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SUSTAINCO The Green House Case Study

Wyróżniony Przypadek November 2013
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This case study is from the SUSTAINCO project. The Green House - a privately owned, retrofitted home in the village of Cusop, near Hay-on-Wye in Herefordshire. It is a highly energy-efficient building which makes best use of its natural surroundings and resources, including light and solar gain.


Building description:

The property was originally a 1960’s built dormer bungalow, but has essentially been encased in a new highly insulated and efficient shell with solar thermal providing domestic hot water and rain water harvesting. The original dormer bungalow was typical of its time in vernacular construction type. It was built in the traditional 300 mm cavity wall construction, single glazed, with low levels of thermal insulation and heated by an old and inefficient oil boiler. The new building is wrapped around the original structure therefore maintaining the exact same floor plan. The walls were significantly increased in width to include 300 mm thickness of thermal insulation. The original roof has been replaced with a zinc barrel-vaulted structure containing 400 mm of insulation, which is higher than the original roof ridge line. The windows and doors which were installed are high performance double glazed components and are argon filled. The new elevations make the most of passive south facing exposure to sun light which is used to both day-light and heat the building naturally. Most of the lighting is low energy. Heating is provided by a town gas powered condensing boiler and domestic hot water is provided by solar thermal panels supplemented by the boiler.


General data:

Name: The Green House

Net heated floor area: 236 m2

Owner: Private

Designer: CO2 Architects Ltd.

New/retrofit: Retrofit

Use: Residential

Location: Cusop, Hay-on-Wye, Hereford, England

Climate: “Sheltered”

Investment cost: £ 265 000;



Approach: A high energy-efficient building which makes best use of its natural surroundings and resources including light and solar gain.

Construction typology: Retrofit of masonry and timber, hung on a Glulam frame which utilizes the original masonry structure as thermal mass, with high levels of insulation.

U-value walls: 0,24 W/(m2K) (ground floor), 0,12 W/(m2K) (first floor)

U-value roof: 0,11 W/(m2K)

U-value windows: 1,70 W/(m2K)

Heating strategies: The south elevation has the largest glazed area which has a mezzanine floor between ground and first floors thus creating a space to collect heat gained from the sun.

Key points:

  • Minimized waste of construction material
  • High levels of insulation
  • Robust construction details
  • Utilization of natural light and solar gain
  • Low emissivity glazing
  • Low energy lighting
  • Renewable energy for hot water
  • Reduced water use (rainwater harvesting) and wastage 


What is special?

It is unusual to see such a radical retrofit to an existing building. To the untrained eye, it would not be unfair to assume that the original building was completely demolished and a new structure put in its place as there are very few visible signs of the original dormer bungalow. Instead, the architect has locked in the embodied energy of the original structure (thus minimizing waste and utilizing its thermal mass) and used this as the framework on which to hang the new highly insulated facades. The initial preference for steel to be used as the first floor external cladding was to reduce maintenance to a minimum. The client was an employee of a steel manufacturer and wished to promote the sustainable use of their products, however, the steel was changed to timber for technical reasons as it could not be bent to the curves of the elevation.



Delivered energy demand: (according to bills data collection, average values of years 2011 and 2012)

Electricity: 646 kWh/y; 2,7 kWh/(m2 y)

Gas (for heating and domestic hot water): 4503 kWh/y; 19,1 kWh/(m2 y)

Primary energy demand: 26,2 kWh/(m2 y)
(calculated according to the Interim Guidance paper GP 3.5 by the British Department of Energy & Climate Change (DECC): delivered to primary conversion factor for electricity = 2,6; delivered to primary conversion factor for gas = 1)

CO2 annual emissions: 1167,4 kg CO2/y; 4,95 kg CO2/(m2 y)

Renewable energy production: A 2-panel (4,2 m2) - solar thermal installation with a 210 liter twin coil solar thermal store.

National energy certification level: Commercial Energy Performance Certificate (EPC) rated: B (82)

Building emission rate: B (83)

Energy use for lighting: 1,2 kWh/(m2 y) (based on full-time occupation)

Heating system: Wall mounted conventional condensing gas boiler with: Worcester Greenstar 30CDI. Distribution via a mixture of wall hung radiators with TRVs and underfloor heating which have grill holes throughout the floor and are controlled by ‘zoned’ heating controllers (2 on the first floor and 3 on the ground floor) with room thermostat and boiler timer/programmer. Heating can be supplemented with multi-fuel boiler in lounge.

Domestic hot water generation: Solar thermal collectors: 4,2 m2 feeding thermal store – 210 liters supplemented by gas condensing boiler and electric immersion heaters.


Acronym of the case

The Green House Case Study

Lessons learnt

If there was a next time: • More mechanical control of natural ventilation such as air extraction could have had heat recovery • All artificial lighting should have been LED or a least CFL (there is a mixture of halogen and tungsten filament lighting also mixed in) • The availability of on-site mains-gas made the option to include renewable energy for space heating too expensive to operate back in 2007. If this project was commissioned now then consideration for a wood pellet boiler - instead of a gas condensing boiler - would have been a viable option because of the Renewable Heat Incentive (RHI) payments • Similarly, photovoltaics would be economically viable as a result of the introduction of the Feed-in Tariff (FiTs) • Installation of Cavity wall insulation to the original ground floor walls to stop heat transfer through an empty cavity • Aerated shower heads and taps – not fitted as an oversight but can be easily added • Advise client on low energy appliances e.g. refrigeration, freezing and washing 10

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