31 May 2021
Four practices at the forefront of future living
Photo: Passivahus Homes Ltd.
While TRADA eagerly anticipated the low energy, low carbon timber and hybrid timber designs submitted by student and graduate teams as part of the Riverside Sunderland: University Design Challenge that ended last Friday, our university engagement manager Tabitha Binding compiled some of the guidance given by four practices at the forefront of future living to the participants.
Tabitha will also provide her own take on big questions about future homes and future living, and provide answers to... why carbon neutral, which targets, why timber and which framing options.
The world is warming. The Paris Agreement adopted at the end of 2015 aims to limit the increase in global average temperatures to 1.5°C compared to preindustrial levels in order to mitigate the twin catastrophes of climate and biodiversity breakdown by reducing global greenhouse gas (GHG) emissions. The construction and operation of buildings account for 39% of these emissions, which is why those of us in the built environment sector have a significant role to play by designing buildings that minimise embodied and operational carbon.
LETI, in its Climate Emergency Design Guide, suggests that the UK construction industry can have a bigger impact as UK emissions are even higher than the global average at 49%.
Routes to carbon neutral
‘Net zero’ or ‘carbon neutral’ refers to achieving an overall balance between emissions produced and emissions taken out of the atmosphere.
In June 2019, the UK set in law a target to bring all its greenhouse gas emissions to net zero by 2050 – one of the most ambitious targets in the world. This net zero target was recommended by the Committee on Climate Change, the UK’s independent climate advisory body.
To achieve a net zero carbon building, the sum total of the Whole Life Carbon (WLC) emissions – both operational and embodied, over the buildings life cycle, including disposal plus offsets – equals zero, as set out in RICS’ Life Cycle Assessment for the Built Environment stages or modules (LCA).
With no clear legislation on getting to net zero by 2050, RIBA and LETI have taken the initiative to draw up a matrix of measurable targets and indicators to aid designers.
The Challenge sets a series of targets for practices to adopt to reduce operational energy, embodied carbon and potable water use. It also encourages sites to be left with significantly enhanced biodiversity and more green cover than before development, and promotes the importance of meeting targets on temperature, daylight and indoor air quality in avoiding unintended consequences of poor health and wellbeing.
LETI’s Climate Emergency Design Guide has set out a route map and key performance indicators as to how new buildings can meet UK climate change targets and get to net zero by 2050. Small and medium scale housing are two of the four archetypes considered.
Key findings and recommendations from the UK Committee on Climate Change’s (CCC) report UK housing: Fit for the future are:
- The UK’s legally binding climate change targets will not be met without the near-complete elimination of greenhouse gas emissions from UK buildings.
- New homes should be built to be lowcarbon, energy and water efficient, and climate resilient.
- Improve focus on reducing the whole-life carbon impact of new homes, including embodied and sequestered carbon. Using wood in construction to displace high carbon materials such as cement and steel is one of the most effective ways to use limited biomass resources to mitigate climate change.
The detailed understanding of the emissions savings that could be achieved at an individual unit level and an economy wide level through greater use of timber framed techniques as well as newer engineered wood products can be found in the CCC commissioned report Wood in Construction in the UK: An Analysis of Carbon Abatement Potential.
Building with timber reduces the embodied energy of a new home and sequesters carbon for the life of the timber products within the home. Building to an energy standard such as Passivhaus reduces operational energy. Building to the Passivhaus standard with timber will harness all three benefits.
Timber frame options
Timber comes in different species, different sizes and different products. To select appropriate products that cause minimum environmental harm and implement optimum solutions to address the environmental crisis we face, we asked professionals at the forefront of future living to talk about the timber frame systems they use in order to inform and guide our Riverside Sunderland: University Design Challenge participants.
Mikhail Riches, Goldsmith Street – twin wall timber frame
Designed by TRADA member Mikhail Riches for Norwich City Council, whose aim was to alleviate fuel poverty for the occupants, Goldsmith Street is the largest Passivhaus scheme in the UK. Behind the brick façade, it is timber frame with cellulose insulation.
