Bristol Retrofit

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A full thermal retrofit of a mid-century, three-storey property built into a hill and located within a Conservation Area in Bristol. The project included an extension, designed and built to a very energy efficient specification. The extension was needed, both to accommodate home-based business facilities, and to improve access to the lower floor, which is now the kitchen/dining area. A key challenge for this project was to address the amount of thermal bridges created by the excess amount of concrete used during the period of the original construction. Concrete gutters and a balcony have been removed. External wall insulation has been used throughout (U
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CO2 emissionsPrimary energy requirement
Energy target

Energy and fuel use

Fuel use by type
Primary energy requirement
CO2 emissions

Measured data from renewable generation is not yet available.

Fuel use

Electricity use 4155 kWh/yr 3500 kWh/yr 2866 kWh/yr
Natural gas use24367 kWh/yr 6500 kWh/yr 4871 kWh/yr
Oil use- - -
LPG use- - -
Wood use3350 kWh/yr 500 kWh/yr 1203 kWh/yr
Other Fuel - - -
Primary energy requirement 208 kWh/m².yr 83 kWh/m².yr 70 kWh/m².yr
Annual CO₂ emissions 37 kg CO₂/m².yr 17 kg CO₂/m².yr 13 kg CO₂/m².yr
Annual space heat demand 140.3 kWh/m².yr 16.93 kWh/m².yr 14.2 kWh/m².yr

Renewable energy

Electricity generationForecastMeasured
Renewables Technology--
Other Renewables Tech--
Electricity consumed by generation --
Primary energy requirement
offset by renewable generation
83 kWh/m².yr 70 kWh/m².yr
Annual CO₂ emissions
offset by renewable generation
17 kg CO₂/m².yr 13 kg CO₂/m².yr

Calculation and targets

Whole house energy calculation method SAP
Other whole house calculation method-
Energy target EnerPHit
Other energy targets-
Forecast heating load -


Pre-development air permeability test01 May 201218.4m³/m².hr @ 50 Pascals
Final air permeability test20 October 20131.85m³/m².hr @ 50 Pascals

Project description

Start date10 September 2012
Occupation date03 May 2013
Location Bristol Bristol  England
Build typeRefurbishment
Building sectorPrivate Residential
Property typeDetached
Construction typeMasonry Cavity
Other construction type60mm cavity brick-brick
Party wall construction
Floor area 201.9
Floor area calculation method Actual Floor Area (SAP)
Building certification

Project Team

OrganisationFour Walls Consultants
Project lead personIan Mawditt
Landlord or ClientIan Mawditt
ArchitectTaylor Kay
Mechanical & electrical consultant Four Walls Consultants
Energy consultantFour Walls Consultants
Structural engineer
Quantity surveyor
ContractorGreenheart Sustainable Construction

Design strategies

Planned occupancyTwo people with occasional guests. Both occupants work full time from home.
Space heating strategyRe-use existing (A rated) boiler with new 3 zone distribution using panel radiators. 4kW installed capacity + 5kW wood burner to living room.
Water heating strategyNew 250 litre mains-pressure, indirect coil hot water cylinder. Primary source is system boiler.
Fuel strategyMains gas for heating and cooking; mains electricity
Renewable energy strategyNone present
Passive Solar strategyLarge amount of glazing (70%) to rear (east) allowing good winter solar gain.
Space cooling strategyNew glazing using high quality triple glazed units. These have a double low-e coating and offer a good degree of solar control. Light colour render and ventilated timber facades help to reduce solar insolation. Lower ground floor (new kitchen) will has approximately 1 metre overhang from balcony above, with planting. Exposed internal thermal mass wherever possible. Live/work from home window management is key in summer.
Daylighting strategyHouse is very well day-lit as many mid-century homes tended to be. Glazed area has reduced slightly with the extension/renovation, but occupied rooms have a daylight factor >4, which means that do not need electric lighting during the day.
Ventilation strategyWhole house heat recovery ventilation system using Paul Novus 300 with frost coil. Ducting is rigid steel with double seals at each connection. System provides 220 m3.h-1 in normal mode and 270 m3.h-1 in boost. Have the option of automatic summer bypass, but in reality system is switched off and windows are permanently open (secure) on ground (rear) and upper floors (all).
Airtightness strategy Wet plaster coating to interior walls. Render under-/slurry-coat applied to outer brick layer prior to applying EWI. Airtightness detail around window and door openings and junctions between floors, walls and roofs, by use of airtightness membranes and tapes. Around openings, an adhesive-backed airtightness tape will be fitted to surrounding structure. Existing intermediate floor joists (penetrating the cavity) will be individually sealed using proprietary air barrier sealant and/or membranes and parging
Strategy for minimising thermal bridges In general, wrapping the house in 120mm of external wall insulation will eliminate many of the existing thermal bridges. Extending insulation in the extension to foundation level reduces psi-values to negligible levels.
Modelling strategyTherm modelling for thermal bridges only. Although a full retrofit, the entire strategy is based upon an elemental approach (e.g. all elements U=
Insulation strategy120mm external wall render insulation system to extension and majority of existing house. South elevation has external concrete stairs and EWI will not be possible. Internal wall insulation (100mm) fitted to this elevation at ground/lower ground. Roof (cold loft) has between 400mm and 600mm Warmcell (excessive, but was also to envelop MVHR ducts). Extension roof, tapered PIR, min 100mm. Extension floor 100mm PIR; existing floor 100mm batts between joists
Other relevant retrofit strategiesFor more information go to
Contextual informationThe house is in a Conservation Area

