The Autonomous House
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as PDF This may be one of the most energy-efficient houses in the UK. Modelled using the Passivhaus Planning Package it is a super-insulated, thermally heavy design: the structural core stores heat in summer and gives it back slowly in winter. Extreme airtightness detailing and eradication of cold bridges minimise heat loss. Specially designed south-facing triple-glazed windows maximise solar gain and it is ventilated by a very efficient MVHR. No heating system of any kind has been installed and there is no connection to mains water or drainage. Rainwater is harvested and filtered to drinking standard; there is a composting toilet system, which uses no water for flushing. Roof-mounted solar panels provide hot water, supplemented by a low-powered immersion heater, usually powered by the grid-connected PV array - the only connection to outside services. Low-energy lighting and appliances are used throughout. Some are powered by a 12V electrical system supplied by batteries charged by the PVs.
The Autonomous House : Project images
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Energy and fuel use
Renewable electricity generation This project has used the contributions from renewable electricity generation equipment to either meet the Retrofit for the Future target or otherwise reduce the Primary energy requirement and CO₂ emissions associated with the project.
Measured data from renewable generation is not yet available.
Fuel use
| Pre-development | Forecast | Measured | |
| Electricity use | - | - | 2818 kWh/yr |
|---|---|---|---|
| Natural gas use | - | - | - |
| Oil use | - | - | - |
| LPG use | - | - | - |
| Wood use | - | - | - |
| Other Fuel | - | - | - |
| Pre-development | Forecast | Measured | |
| Primary energy requirement | - | - | 44 kWh/m².yr |
|---|---|---|---|
| Annual CO₂ emissions | - | - | 10 kg CO₂/m².yr |
| Annual space heat demand | - | 5 kWh/m².yr | - |
Renewable energy
| Electricity generation | Forecast | Measured |
|---|---|---|
| Photovoltaics | - | 2000 kWh/yr |
| Other Renewables Tech | - | - |
| Electricity consumed by generation | - | - |
| Primary energy requirement offset by renewable generation | - | 13 kWh/m².yr |
| Annual CO₂ emissions offset by renewable generation | - | 3 kg CO₂/m².yr |
Calculation and targets
| Whole house energy calculation method | PHPP |
|---|---|
| Other whole house calculation method | - |
| Energy target | PassivHaus |
| Other energy targets | Primary energy requirement from PHPP: 11 kWh/(ma) total deman on heating installation, domestic hot water, household electricity and auxiliary electricity |
| Forecast heating load | 7 W/m² demand |
Airtightness
| Date | Result | |
| Pre-development air permeability test | - | - |
|---|---|---|
| Final air permeability test | 13 January 2012 | 0.36m³/m².hr @ 50 Pascals |
Project description
| Stage | Occupied |
|---|---|
| Start date | 17 June 2009 |
| Occupation date | 30 July 2011 |
| Location | Cropthorne, Pershore Worcestershire England |
| Build type | New build |
| Building sector | Private Residential |
| Property type | Detached |
| Construction type | Masonry Cavity |
| Other construction type | Brick and block construction with 375mm insulation in cavities |
| Party wall construction | |
| Floor area | 160 m² |
| Floor area calculation method | Treated Floor Area (PHPP) |
| Building certification |
Project Team
| Organisation | Mike Coe |
|---|---|
| Project lead person | Mike Coe and Mike Neate |
| Landlord or Client | Mike Coe and Lizzie Stoodley |
| Architect | Neill Lewis Chartered Architect, Malvern |
| Mechanical & electrical consultant | Andy Martin, AJM Electrical Services; Andrew Farr, Green Building Store (MVHR) |
| Energy consultant | David Olivier |
| Structural engineer | Stuart Derbyshire |
| Quantity surveyor | Steve Bowen |
| Consultant | |
| Contractor | Mike Neate of Eco-DC |
Design strategies
| Planned occupancy | Designed as a modest house for a family of four. |
|---|---|
