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Istop dead amid an army of old dears and eager tourists. Glancing down with some trepidation, I realise that all is not what it seems. Fractured grave markers stretch out before me like a series of grim stepping stones. I am standing on one of them.

As the musty odour of damp stone works its way into the back of my nostrils, however, I almost relish the macabre absurdity of my situation: I am taking cream tea in a converted crypt.

Due to its imposing arches, rich historical ambience and pure novelty value, the Caf茅 in the Crypt has been a popular attraction at St Martin-in-the-Fields Church on Trafalgar Square for quite some time. But it has now benefited from a major refit as part of the church鈥檚 拢36m renewal and extension programme, which includes 拢4m worth of m&e services installed by Imtech Meica.

Designed by architect James Gibbs, St Martin-in-the-Fields was built at the beginning of the 18th century and became a monument of national and religious importance. In fact, when Nelson鈥檚 column was conceived in 1843, its design had to be reduced by 14 feet so that it was not higher than St Martin鈥檚.

Even today, the church heaves with life every day of the week and has more than earned its reputation as 鈥榯he church of the ever-open door鈥�.

The stunning place of worship may be grade I-listed, but it had long been in desperate need of some tender loving care, not least on the services side. The last major refurbishment took place in the Victorian era.

What鈥檚 more, expansion was firmly on the cards. But how to achieve such a feat when space is at a premium in this part of central London?

Excitingly, Eric Parry Architects decided that the answer lay in going underground. The crypt-cum-caf茅 now opens out directly onto an airy white-walled reception area, complete with contemporary glass elevators and arty spiral staircase. The effect is more akin to a modern day office space than anything else.

Yet incredibly, until its transformation began in April 2006, this was part of a perennially leaky Victorian burial vault that housed the services for the original church building. The unmonitorable warren was a maintenance firm鈥檚 nightmare, and modernising services was unfeasible.

Nick Cramp, project engineer for m&e consultant Max Fordham, explains: 鈥淚t was in a state. It was more like an underground car park 鈥� filthy, with just a single light bulb. It didn鈥檛 need to be upgraded, it needed to be replaced.鈥�

Eric Parry鈥檚 innovative solution was to transform much of the vault into a subterranean world of offices, music rooms, chapel and even a shop.

According to Nick Ashcroft, project manager for m&e contractor Imtech Meica, installing plant in such a constricted space was no easy task.

鈥淭here are two new plant rooms crammed into the north range in the roof spaces, and the space in there is exceptionally tight. That was very difficult, and it was incredibly shoehorned. We overcame that with good co-ordinated drawings and a team of people who were prepared to work to the detail,鈥� he says.

鈥淓verything was pretty much designed when we came onto the project, but we had to modify the design in every plant room because the equipment didn鈥檛 fit correctly. The correct co-ordination hadn鈥檛 been taken on board.

鈥淚 think it would have been better to have been involved earlier. The main thing to take from it is: try to get people to work with you from the design stage so that you can say from the benefit of your experience what works and doesn鈥檛 work,鈥� he says.

The vault鈥檚 transformation is surprisingly spacious 鈥� it doubles the church鈥檚 original floorspace 鈥� and the vault is flooded with natural light from a huge central light well surrounded by a series of reflectors. This glazed centrepiece also provides an aesthetic focal point, offering striking views of the newly restored church spire from its base. A dozen or so luminaires are dotted around the well for added emphasis. Like most off the new lighting scheme, these are low-energy.

As if that wasn鈥檛 impressive enough, the light well also plays a crucial part in the building鈥檚 ventilation system.

Cramp explains: 鈥淭here is a void underneath the whole of the new-build area. That鈥檚 at least 10 metres underground, and the temperature is actually quite stable down there. It provides lots of free cooling.

We suck in air from the light well and then it鈥檚 redistributed at high levels to all spaces.鈥�

Night-time precooling is also applied. The underground void is precooled throughout the night using a bore hole cooling system, which reduces the temperature of the air throughout the day and heats it up during the winter months.

The system runs without heat pumps, taking cold water from an aquifer, which is about 150 metres down, and running this directly through coils and air-handling units to achieve what Cramp describes as 鈥渁lmost free cooling 鈥� there鈥檚 just the cost of the pumping鈥�.

At street level, a curved glass pavilion mirrors this circular light well, delivering conscious architectural symmetry and providing an entrance way with stair and lift access to the new suite of spaces.

The pavilion is a design feat in its own right. Its outer pane has a metallic coating to reduce solar gains and enhance reflections.

Heating is provided by plastic hot water pipes behind the vertical panels of limestone that line the internal faces of its plinth.