Designed, manufactured and installed by TRADA member Cygnum Timber Frame, the highly insulated timber frame structure was built to Passivhaus standard. Cygnum’s Passive system features a twin-wall sheeted on both sides, with a membrane on the inside forming the airtight layer. The build-up featured an 89mm stud, and a second 67mm stud, fully cellulose-insulated with an overall wall thickness of 399mm, which is designed to be thermal bridge free. The intermediate floors and roofs were factory assembled cassettes with the cellulose-insulated zone to the roof using 400mm engineered joists.
To achieve passive certification, they had to ensure that everything – materials, detail design and factory quality control standards – was measured and monitored. Four key factors had to be addressed: U-values, thermal bridging, interior surface temperatures and airtightness.
Airtightness is important for many reasons, including reducing heat loss, improving comfort and protecting the building fabric. Airtightness is achieved by sealing a building to reduce infiltration – which is sometimes called uncontrolled ventilation.
Airtightness is measured by monitoring the amount of air that escapes or enters a building at a pressure of 50 Pascals. For Passivhaus calculations, this measurement is expressed in air changes per hour (ACH) i.e. the number of times an hour that the air in the building changes when it is pressurised (either negatively or positively). Goldsmith Street’s airtightness was measured at 0.44 ACH @ 50 Pa. which exceeds the Passivhaus airtightness standard of ≤ 0.6 ACH @ 50 Pa. See Demystifying Airtightness by the Passivhaus Trust.
The Goldsmith Street scheme won the RIBA 2019 Stirling Prize. Post occupancy evaluation is being undertaken to see if the aim of annual fuel bills of £150 has been achieved.
Watch the design team – which was composed of James Turner, Associate Director, Mikhail Riches; Harriet Bourne, Director, BBUK Studio; and Sally Godber, Director, WARM – as well as Andrew Turnbull, Housing Development Manager Norwich City Council, discuss Goldsmith Street
LEAP, Larch Corner – CLT
Larch Corner achieves zero carbon status because, according to the EPC, emissions are -2.2 tCO2e/yr. Cross-laminated timber (CLT) is the structural system chosen by LEAP Principle and Passivhaus Architect Mark Siddall for Larch Corner, a contemporary 3-bedroom single storey home in Warwickshire. Almost every fibre of this home – including its structure, insulation, cladding and light fittings – is sustainably sourced timber, reducing embodied carbon emissions during processing, manufacture and construction.
Materials include cross-laminated timber (CLT), engineered timber I-beams, woodfibre insulation, Larch cladding and Redwood timber triple glazed windows from Green Building Stores’ ULTRA range, which achieved a whole window U value of 0.74 W/m2K. The timber frame of the ULTRA windows was integrated into the wood fibre wall insulation to minimise thermal bridging.
By careful design, using CLT as the air barrier and with exemplary workmanship, the Air Permeability was measured at 0.041 m3/hr/m2@50Pa. This, the UK’s most airtight home, is 244 times more airtight than Building Regulation requires.
The AECB Water Efficiency Standard was adopted for the project to help minimise the amount of energy used to provide hot water. Measures included minimising the volume of ‘dead legs’ and a super-insulated storage cylinder.
Space heating and hot water is provided by an air source heat pump. As a certified Passivhaus integrating 9.3 kW photovoltaic array, Larch Corner achieves zero carbon status because, according to the EPC, emissions are -2.2 tCO2e/yr. The ‘excess’ energy is used for charging a fully electric car.
PH Homes, Akerman Rd – I-Joist PH15 System
PH Homes utilise timber I-joists in their PH15 optimised complete construction system that details and supplies the thermal airtight shell of projects, including timber framed triple-glazed windows and a heat recovery ventilation system, delivering net zero or carbon neutral homes. PH15 pre-cut the timber frame off-site.
Designed to simplify all the technical aspects of building up to the 2025, 2030 and 2050 Climate Change carbon targets, PH15 can be net zero carbon for operational energy, using the Passivhaus energy efficiency standard and renewable on-site energy, and net zero for embodied carbon using timber frame and non-fossil fuel-based insulation that sequester carbon and offsetting the residual embodied carbon with tree planting in the UK.
