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Fawley Passive House: Design process

Updated: May 23, 2023

This new home, in many ways, exemplifies the ideal project for GreenTrace Architect: a modest yet joyous passive house built with low carbon, natural materials...on a relatively tight budget. The home is designed to be so well insulated and airtight that it will only require 2 towel rails for all the space heating! Most of the year the house will not need any heating at all, thanks also to the solar passive design and thermal mass.

Rear view showing how the roof lifts over the entrance

Conceptually, the house is inspired by a nut, with a soft inner environment protected by a hard, tough outer shell. The nut opens over the entrance to reveal the warm inner space.

The concept is inspired by a nut

The planning strategy for this project included obtaining a certificate of lawful development for an existing timber shack that had been occupied by a farmer's friend. Once this shack was given lawful dwelling status, a replacement dwelling application was then submitted which also sought to reposition the dwelling in a better location fronting the road and making the most of the beautiful orchard views to the south. Being relatively confident that planning consent would be granted, a decision was taken to progress the technical drawings whilst the application was pending a decision.

Planning drawing showing the new dwelling relating to the street

The clients have strong ambitions for a sustainable, future-proof dwelling with low running costs. The gold standard in energy efficiency is the Passive House standard but achieving can add costs that would stretch the client's budget. However, the pioneering mortgage providers - the Ecological Building Society, confirmed that they would lower their interest rates if certification was achieved; an applaudable incentive which encouraged the clients to aim for certification.

Passive House certification requires consideration right from the outset of a project to ensure that key decisions such as the form factor and orientation of the building are optimised to encourage heat gains and thermal efficiency. In this case, even though achieving certification was a decision that came later, the design submitted at planning stage aimed to maximize energy efficiency by adopting the passive house principles. Reaching the stringent targets for operational energy use were therefore possible with only minor alterations.

Achieving the standard requires in depth calculation of the predicted energy use which is dependent on the form, fabric and services specified. GreenTrace Architect carried out the calculations using the Passive House Planning Package (PHPP) - an incredibly detailed excel spreadsheet that does much of the calculation but requires a good working knowledge of building physics and the passive house principles.

Extract from the PHPP excel spreadsheet summarising the heat balances etc

The PHPP model indicates that the house can achieve passive house certification and design-stage approval has been granted. Below is a breakdown of the key elements:

  • The form of the house is relatively simple with a form factor of 3.5. This is the ratio of usable floor space to external surface area and is an indication of how efficient the form is at conserving energy.

  • The fabric has U-values ranging from 0.1 to 0.12W/m2K. These low values are achieved with natural materials such as wood fibre insulation, clay plasters and recycled newspaper insulation.

  • The timber windows are triple glazed and super air tight. The windows are from The Green Building Store and have U-values in the range of 0.67W/m2K.

  • The domestic hot water is provided by an Ariston Nuos Air Source Heat Pump with integral insulated hot water tank. This dramatically reduces energy demand; with a coefficient of performance of 3.05!

  • Highly air tight buildings require mechanical ventilation, and it is important that any heat expelled is recaptured via a heat exchange. The dwelling therefore includes a Ubiflux Vigor MVHR unit to ensure adequate ventilation, a healthy environment and minimal heat losses.

  • The space heating is predominantly achieved by solar gains, but for the few winter months heating is provided by 2 towel radiators connected to the ASHP.

GreenTrace Architect will be working closely with the clients and the contractor during the construction phase. Our role is to monitor the construction and accumulate the evidence required to demonstrate that the building is constructed to the high levels required to meet the Passive House standard; taking care to ensure extremely good levels of air-tightness and thermal bridge free detailing.

Detail of the eaves/wall junction that is thermal bridge free (less than 0.011W/m2K).

The project is in good hands with the contractor The Passive House Builder Ltd. Dai Rees has built many Passive houses including his own and is well aware of the level of workmanship required.

The clients deserve praise too; their aspirations and determination to make this a low carbon, sustainable dwelling is to be applauded. In addition to Passive House certification, they have also agreed to take part in the RIBA 2030 challenge which sets minimum targets for operational energy and embodied carbon.

We have huge respect for them and thank them for sharing our goals for low-impact architecture, and seeking to achieve such as high standards for the project.

We will be blogging more about this project as the construction progresses. Next blog will focus on the low embodied carbon of the natural building materials.

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