building.
AIG: This is an extremely heartening story: there is no merit in waste given the demands of today; the buildings should be cosy in the winter and the air handling units should provide fresh air into every room that has had the warmth from the exhaust air extracted to again ensure that waste is minimised. Even the cooker hoods, for example, are part of the scheme as they remove grease and moisture and then heat is extracted to warm the fresh incoming air.
Univ North site showing three of the five buildings in construction, September 2024
The air heating load is less than 20% of what it would be otherwise. Let us finish by looking at the built environment holistically. Univ students have been living at “Stavertonia” for 50 years, and the development will open its doors to a new children’s nursery, and an integrated site next to our neighbours at the Fairfield Residential Home. How is this multigenerational community expected to benefit from the new buildings and enhanced environment all set in the North Oxford Victorian Suburb Conservation Area?
RT: The developed site will be fabulous. It will offer a genuine opportunity for enhanced living and improved wellbeing. The architecture is “placemaking”, if you’ll forgive the jargon. The potential for the old, the young and Univ’s students and staff in between, to find points of connection and interaction in that built environment and breathe life into a vibrant, happy location is very real. Univ North will truly be a complementary site to the College’s High Street location.