Evidently, the St Martin鈥檚 project was a particularly complex one, with an unusually large number of diverse facets. The experience brings with it words of caution from Cramp.

鈥淢y main tip for contractors and clients is to avoid partial handovers because they鈥檙e very complicated, and this job was obviously going on for a long time.

鈥淎s for designers, my advice is to put in more than you need at an early stage, because we鈥檝e been fighting against the budget throughout. So that approach gives you some room to manoeuvre through the course of a job like this, which involves a lot of risk because you鈥檙e digging in the ground and you don鈥檛 know what鈥檚 in there, and also because there鈥檚 an existing building that needs to be understood.鈥�

The original parts of St Martin鈥檚 required an extremely delicate touch, due to the church鈥檚 age and listed status.

鈥淲ithin the church the main challenge was dealing with the age of the fixtures and fittings and the fragility of the building without causing damage, and keeping within the character of the building,鈥� says Ashcroft.

鈥淚t鈥檚 a mark of national importance, and you can鈥檛 do anything too intrusive. So what you try to do is just make the best improvements possible to what鈥檚 there already,鈥� he adds.

One of the main challenges facing the m&e team was evident from the start: how would they manage to conceal all that ugly wiring within an 18th-century church building with the strictest of preservation policies?

鈥淲ith a listed building, you try not to hide things because that damages them. You just run them along the surface and use nice wiring, like mineral-insulated copper-clad cable (MICC).

At St Martin鈥檚, though, we were quite lucky in many places. There鈥檚 a lot of panelling on the walls, and wooden floors that have voids underneath them, so you can hide it all there. But you鈥檙e certainly not allowed to drill into the walls,鈥� laughs Cramp.

Loughborough-based Clymac designed and installed the fire and security systems throughout the project. The firm opted for a mixed radio and hardwired system, with radio in the church and crypt and hardwiring in the north range and back-of-house areas.

Bare MICC was chosen for voice sounders in the church and crypt areas.

In addition, aspirating detection was fitted in the large void above the church due to the height of the ceiling. This enabled the system to achieve a high level of sensitivity and eliminated the need to gain access from the church for maintenance purposes.

Aesthetically discreet capillaries were also used in the church ceiling.

Original and new-build church spaces are linked by a single building management system (BMS) which controls fire and security as well as lighting.

The two elements of the project did not exist independently of each other, and combining old and new structures requires different skill sets, and often modern software technology.

鈥淵ou had to have this light touch with the old building, but a more rigorous approach with the new-build to achieve modern standards. Blending those two was a challenge. We had to dig under the crypt to put in new air ducts, but we didn鈥檛 want to destabilise the existing structure, so that was quite a difficult thing to do. In the end, it just came down to very careful planning,鈥� says Cramp.

鈥淎 major piece of advice for those in m&e who are embarking on this type of project is to learn the building very, very well before you start,鈥� he adds.

Careful planning is Cramp鈥檚 forte. He specialises in building physics and undertakes computational fluid dynamics (CFD) and lighting analysis work for many of his projects. This proved crucial in creating a comfortable environment below ground level.

鈥淲e used light modelling to look at the balance of daylight and artificial lighting within the spaces.

鈥淲e did some computational fluid dynamics in 3D to look at ventilation, and we did a lot of our CAD drawings in three dimensions,鈥� he says.

A meticulous forward planner he may be, but is there anything Cramp would aim to do differently if he could start the project again from scratch?

鈥淲ith projects like this, the client always changes what they want as things start to progress, so they end up adding in elements that you could have designed 鈥� and to a superior standard 鈥� at an earlier stage.

鈥淚 think we probably would have added more central systems, knowing now what they want 鈥� air-conditioning systems, for example, in spaces that have become commercial spaces that weren鈥檛 originally intended to be so.

We would have designed the lighting differently, for the same reasons. But things change. It was quite a long job.鈥�

The finished result makes little of these issues, and the whole design concept works superbly.

As I wander around the former vaults, however, I can鈥檛 help but recall some of Cramp鈥檚 earlier, more unsettling words: 鈥淲e didn鈥檛 find any bodies in this one. Well 鈥� only a couple anyway.

We鈥檝e had bodies in other jobs of this type though. When we worked on the London Symphony Orchestra鈥檚 St Luke鈥檚 Church, there were 1900 bodies down there. That鈥檚 horrible, isn鈥檛 it?鈥�

I eye one or two of the old dears with suspicion. I鈥檇 thought there seemed an unsettling number of them shuffling around.

This article was originally published in EMC February 2009 as What Lies Beneath