The timber frame is designed in 3-D, which includes structural engineering. The shell includes the intermediate ‘open webbed’ floor joists and the floorboards, as well as the stud work for the internal walls (no stairs). The engineered I-joist frame, LVL beams and lintels are all pre-cut off-site and arrive flat-packed ready for erection, with easy-to-read frame erection drawings.
Other elements included are the high-quality air-tightness tapes, membranes and grommets, the guaranteed airtightness/racking boards, all the natural insulations including external wood fibre boards and the blown-in insulation (also installed on site). The PH15 wall/roof construction comprises over 85% insulation, giving PH15 a very low timber fraction. And to top it off, two air-tightness tests are included with the system purchase.
A choice of triple-glazed timber windows and energy-efficient doors, typically clad externally in aluminium for low maintenance, and a fully designed, supplied and commissioned ventilation system are also provided.
PH15 was used in a social housing project for Lambeth Council at Akerman Rd, London. The site is an urban infill on a typical London street. Designed by Anne Thorne Architects, the finishes are a mix of lime render, brick entrance porches, brick slips and reconstituted slate roofs. This project was the first use of the PH15 for social housing provision and came in under £450/m², achieved in part due to the excellent Form Factor (1.8) from the three storey terrace layout. Airtightness test results 0.34, 0.39 and 0.4 ach @ 50Pa were achieved. A post occupancy study by UCL highlighted health benefit outcomes.
Watch Chartered Architect and Director of Passivhaus, Jae Cottrell, speak about the PH15 system and Akerman Road
MAKAR, Geanaisean – home-grown timber
MAKAR Ltd is an architect-led design, manufacture and construction company established in 2002. Existing to design and deliver low carbon, healthy, comfortable, beautiful and low-energy homes, they only use environmentally friendly building materials, including Scottish-grown timber. Over the last 18 years, they have designed and built 100+ homes and buildings, incrementally improving their knowledge and designs which use their closed-panel off-site manufactured natural SIP (n-SIP) system. This nSIP system utilises several layers of hydroscopic materials – materials which insulate and are airtight but allow the safe passage of moisture in a controlled manner without degradation while avoiding interstitial condensation.
Geanaisean is MAKAR’s first certified Passivhaus home. Their client was keen to achieve the Passive house standard using a healthy, ecologically-sound construction system, which perfectly matched Makar’s ethos.
The construction is an enhanced version of the standard Makar timber kit, with an additional thermal insulation jacket formed of wood fibre board. OSB board was used internally as the air-tight layer, with the joints taped to preserve the air-tight envelope. This meant that membranes were not needed to achieve the airtightness levels required. Airtightness was measured at 0.36 ACH @ 50 Pa.
Although Passive Houses require almost no space heating, the client was keen to use the woodland resource around his home. Therefore, a wood burning stove with back boiler to a large hot water store was installed. This provides all the hot water and space heating to 2 radiators and towel rails, which is the entirety of the heating system in the house.
In addition, PV (electric) and solar thermal panels were installed on the roof, both of which off-set any energy required for hot water heating. A rainwater harvesting system feeds both WCs, the washing machine and the garden taps. Air quality is maintained in the house by way of a mechanical ventilation with heat recovery (MVHR) system, which replaces stale air with pre-warmed fresh air. Every room had at least one openable window to avoid overheating on really warm days.
The house was recognised at the annual awards for the Alliance for Sustainable Building Products in 2020, both for Best New Build and Best Product.
Watch Architect, Founder and Director at MAKAR Ltd, Neil Sutherland, as he talks about the Makar system and Geanaisean build
About the author
TRADA’s Tabitha Binding drives our busy University Engagement Programme, which proactively seeks to encourage lecturers of architecture, engineering and other building-related courses to teach timber as an equal to other materials. To facilitate this, TRADA has developed a range of free teaching materials, runs high-profile design competitions for students, and creates opportunities to collaborate, uniting universities with members of the timber industry.
24 May 2021