| Space heating strategy | No heating system to be installed: no boiler, heat pumps or woodstove. The thermally massive structural core of the building would store heat from the sun, gained through the triple-glazed windows in the summer, and give it back slowly into the winter months. Super-insulation and extreme airtightness detailing would ensure that as little heat was lost as possible; an efficient MVHR system to reclaim heat from outgoing air. Incidental gains from body heat, cooking, appliances etc would be enough to keep the internal temperature comfortable for most of the winter. Possible future purchase of a bio-ethanol stove for back-up heating, if necessary. Living spaces upstairs, so they would remain warmer in winter. |
| Water heating strategy | A super-insulated 500 litre water tank in the basement, heated by four large roof-mounted solar panels. This would be supplemented by a low-powered immersion heater, usually powered by the grid-connected PV array in the garden. |
| Fuel strategy | Electricity for cooking induction hob; electric oven. No other fuel needed. |
| Renewable energy strategy | A standard 2.3kW grid-connected photovoltaic array mounted on a pergola in the garden. This would be connected to a 12V electrical system powered by batteries, which would be charged during the day by the PV array and would supply most of the lighting in the house, plus power for the 12v pumps, smoke alarms and router. |
| Passive Solar strategy | Maximum possible glazing installed to the south and west faces of the building (window positions and proportions optimised using PHPP). |
| Space cooling strategy | Natural ventilation from opening windows in the summer. MVHR used for night purging during heatwaves. Bedrooms downstairs to remain cooler in summer. |
| Daylighting strategy | The number and proportions of all windows in the house optimised using the PHPP to ensure maximum possible daylight for as long as possible. Window reveals splayed to increase the amount of light entering rooms. |
| Ventilation strategy | MVHR system, to keep the house well-ventilated throughout the year. Windows can be opened in hot weather. |
| Airtightness strategy | Internal walls: highest density concrete blocks, lime-plastered to provide a breathable airtightness barrier. Outer skin: lime plaster on wood wool boards over an airtightness membrane. Roof: a sealed airtight membrane. Penetrations in roof or walls to be kept to a minimum; any that are made to be sealed carefully afterwards. |
| Strategy for minimising thermal bridges | Continuous insulation to be maintained throughout and care taken at all junctions and interfaces, including special care taken around dormer window. Brick plinth stabilised with non-conductive wall ties. Triple-glazed windows with insulated frames, enclosed within the surrounding walls. Non-conductive spacers between the panes of glass. Cut-outs in external walls for light switches and sockets parged behind the metal wall boxes. A layer of insulating blocks at junction of ground-floor walls with basement ceiling. Composting toilet chamber (in basement and outside thermal envelope) to have insulated cavity walls and connected chutes to be insulated. |
| Modelling strategy | Whole house modelling in PHPP. |
| Insulation strategy | Cavity walls containing 375mm insulation; 450mm in the roof; insulation applied to the ceiling of the basement (outside the thermal envelope). Insulated frames on windows. |
| Other relevant retrofit strategies | |
| Contextual information |
Building services
| Occupancy | Designed for a family of four; present occupancy, two. |
|---|---|
| Space heating | No heating system installed (no boiler, woodstove, heat pumps et al). The thermally massive structural core of the building stores heat from the sun, gained through the south and west facing glazing in the summer, and gives it back slowly into the winter. Super-insulation and extreme airtightness detailing ensure that as little heat is lost as possible; an efficient MVHR system reclaims heat from outgoing air. Incidental gains from body heat, cooking, appliances etc are enough to keep the internal temperature comfortable for most of the winter. In extreme conditions (long overcast periods in the coldest months), two 1kW electric heaters can be deployed. Living spaces are upstairs, so remain warmer in winter. The double-height conservatory on the south side of the house (outside the thermal envelope) is used to provide passive heating, on sunny days in winter, by opening the ground floor and first-floor doors into the house. |