Building services

OccupancySee design strategy
Space heatingSee design strategy
Hot waterSee design strategy
VentilationSee design strategy
ControlsThree zone heating controls: one thermostat and programmer per floor. Ventilation central controller in study with boost buttons in kitchen and outside bathroom. Controls are programmable: 4 different speed settings, set according to occupancy and time of day.Hot water programmer set for one hour per day.
CookingMains gas hob - daily use; electric Oven - approx. twice-weekly; microwave - approx. once weekly
Lighting70% LED; 30% CFL/T5
AppliancesA rated electric oven; A+ rated fridge freezer; A+ rated dishwasher; A++ rated washing machine; A+++ rated tumble drier (heat pump)
Renewable energy generation systemNone fitted yet - fabric first approach followed by a period of monitoring and refining. Then consider options for micro-generation.
Strategy for minimising thermal bridgesSee design strategy

Building construction

Storeys 3
Volume 581.5
Thermal fabric area 470
Roof description Original roof: cold loft; timber with concrete tiles. Insulated using Warmcel blow in to a depth of 300mm, with a further 300mm ontop/around MVHR ducting, resulting in A.W U=0.1. New roof to extension: flat, warm roof construction made up with 150mm Kingspan Thermaroof TR26 and Cefil single ply membrane. Sedum layer dressing. (U=0.14)
Roof U-value 0.10 W/m² K
Walls description Existing masonry CW: Infilled with Ecobead+ (nominal 60mm depth). Cavities edge-sealed at top. EWI applied using 2x60mm layers of Kingspan K5 over a sand/cement slurry coat (air barrier). Finished in render or ventilated timber cladding. (U=0.12)Extension walls: solid AAC using 190mm 7.3n/ blocks. insulation/air barrier/finishes as per existing walls. (U=0.14).Timber infil sections (U=0.1). Internal insulation areas on south elevation at ground and lower ground using 100mm Pavadentro (U=0.17)
Walls U-value 0.12 W/m² K
Party walls description N/A
Party walls U-value 0.00 W/m² K
Floor description Original LGF slab removed and floor lowered. New slab to existing and extension LGF built up using 100mm Kingspan Kooltherm K3 with a 30mm liquid flow screed over
Floor U-value 0.13 W/m² K
Glazed doors description Two x TG tilt/slide units and one TG bi-fold door set by Green Building Store EcoContract+ range
Glazed doors U-value 0.90 W/m² K installed
Opaque doors description Two opaque doors by Green Building Store EcoContract+ range
Opaque doors U-value 1.00 W/m² K installed
Windows description All timber frame triple glaze with 2xlow-e coatings. All by Green Building Store EcoContract+ range
Windows U-value 0.90 W/m² K installed
Windows energy transmittance (G-value) 0.49 %
Windows light transmittance -
Rooflights description 3 x Velux GGU-62 units in extension roof
Rooflights light transmittance 0.69%
Rooflights U-value 0.81 W/m² K