Geothermal boreholes passive house
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The Hertfordshire passive house using vertical geothermal boreholes

This villa named Ashraya and submerged in the english countryside is designed in laminated timber and concrete for exchanging heat 90 metres underground

Kirkland Fraser Moor

Geothermal boreholes passive house
By Redazione The Plan -

I'm going to move to the countryside. How many times have we thought that? Boosted by the recent complicated times and working from home, what could be called the shift to the country is making a lot of rustic dreams come true. Nature at your fingertips, silence, clean air ‒ these are some of the attractions of villages, alongside the chance to experience new and different ways of living.

Let's look at the Ashraya example. This low-carbon house in Aldbury, in the English Chiltern Hills (Hertfordshire), was designed by the Kirkland Fraser Moor (KFM) architecture studio. It is a three-level passive house whose green roof is an extension of the meadows surrounding it on all sides. Nevertheless, its unusual feature is its geothermal system including 90m-deep vertical boreholes. This makes it an extremely eco-sustainable and energy-efficient building, which sets heat exchange a cornerstone, blending it harmoniously with the English countryside

Casa Passiva Ashraya - Kirkland Fraser Moor © Edmund Sumner, courtesy of Edmund Sumner


The Hertfordshire passive house using vertical geothermal boreholes

The KFM design for Ashraya extends the long straight time-honoured flint perimeter wall of the neighbouring garden that stands above the arched roof, thus creating perfect harmony with its environment, and vaguely echoing 1960s land art. The layout of this house looking out onto courtyards on either side is a key aspect in the Ashraya project. The western part of the garden can be seen from the road, while the eastern courtyard offers a more secluded area. 

The choice to build a passive house here was not by chance: the site lies one kilometre north of the village of Aldbury, in the Stocks estate, whose Georgian architecture expanded on an edifice dating back to 1270, with manor house and walled gardens retaining their elegance still today. Ashraya stands just south of this estate, and the tall perimeter walls and bordering hedges further underline the architecture chosen by the designers, creating continuity and connection between the old and new walls (which are nonetheless identifiable from their different construction styles).


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How the Ashraya geothermal boreholes work

The stone-clad roof covered in turf in continuation of the surrounding garden is interesting, but so too is the structure of the house, which is cross-laminated timber supported by a precast concrete base.

Yet what makes Ashraya an authentic passive house is its geothermal system, which uses 90m-deep vertical boreholes. As a principle, a system of this type utilises the land or the water it finds underground as a heat source or disperser, and carries this heat by means of the water or an antifreeze liquid. The pipe system running through the ground may be open or closed: it is open in the case of Ashraya. 

What does this mean? That the groundwater is used both as a source of thermal energy and as a fluid flowing in the circuit exchanging energy with the heat pump, and the boreholes are fed by the aquifer itself. Water is drawn from this and sent to the heat exchanger, which is connected to the heat pump. Then the water is pumped back into the aquifer using another borehole. When the same borehole is used, the return water is pumped in at the borehole surface while the feed water is drawn from the pressurised bottom. In this case, the pipes are placed vertically in designated pre-perforated boreholes making the system a vertical one. 


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Casa Passiva Ashraya - Kirkland Fraser Moor © Edmund Sumner, courtesy of Edmund Sumner


Walls made of straw, clay and chalk blocks

The interiors display a contemporary style, with natural materials worked by local craftspeople. The bedroom walls are built of various blocks mixing straw, clay and chalk plus a little natural lime. This detail too is not by chance: the clay blocks have one of the lowest carbon footprints among the various construction materials on the market, with a level averaging at just one-tenth of their concrete counterparts. Instead, the local clay plaster has been hand-finished in the day zone to render an effect similar to stone.

The layout includes a basement space with a study and a billiards room, while beyond the entrance hall, on the ground floor, stands a west-oriented full-height glass expanse partially fitted beneath the timber ceiling. A chimney rises above the roof, creating a break in the curved design. The kitchen is a large open space and extends to the dining room, made up of a semicircle protruding into the eastern courtyard. The night area on the first floor offers two small bedrooms.


Casa Passiva Ashraya - Kirkland Fraser Moor © Edmund Sumner, courtesy of Edmund Sumner


Ashraya is an innovative piece of rural architecture that is harmoniously at balance with its surrounding landscape and ecology. It continues and enhances the strong vernacular regional tradition without resorting to pastiche, commented David Kirkland, founder and partner at the KFM architecture office.




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Location: Aldbury, Hertfordshire
Site area: 1.1 ha
Project by: Kirkland Fraser Moor Architects
Contractor: Husker
Photography by Edmund Sumner, courtesy of Kirkland Fraser Moor Architects

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