| Hot water | 4 x Velux 2m roof-mounted solar panels, connected to a 500-litre super-insulated storage tank in the basement. This is supplemented by a 1kW immersion heater fed by a SolarImmersion intelligent controller to improve performance at marginal times of the year. |
| Ventilation | Paul Thermos 200 whole house MVHR system based on Lindab rigid aluminium ducting, with a 30m ground tube on the air input made from Rehau Awaduct. Keeps the house well ventilated throughout the year; reclaims heat in winter. Used for night purging in heatwaves. (Windows are opened in very hot weather.) The MVHR is also used to ventilate the composting toilet system and reclaims the small amount of heat from the composting process. |
| Controls | MVHR boost switches installed near bathrooms and in kitchen - used when frying food or showering to put system into 15-minute full-power mode (works very well). |
| Cooking | Induction hob and electric double-oven (small oven used most of the time). |
| Lighting | Low-energy lighting used throughout. A 12 volt electricity supply, powered by two leisure batteries charged in the day by the PV array, powers some of the lighting in the house providing some resilience to power cuts. |
| Appliances | Energy-efficient Indesit induction hob and electric oven. Fridge and freezer not currently A-rated, but will only be replaced when they wear out. Bosch dishwasher chosen because it can be programmed to take in hot water (from the solar hot water system). Miele washing machine reasonably energy-efficient. |
| Renewable energy generation system | Solar hot water, as described above. A standard 2.3kW grid-connected photovoltaic array mounted on a pergola in the garden. There is a separate 12V electrical system, powered by two leisure batteries, charged in the day by the PV array. The 12V system powers some of the lighting in the house, the 12v pumps in the rainwater harvesting system, the smoke alarms and the router. |
| Strategy for minimising thermal bridges | Great care was taken to minimise thermal bridging. Continuous insulation was maintained throughout and care taken at all junctions and interfaces (special care was taken around the dormer window). The brick plinth was stabilised with non-metallic wall ties. The triple-glazed windows have insulated frames, which were enclosed within the surrounding walls to avoid cold bridging. The spacers between the panes of glass are made of non-conductive material. Cut-outs made for light switches and sockets on external walls were parged behind the metal wall boxes. |
Building construction
| Storeys | 3 |
|---|---|
| Volume | - |
| Thermal fabric area | 160 m² |
| Roof description | Clay tiles over membrane, over 450mm Knauf Dritherm insulation. Supported by I-beams with insulating thermal packing. |
| Roof U-value | 77.00 W/m² K |
| Walls description | Internal high-density concrete blocks, 375mm Knauf Dritherm insulation, membrane, wood wool boards, external render. Boards supported on Boise Cascade I-beams with thermally insulating packer. |
| Walls U-value | 97.00 W/m² K |
| Party walls description | |
| Party walls U-value | - |
| Floor description | Poured in situ 280mm suspended concrete slabs. The ground floor slab forms the ceiling of the basement plant room below. Insulation on plant room ceiling: 300mm Jablite (EPS) panels. |
| Floor U-value | 94.00 W/m² K |
| Glazed doors description | Optiwin Alu-2-Wood triple-glazed doors, south-facing aluminium outside; wood inside. Bespoke glazing specification for maximum solar gain (65%). |
| Glazed doors U-value | 0.51 W/m² K installed |
| Opaque doors description | Front door: Entrance door IV78 in ash by Greensteps Ltd, with external passivhaus aluminium cladding and Gutmann Miratherm insulation. |
| Opaque doors U-value | 8.00 W/m² K installed |
| Windows description | Optiwin Alu-2-Wood triple-glazed windows (U: 0.6 W/mK g: 52%). South elevation: standard Optiwin frames but with bespoke glazing configuration for this house to ensure maximum solar gain from the south (U: 0.51 W/mK g: 65%). Designed in collaboration between Optiwin Austria, supplier the Green Building Store, and energy consultant David Olivier. |
| Windows U-value | 0.51 W/m² K - |
| Windows energy transmittance (G-value) | 6552 % |
| Windows light transmittance | - |
| Rooflights description | |
| Rooflights light transmittance | - |
| Rooflights U-value | - |