So much more than just a Garden Room

Garden Room

There’s more to the Garden Room, at Cwm Barn, than its name suggests. Arbor Architects designed an elegant, timber-framed annexe with Japanese influences. It’s a space to get away from the house – the perfect complement to a family home in rural Peterchurch, near Hereford.

The clients wanted a flexible space set a little apart from their barn-conversion house, Cwm Barn. For the three teenagers in the family, the new annexe provides a place to hang out with their friends. It is also a very comfortable space for guests to stay overnight, and features a home office room, kitchenette and a small gym.

External view of the Garden Room at Cwm Barn

The walls are clad in zinc standing seam

A chance to upgrade
Elly Deacon-Smith and her colleague Matt Hayes – both of them Directors and Co-Founders of Arbor Architects – explain that the clients also wanted to make the whole site more energy efficient. Their house had been converted from a barn in the 1990s and was heated by a large oil tank.

They wanted to move away from fossil fuels, reduce their heating bills and build up their home’s energy resilience – the site is quite remote and there are often power cuts in the winter. From a quality of life perspective, the new annexe was an opportunity to really make the most of the beautiful site – perched on a hill overlooking a valley.

A big part of the brief was that the Garden Room should connect with nature. Arbor Architects’ design sought to celebrate the rural location and they incorporated their clients’ suggestions of a Japanese aesthetic into the project’s look and feel.

A timber frame and a panel system
Arbor always try to build with fabric-first principles, and this new annexe was to be no exception. Firth Construction was the main contractor, supplying and installing the panel system that made up the structure of the annexe.

Everything about the design was focused upon the interface between the building and its surroundings.

The walls were built offsite from a twin-stud panelised system. Posi-joists were used in the roof and were installed onsite. The timber frame was spruce and is fully filled with Warmcel – a recycled newspaper insulation which is a breathable and helps regulate moisture.

“We used the construction method we follow on most projects, which essentially follows Passivhaus methodologies: an insulated raft slab at the base, with the insulation wrapping around this and joining up with the insulation in the walls and over the roof. So the whole building basically has a duvet layer of insulation, which reduces thermal bridges and really brings down its energy demand.”

The structure
The raft slab is concrete with Isoquick insulation underneath it and wrapping up the perimeter of the slab: the Isoquick essentially being the formwork for the concrete as well as the insulation, Elly states.

“The twin stud structure in the external walls allows the cavity to be fully filled with insulation, minimizing thermal bridges, much like using a Larsen truss system,” explains Matt. “ The inner line of studs are structural and sit on a wall plate around the perimeter of the slab. This wall plate sets the dimensions. Then the timber frame was built offsite, to those dimensions. Firth Construction are able to do a little bit onsite, and a little off site in their factory. The factory of course, allows for achieving better tolerances and keeps the timber dry.”

Local oak was used for the flooring but also for this striking TV cabinet

The site is sloping, so gabion baskets were installed to the north of the plan where the land falls down, and beneath the area where the decking sits.

Using wood wisely and sensitively
“We worked closely with structural engineers Build Collective,” Elly says. “Wherever possible we tried to take the specified timber down from C24 to C16 to allow us to specify homegrown timber.” Chestnut from local woodland was used for the decking, there is larch cladding at the gable end of the building, and internally, oak was used for floor and wall finishes.

Douglas Fir was used for the exposed beams. Above the full-width bifold door, LVL (laminated veneer lumber) was used rather than steel.
Arbor took on board their client’s interest in the Japanese philosophy of wabi-sabi, which celebrates the marks of time and age in the materials used.

As you come into the Garden Room you encounter a dark, small space that uses reclaimed timbers – which have texture and age embedded in them. The doors are “hidden” in panels, with simple Arne Jakobson handles. Going through them, you then enter into a contrasting brighter space.

The beams are made from Douglas Fir. Much of the wooden furniture, including the side table shown, was made locally by Barnby Design.

The beams are made from Douglas Fir. Much of the wooden furniture, including the side table shown, was made locally by Barnby Design.

Internal joinery
Firth appointed a local specialist subcontractor for the internal joinery, who achieved striking results with homegrown oak for the paneling in the main multi-function room. He also used Valchromat (a formaldehyde-free organically dyed MDF board) in several places: one being a storage wall in which a pull-down bed is concealed.

Locally-sourced oak was used for the flooring, window cills and the TV cabinet. Whitney Sawmills supplied the wood from their woodlands near Hay on Wye. Some of the furniture is also local and wood-based too, made by Barnby Design.

Roofs and rainfall
The site is quite exposed and the house receives a lot of rainfall. Drainage was therefore a big concern for the client – when they get storms there’s a lot of run-off in surrounding fields. All around the perimeter of the building land drains were installed to take water away from building.

“The pitch of the roof was determined by the necessity of getting rainfall off it and away from the building,” remarks Ellie. “The gutters are oversized, with a chute coming down at the centre point. This falls into an expressed corten container, which in turn falls into a rill that runs away down the slope.”

The pitch also had to be right for the photovoltaic panels (PVs) that were to be installed and the roof was designed with their load in mind. It’s clad in zinc, and the clips of the cladding’s standing seam allow the PVS to be fixed down – so there is no penetration through the finish of the roof. And beneath the standing seam there’s a ventilated zone so any water that does get into this space is wicked away.

The cladding: zinc and splayed larch
The majority of the external walls are clad in zinc. All the standing seams line up perfectly with the edges of windows and doors, and with the standing seams on the roof. But on one side of the building – where full-width bi-fold doors lead into the multi-function room – the cladding is splayed larch. This was treated with a grey finish, by Osmo, to pre-weather it.

A chic and discreet Arne Jacobsen door handle sits on a door of local oak.

Shifting away from fossil fuels
The annexe features a fairly large plant room. That’s because it supplies the entire site’s energy – so it needs to house both battery storage and water tanks for two air source heat pumps (one for each building), as well as an MVHR unit.

Ultimately, only modest upgrades were made to the existing main house, such as improved glazing, in order for the heat pumps and PV array to meet its needs.