The face of many historic cities has changed over the past few decades with varying architectures and newer buildings mushrooming across whole areas of towns.

Today, ancient centres and buildings, such as pre-war office blocks and churches, intermingle with futuristic skyscrapers and ultra-modern financial districts.

"Upgrading the insulation is key to improve the energy footprint of old buildings."

As town planning problems like housing density, traffic pollution and social inequality progressively increase worldwide, the modern use of historic buildings and the preservation of architectural diversity has gained widespread support in the industry.

Historic buildings can help overcome some of these issues. Usually an integral part of a city they contribute to the townscape and local urban grain. Their refurbishment maintains not only the status quo and character of cities but also has the capacity to strengthen an area's architectural character, benefitting the building's owners, the users and public alike.

Finding ways to retain and reuse old structures despite structural and planning challenges and diversified conservation guidelines, is therefore an integral part of today's architecture world.

The combination of old and new architecture

"The attraction of the modern city is the blend of old and new," said John Robertson Architecture (JRA) principal director John Robertson. The UK-headquartered architecture, design, planning and strategic consultancy has made historic projects a vital part of its business in London, following the strategy of 'contextual civic modernism', which recognises that much of a building's value lies in how it interacts with its context.

"In central London the legacy of the past is as important as the pressure to provide buildings equipped for future needs of international business. Restoring historic buildings has offered us an opportunity to combine all the benefits of contemporary office construction with attractive historic features, often of very high architectural and cultural value," he added.

According to Robertson, the work on historic constructions is about respecting the past while providing a worthy legacy for the future.

The challenges however are plentiful: while works on the exterior of a building can usually be carried out using original materials such as brick, metals, earth materials, stone and stucco, the internal structures and energy performance quickly becomes the main scope of work.

In order to become viable for modern usage, the buildings often have to be upgraded with equipment such as wires for new information and communications technology or have to be made accessible for disabled people - all without changing the face of the construction.

"This usually means re-planning parts of the building to incorporate lifts and improved sanitary facilities," explained Robertson. "Historic buildings often have steps at their entrance and finding discrete new ways of providing access for disabled persons requires careful thought."

The installation of air conditioning and fire systems can also be a major challenge as floor-to-floor heights are usually less high than in newly constructed buildings. Especially important is the installation of effective fire detection and suppression systems.

Old constructions are particularly prone to fire damage with a large number of listed buildings damaged to an irreplaceable level every year.

As the impact on the design and structure of the buildings should be as small as possible, the design of the fire systems for historic buildings requires a completely fresh approach. Bulky pipes should be replaced with plastic sprinkler pipes with solvent-welded joints that will not harm the delicate fabric of an old building, while smaller and modern sprinkler heads should match with the surrounding decorations to minimise the visual impact on the rooms. Alternative forms of fire suppression that offer the prospect of less water damage when going off are being developed.

Shrinking the energy footprint

The improvement of the environmental performance of a historic building is paramount for its everyday usage. Older structures tend to release a large amount of carbon dioxide into the environment, which is not only harmful for the environment but also impacts energy expenses.

"The work on historic constructions is about respecting the past while providing a worthy legacy for the future."

Measures to enhance the energy performance can include the modernisation of heating and lightning (replace lightning installations, replace boilers), taking advantage of daylight, adjusting time switches and thermostats such as boiler controls, installing or improving thermal insulation and draught proofing, installing good systems to prevent unnecessary heating and implementing an energy saving regime.

Upgrading the insulation is key to improve the energy footprint of old buildings. The insulation of hot water pipes, valves and joints can reduce energy loss from the pipe by 70%, which can save around 5% of the heating bill. Also roofs, walls and floors can be insulated and reduce the energy loss by around 70%.

Windows can be another source for extensive heat and energy loss. Single glazed windows transfer from surface to surface well, leading to loss and condensation, while rattling windowpanes and gaps in frames cause draughts, with damp air adding to the chill, especially during the winter months.

The use of double glazing windows is inappropriate for most historic buildings because of their aesthetic impact and the need to fundamentally change the historic fabric. Draught proofing however will reduce much of the loss.

Call for European standards for conservation

In Europe, with a general trend toward more comprehensive standards across the EU, it seems sensible for many industry members that building preservation should develop area-wide standards and codes of practice as the wide variation of national guidelines are seen as a barrier to trade between member states. In 2011, the European Committee for Standardization CEN approved the first set of guidelines to be introduced within the next three years.

The first standard regulates the indoor climate of churches, chapels and other places of worship. Guidelines currently under approval or under drafting cover surface protection for porous inorganic materials, standards on movable and immovable heritage, guidelines for the management of environmental conditions, for the methodology for sampling from materials of cultural property, for exhibition lightning, procedures and instruments of measuring air humidity and moisture exchanges, as well as standards for the determination of drying properties.

However, as Building Research Establishment (BRE) Centre for Construction leader Tim Yates wrote in his article 'British and European Standards for Heritage and Conservation' published on the Building Conservation website, there is always the risk that internationally developed standards do not reflect local or regional good practice as this could suppress innovation and the development of new skills. Therefore the industry must ensure its views are represented on both national and international levels.

Case study: Daily Express building, London

"Key to restoration was reinforcing the integrity and autonomy of the Daily Express Building while bringing it up to modern standards."

One outstanding example of blending new and old architecture into one building has been delivered by JRA when it restored the £10m Sir Owen Williams Grade II-listed Daily Express building on Fleet Street in London, completed in 2000.

Client Itochu's and the City of London's brief required the 12,000m2 building should be retained and the whole development should work within the context of the Fleet Street Conservation Area.

"Key to the restoration was to reinforce the integrity and autonomy of the Daily Express building while bringing it into line with modern standards," said John Robertson.

The building's glass façade was London's first example of curtain walling when it opened in 1930, but had lost its previous allure and was in a very poor state of repair when the conservation project started in 1997.

According to Robertson, the building did not even reach approximately modern standards of acoustic or energy isolation. The firm therefore used hollow, thermally broken aluminium section, double glazing and curtain walling, post-tensioned with steel rods inside to increase their strength, to make the wall thicker, heavier and insulate the whole building.

Also the historic interior was in a very poor state of repair and was completely recreated, as Robertson explained: "Old photographs were the main source for recreating the entrance hall which had suffered considerably over time, helping to establish the floor pattern which was remade in terrazzo, and the only source for redesigning the serpent handrails which had disappeared.

"The work included reinforcement and gilding of the fibrous plaster ceiling and the restoration of the dramatic Aumonier wall panels. Today this entrance stands as perhaps the best British example of the Art Deco style."