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How Prewett Bizley Architects balanced heritage and building performance

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Prewett Bizley Architects’ Bloomsbury House had ambitious aspirations to be a Passivhaus retrofit, says Yogini Patel 

ARCHITECT’S VIEWPROJECT DATA • PLANS • SECTION • ISOMETRIC • DETAILS • COSTS

Bloomsbury House, recently shortlisted in the 2016 AJ Retrofit Awards, manages to find an equilibrium between architectural preservation and building performance in a constraint-ridden upgrade that could easily have ended in a clash between historic conservation and comfort.

The client enlisted Prewett Bizley Architects with initial aspirations of a Passivhaus retrofit – no small ambition for an 18th-century Grade II-listed Georgian terrace located in London’s Bloomsbury Conservation Area. After many years as an office, the five-floor building has been returned to use as a family home, suitable for seven occupants. Conversion work included upgrading and restoring the original 1820s features, along with updating a 20th-century rear extension. It focused on three key themes:

  1. Improving the building’s energy performance.
  2. Preserving the historic architectural characteristics.
  3. Pulling the home back into modern-day living.

Performance and building fabric upgrade

Being mid-terrace, the building has a highly favourable form factor ratio – the external envelope through which heat can escape is small, compared to the area of the usable building, also known as treated floor area, which is approximately 370m2. A good ratio was important for improving thermal performance.

The existing external walls are solid brick, becoming progressively thinner with increased height, but preservation of the streetscape ruled out external wall insulation. Using the Passivhaus Planning Package (PHPP) as a fundamental design tool, the energy strategy relies on intricately planned and installed internal wall insulation, which incorporates a number of different wall build-ups.

Choice of insulation was restricted in areas where space was limited by the need to maintain internal features. The architects took care to ensure wall build-ups were breathable to minimise the risk of any detrimental moisture damage to the existing fabric. Lime render and plaster were restored where appropriate. Prewett Bizley director Robert Prewett says using wood fibre with aerogel proved an effective insulation material.

This upgrade of the building’s thermal performance meant the practice could remove conventional radiators, helping restore the rooms to their original character. They were replaced with a low-output underfloor heating system, driven by an air source heat pump (ASHP). Hot water is provided by another ASHP in combination with solar thermal panels.

Mechanical ventilation systems with heat recovery (MVHR) minimise heat losses and provide a constant supply of fresh filtered air. The building has two such systems, which is unusual for a single home, but necessary since the network of ducting could not damage or penetrate any of the original building fabric. A top-floor Paul Novus 300 MVHR unit services the bedrooms with the majority of ductwork positioned within a ceiling void at roof level. The second Paul Novus MVHR is on the first floor in a purpose-built service riser to the rear of the home. This also houses waste pipes and electrical services. Air valves are concealed behind metal grilles or placed discreetly where possible. 

Bloomsbury House by Prewett Bizley Architects

Bloomsbury House by Prewett Bizley Architects

Source: Alan Parker

Preservation and patience

Although the sash windows were not original, having been replaced in the Victorian era, they nevertheless proved a sticking point for the local authority conservation officer and had to be maintained. A triple-glazed Passivhaus certified sash system is available, but was deemed unsuitable because of its frame thickness. It took a year and a half of careful negotiation to allow the original window surrounds to be dismantled and reassembled with secondary glazing incorporated between the restored sash window and shutter, which helped partially conceal the new frame. Thermally insulating evacuated glass has been set into a slim timber frame for improved performance and narrow sightlines.

‘It took three more applications until eventually permission was granted after we had been on site for eight months,’ recalls Prewett. ‘The evacuated glass means that the u-value for the whole window is likely to be close to 1W/m2K.’ All new windows and rooflights meanwhile were triple glazed.

Restrictions on the sash windows along with challenging airtightness considerations persuaded the architect to aim for the more relaxed EnerPHit Standard, specifically created for challenging retrofits, rather than the Passivhaus standard. The table below shows the targets required, and how the scheme currently fares.

  Passivhaus EnerPHitBloomsbury House 
 Thermal Energy Demand (kWh/m²/year) 15  25  20 
 Thermal Energy Load (W/m²)  10 15  15 
 Primary Energy Demand (kWh/m²/year)  120  120 + heat load factor 120 
Airtightness (n50 ACH@50 pascals)  0.6  1.0  1.1 

While it is close to meeting the EnerPHit standard, Prewett suspects a final airtightness test may suggest remedial work is needed where the old building meets the extension, to bring the figure down to 1.0ACH. ‘While getting below 1.0ACH airtightness is not yet quite achieved, the level of performance is still impressive and lessons learned will go into future projects,’ he says. 

Bloomsbury House by Prewett Bizley Architects

Bloomsbury House by Prewett Bizley Architects

Comfortable living

The house has been occupied since October 2015. As it was previously an office, there is little data to compare building performance, although monitoring equipment is being installed.

The home feels light and fresh throughout. Just as I was initially impressed with the grandeur of the Georgian proportions, I was equally taken with the newer rear extension, which has a very distinct feel from the old but continues the flow of spaces with a similar palette of colour and materials. It is flooded with daylight via cleverly designed lightwells and well-positioned light scoops. Surprisingly the large rooms to the front of the house are quiet, with no sounds from the main road.

