Project team

Architect – Grain Architecture | Design consultant – Hartwyn | Engineer – FK Leung | Contractor – Hartwyn

  • Foundations – recycled compacted concrete aggregate, Limecrete, concrete.
  • Plinth walls – locally reclaimed brick, lime mortar.
  • Structure – load-bearing timber frame, with local chestnut roundwood section.
  • Roof – timber frame, profiled metal sheeting.
  • Insulation – straw bale walls, foamglas® foundation and subfloor, wool and wood fibre roof and reveals.
  • Airtightness – taped joints at openings and junctions, membrane to the roof, fully plastered inside and out for main structure.
  • Finishes – Oak joinery, earth floors, clay paint, limewash, linseed oil and beeswax.

The Brief

This project needed to serve several purposes. Two workshops; one for jewellery, one for pottery. A covered outside eating area, a log store, garden store/workshop, and a kiln room. The clients wanted it to be sustainable, low impact, and ultimately a beautiful addition to their property.

The new building replaced an existing 100-year-old timber barn that was in derelict condition. The initial use is recreational and functional, but we always design our buildings with the future in mind. Straw bale buildings are designed to last and will inevitably be used for different functions over time. We built this structure to meet and exceed building regulations standards so that it can be as versatile as it needs to be throughout its life.

As with all of our builds, we aim at very low carbon, both in the embodied carbon of the materials and the operational carbon in the building’s use. This strategy works well when we are involved from the very early stages, but is more challenging if we join later in the project.

The design for this straw bale building went through various revisions with the original architects, and while it had a good selection of materials aimed at low operational carbon, it was complicated and had missed opportunities to engage with low carbon design.

When put out to tender, it always came back well above the intended budget, which led to value engineering and moving further away from the clients brief.

Specs

Footprint: 160 m2

Wall u-value: 0.13w/m2k

Roof u-value: 0.14w/m2

Build cost: £1500/m2

A fresh start

Grain Architecture took over the project and brought us in. We worked closely together to reimagine the design using our palette of local natural materials; bringing back the elements that we lost in the previous value engineering, lowering the upfront carbon commitment, and keeping the values of efficiency and aesthetics close.

Combining these materials effectively can be complex. We hold ourselves to very high standards both in terms of the craft and the performance. We consider whole life carbon to be a key part of the design, so we think about how the materials can be repurposed at the end of the buildings life – fixing with screws to allow easier disassembly, using materials that can be upcycled or composted, considering re-use over recycling – this building is not a fixed point; it will change and move and take on different functions, so it needed to be flexible in every sense of the word.

A slate DPC was used to minimise any moisture ingress travelling up from the porous brick.
A slate DPC was used to minimise any moisture ingress travelling up from the porous brick.

Foundations

The foundations presented a challenge. Ground conditions were poor and wind uplift was a significant concern due to the coastal location. The aim was to reduce concrete use at every turn so a complex hybrid solution was designed with eight distinct foundation build-ups using a mixture of:

  • Limecrete
  • Free-draining compacted recycled concrete aggregate
  • Minimal cast-in-situ concrete

Weighing up priorities at this stage was important. Achieving the most sustainable option was by no means simple or inexpensive, though the material costs were mitigated somewhat by on-site processing.

Straw bale walls need to be elevated to protect them from water ingress and a plinth wall is usually used for this. In this case, we specified an outer leaf of reclaimed local brick bedded on lime mortar and an inner leaf of structural foamglas block to eliminate thermal bridging along with foamglas aggregate cavity infill.

Superstructure

The superstructure was a dimensioned timber loadbearing frame, with a roundwood section (local green chestnut). When designing timber frames to suit straw bale we want to reduce the timber fraction as much as possible. This means that racking details are very important to get right. Working around window and door openings presents it’s own challenges, but our experience and a good engineer always find a way.

Our experience and a good engineer always find a way.

– Hartwyn

The result was an affordable project that delivered the clients brief, on time and on budget.
The result was an affordable project that delivered the clients brief, on time and on budget.

Insulation

Insulation is one of the main ways we can sequester carbon in our buildings by utilising natural materials. These have low embodied carbon from production and processing, and also trap a certain amount of carbon for the duration of the building’s life.

This is a careful balancing act; it’s not as simple as ‘high embodied carbon = bad’. Where has that carbon come from? Is it sequestered within the material, or was it emitted as part of the processing or transportation? We took this into account and make decisions based on the available data and our client brief: sustainability, performance, aesthetic, and budget.

We used straw bale for our walls, using the infill method rather than load-bearing. The roof had a combination of woodfibre and wool insulation, which we duplicated at crucial thermal bridge junctions in the eaves and around windows and doors.

Floors make up a significant part of our thermal envelope and we took care of the insulation element with foamglas aggregate. With this style of build, the floor is a really good opportunity to provide useful thermal mass. We specified a poured earth floor, which has a rammed earth sub-base using spoil from the site.

Finishes

The structure of a building is one thing – structure is incredibly important for ensuring longevity. However, it’s the final finishing touches that really helped make this straw bale building perform well. After all, this is mainly how we interact with a building, so the finishes needed to be appropriate, robust, and joyful!

We used clay plaster internally and lime render externally with local green larch cladding. We also used clay paint and limewash for the plastered surfaces and either linseed oil or beeswax for the rest. The windows and doors were handmade oak, with oak window sills.

All of these materials perform well, are non-toxic, promote healthy indoor air quality and provide technical performance through moisture buffering and humidity regulation.

Sustainability was an important factor.

– Grain Architecture