Keeping it simple - Hook Hollow social housing
A Passivhaus terraced development of 10 affordable homes at Seend in Wiltshire has recently been completed for White Horse Housing Association and Seend Community Land Trust. The project has been designed and constructed with robustness, simplicity, and cost-effectiveness in mind. The development has four 2-bed, one 3-bed and two 1-bed homes for rent as well as two 3-bed and one 2-bed for shared ownership sale.The project was the first Community Land Trust affordable housing scheme in Wiltshire to provide rental and the shared ownership homes to be occupied by people with a local connection.
PHT Patron Passivhaus Homes was the Passivhaus Consultant on the project and delivered the scheme using its PH15 offsite timber frame system. The architects had not delivered to full Passivhaus standard previously but understood the importance of simplicity for the overall design of the houses, including having duplication of house type within the mix. Working with Passivhaus Homes the construction detailing was kept as straightforward as possible. This all helped to keep costs down and ensured airtightness was achieved at the preliminary tests.
The PH15 construction means that the insulation follows the pitched roof line (not the ceiling) so that all roof spaces are contained within the thermal envelope. This contributed to cost-savings, as it improves form factor on the three bedroom units and delivered extra useable space in the remaining houses. The smaller homes have a potential staircase designed in and are 'loft conversion ready'. The MVHR unit was located in the warm roof space.
Key stats
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A return to modest terrace housing over larger stand-alone homes is a simpler and efficient approach to building Passivhaus structures. If loft areas are fully utilised, form factors can be kept very low, thereby improving energy performance, reducing embodied carbon, and the construction costs. By paying attention to simplicity and cost sensitive strategies, we can help to enable more Passivhaus affordable houses being built.
Jae Cotterell, Director, PH15 & Passivhaus Homes
Construction
The homes are constructed from pre-cut timber frames which were then assembled, insulated with wood fibre and recycled cellulose and sealed with airtight wind and waterproof tape before the exterior was clad. The dimensional stability of the frame is important to control when using offsite methods such as pre-cutting. The PH15 frames' dimensional stability is enhanced by using laminated veneer lumber (LVL) which is much more dimensionally stable than solid section wood. The PH15 shell was constructed under a tent system which is a full scaffold with temporary tin roof over to give full weather working and avoid regular wetting of the frame. A brick cladding exterior wall was specified as an additional robustness requirement, as requested by the housing association.
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Embodied carbon
The PH15 Construction System uses low embodied carbon materials, with a primary focus on timber. The system uses a range of timber variations including Laminated Veneer Lumber (LVL) for beams, lintels and base plates; wood fibre insulation for air injection into wall and roof cavities; flexible wood fibre or hemp insulation for filling smaller spaces; and a wood fibre sheathing board as an external insulating layer. LVL uses timber more efficiently, 35% to 50% more of the timber can be used when you create an LVL I-joist compared to when using solid section timber. Using PH Ribbon as calculation tool, PH15 projects have been calculated to be typically 70% lower in embodied carbon than a masonry Passivhaus cavity wall construction (A1-A4 Cradle to Site). However, with the specification of the additional external brick wall in this project, the embodied carbon savings were not quite as significant, delivering an approximately 45% reduction.
Roof: 0.097 W/m2K
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Wall: 0.1 W/m2K |
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Floor: 0.12 W/m2K |
Building performance
Designed energy performance |
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Airtightness n50 (≤ 0.6ACH @ 50 Pa)
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0.2 @ 50 Pa
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Space Heating Demand (≤ 15 kWh/m².a)
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15 kWh/m².a
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Heating Load (≤ 10 W/m²)
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9 W/m²
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Primary Energy Renewable (PER) Demand (≤ 60 kWh/m².a*)
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69 kWh/m².a
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Overheating %
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< 1% |
*+/-15 kWh/m².a allowance if offset by energy generation. See Passivhaus criteria.
Services
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Space heating: The client's brief called for gas-free heating but without specification of a full air source heat pump system. A simple and low-cost heating strategy was implemented, requiring two small electric radiators (located in the upstairs and downstairs halls) and electric towel rails.
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Hot water: For the hot water strategy, a hot water cylinder with an integrated mini heat pump was specified, offering a space saving, simple and cost-effective solution. This hot water heat solution has a stated Coefficient of Performance (COP) of 3, so should be delivering hot water energy efficiently.
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On-site generation: 4 solar PV panels were specified for each house. There was potential scope for more but budgetary constraints limited the size of the solar arrays.
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Summer comfort: Overheating risks were designed out in the early design stages. In PHPP this equates to less than 1% of days where the internal temperature exceeds 25C.
Challenges & lessons learned
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Ground levelling: A lot of levelling groundwork was needed on the site, resulting in a more expensive ground floor construction solution. A suspended concrete beam floor then supports the insulation on top of a concrete floor slab.
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Brick cladding: The client specified brick cladding for the timber frame homes for additional robustness & longevity. The brick cladding was not structurally required but needed independent support.
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Project costs: The ground levelinng and brick cladding both elevated the project costs but are unrelated to achieving Passivhaus. It is important for the delivery team to have clarity on design and site costs that are/are not specific to Passivhaus.
Architect’s viewThe layout has been carefully considered, taking into account the site's location, Passivhaus principles, and views into and through the site from the highway and neighbouring dwellings. The dwellings themselves follow the local vernacular with their scale and form. Tom Kent, Director, PKA Architects |
Key team
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The Hook Hollow project is one of four case studies being showcased in our 2025 'How to build a Passivhaus' webinar series. Book a place on the intermediate level course to find out more lessons learned from the project. To encourage take-up of Passivhaus among social housing providers, the webinar series is FREE for housing associations and local authorities.
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Further information
Hook Hollow, Seend: Passivhaus Homes
Hook Hollow, Seend Cleeve: White Horse Housing Association
Community led affordable housing at Seend Cleeve: Seend Community Land Trust
'How to build a Passivhaus' webinar series 2025
Passivhaus Benefits Guide & costs research
Previous PHT Story: Social housing championing Passivhaus at scale - 12 May 2021
28th January 2025
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