One occupant admires how the building has been welcomingly cool – greatly appreciated during the recent heatwave. Prewett admits there were initial issues with balancing the MVHR and ASHP systems, but says these are now working well.

But balancing building performance with architectural preservation has come with a hefty price tag. Such solutions need to become more cost-effective to facilitate their mainstream adoption. Viva the low-energy retrofit revolution!

Bloomsbury House by Prewett Bizley Architects

Bloomsbury House by Prewett Bizley Architects

Source: Alan Parker

Basement plan

Bloomsbury Dwgs  1F

Bloomsbury Dwgs 1F

Ground floor plan

Bloomsbury Dwgs 0F

Bloomsbury Dwgs 0F

First floor plan

Bloomsbury Dwgs 1F

Bloomsbury Dwgs 1F

Second floor plan

Bloomsbury Dwgs 2F

Bloomsbury Dwgs 2F

Third floor plan

Bloomsbury Dwgs 3F

Bloomsbury Dwgs 3F

Roof plan

Bloomsbury Dwgs 4F ROOF

Bloomsbury Dwgs 4F ROOF

Section 

Bloomsbury Dwgs Section

Bloomsbury Dwgs Section

Construction isometric

Bloomsbury Dwgs Iso

Bloomsbury Dwgs Iso

Window details

Bloomsbury Dwgs Window1

Bloomsbury Dwgs Window1

Original window surrounds were dismantled

Bloomsbury Dwgs Window2

Bloomsbury Dwgs Window2

Secondary glazing was added between the restored sash and the window shutter

Bloomsbury Dwgs Window3

Bloomsbury Dwgs Window3

The original sash windows were restored

Bloomsbury Dwgs Window4

Bloomsbury Dwgs Window4

The window surrounds and shutters help to partially conceal the frame of the secondary glazing

Architect’s view

In early 2013 our client approached us with the desire the convert their newly acquired purchased Georgian townhouse in Bloomsbury into a Passivhaus home for themselves and their three young children. 

The listed status of the building meant that the design process began with very detailed research of the building’s heritage value and significant features. In conjunction with this research a feasibility study was prepared to review which energy efficiency strategies might be appropriate and options for adapting the internal spaces to better suit the clients’ needs whilst remaining sensitive to the building’s heritage. 

The heritage status and specific building physics of the property made it necessary to tailor the insulation strategy to each floor. This resulted in the use of five types of vapour open insulation (cellulose, woodfibre, glass wool, aerogel and open-cell sprayed insulation) and three types of vapour closed insulation (XPS, phenolic and closed cell sprayed insulation). The services were also carefully planned so as to not damage the original fabric with the main services run located in the retrofitted rear-extension. The windows, which perhaps proved the most challenging to upgrade due to planning restrictions, were fitted with secondary glazing with evacuated glass. The installation of these units was carefully planned in order to preserve the shutter boxes and align any new window bars with the existing. 

The project presented an opportunity to develop and a number of themes that the office has been developing for some years. On the one hand there was a very technical side to do with energy efficiency and building physics. On the other there was the opportunity explore how historic and contemporary spaces could be woven together along with the technical issues. 

Robert Prewett, Prewett Bizley Architects

Project data

Start on site June 2014
Completion October 2015
Gross internal floor area 400m2
Form of contract JCT intermediate
Construction cost £ 1.58 million
Construction cost per m2 £ 3,946
Architect Prewett Bizley Architects
Client Private
Structural engineer Jonathan Parks
Quantity surveyor Mark Hammond
Interior designer Emily Bizley
CDM coordinator Prewett Bizley Architects
Approved building inspector Oculus
Main contractor Bow Tie
CAD software used MicroStation 2D and SketchUp
Annual CO2 emissions 23kg/m2 calculated regulated and unregulated
On-site energy generation 7 per cent
Annual mains water consumption 45m3/occupant
Airtightness 1.1ACH at 50Pa
Heating and hot water load 35kWh/m2/yr

Costs

 Total costCost per m²Percentage of total 
Demolitions and alterations £89,234 £223.09 6%
Substructure and tanking £52,783 £131.96 3%
Works to floors £30,000 £75.00 2%
Roofing £56,156 £140.39 4%
Internal doors £49,000 £122.50 3%
Internal partitions £89,277 £223.19 6%
Wall finishes £53,000 £132.50 3%
Floor finishes £60,000 £150.00 4%
Ceiling finishes £60,001 £150.00 4%
Electrical £120,000 £300.00 8%
Joinery £140,000 £350.00 9%
Other fittings £150,000 £375.00 10%
External works £90,000 £225.00 6%
Sanitary £40,000 £100.00 3%
Insulation £90,000 £225.00 6%
Air tightness £30,000 £75.00 2%
Existing windows including secondary glazing £75,000 £187.50 5%
Triple glazing £43,000 £107.50 3%
MVHR £30,000 £75.00 2%
Other mechanical £70,000 £175.00 4%
Preliminaries and insurance £160,850 £402.13 10%
TOTAL  £1,578,301  
  • 1 Comment

Readers' comments (1)

  • Outstanding 'tour de force' to have turned this old characterful property into a PassivHaus retrofit. By far the most comfortable Georgian house that exist! (Most comfortable listed building even perhaps?)

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