The glass oasis

As you walk through Manchester's regenerated city centre to visit the spanking new essay in solid geometry known as the Urbis museum, you might well be distracted by a similar triangle on the way. Known as No 1 Deansgate, this building copies Urbis' attention-grabbing ski-slope roofline and its sheath of sleek, untinted glass. And, like Urbis, it is the work of local practice Ian Simpson Architects. But the striking congruence hides the no less striking differences. Most obviously, No 1 Deansgate is twice the size of Urbis: it rises to 18 storeys and is one of the highest buildings in the city centre. Second – and not at all obviously – it is not a museum, nor another city-centre office, but an apartment block.

Third, and most alluring of all, No 1 Deansgate's outer glass skin is almost entirely made of opening louvres. These are not so much sophisticated versions of the lowly bathroom louvres of the 1970s so much as high-tech, transparent shingles. Together, they form a beguilingly diaphanous outer veil to the building.

But perhaps the Urbis museum is not the best building to choose for a comparison. Rather, you could see No 1 Deansgate as Manchester's answer to Richard Rogers Partnership's luxurious Montevetro apartment block in Battersea, south London. With a selling price of £4300/m2, its developer, Crosby Homes, claims that it is the most expensive speculative block of flats outside London.

The block fulfils the estate agents' number one adage by being located in Manchester's city centre, close to the cathedral. Paradoxically, the site itself is anything but prime – it is one of the most inhospitable in central Manchester. The building's extraordinary architectural features, including the louvred facades, are a response to this difficult site.

No 1 Deansgate was conceived as part of the masterplan for the Manchester Millennium Quarter, which has regenerated the city centre after the devastation of the 1996 IRA bomb. Simpson formed part of the consortium, along with urban designer EDAW, that won Manchester council's masterplan competition. Simpson's scheme for the building was then sold on, with the sublease of the site, to Crosby Homes.

"This new quarter was to be predominantly retail," recalls Ian Simpson. "But our strategy was that there should also be a residential component." Predictably, all the best sites in the quarter were snapped up the retail big guns, leaving the least promising area for residential.

No 1 Deansgate was left with a site that was more than unpromising: it was downright hostile. It had been occupied by the brutalist, concrete, 1960s commercial development the Shambles – the IRA's destruction of it was probably their most popular act in the city. The site faces the equally brutalist stained-concrete tower of the Renaissance Hotel, and between them passes the dark canyon of Deansgate with its non-stop torrent of traffic. To make matters worse, the head lease for the site was acquired by the Prudential, which is developing its own retail development over most of the site, leaving just one corner for the residential block.

Simpson's response to these conditions has been to push the living accommodation up to the fifth floor and above, as the four lower ones are shadowed on two sides by the Prudential's stores.

The ground and first floors of the building are taken up by shops that open directly on to the pavement. The dead intermediate space above the shops and below the flats has been partly used by Simpson as a grand double-height reception hall, with concierge, to the apartments. "I wanted to make it a real focus for the residents," comments Simpson.

Simpson has also exploited the excess space in this intermediate zone to express the building's functions and structure, with Martin Stockley Associates as structural engineer. The reception hall is set back from the main facade, clearly differentiating the retail podium from the block of flats above.

More dramatically, 12 pairs of raking struts reach out like outstretched arms to support the flats above. Powerful cylinders of tubular steel, they spring from base nodes on the second floor that are clad in rounded glass-reinforced concrete capping pieces as smooth as chamois leather. Overhead, the soffit of the flats has been curved and faced in shiny white glass-reinforced gypsum, as Simpson puts it "like a soft underbelly". Pockets are left where the raking struts punch into the underbelly, emphasising the effect.

As well as providing a visually arresting appearance, the raking struts play more functional roles. They transfer the structure from 12 reinforced concrete columns rising up through the retail podium to a framework of steel columns and beams supporting the flats above. They also offer more space to the flats by cantilevering them 4 m out over St Mary's Gate at one end.

The apartments themselves are enclosed by clear-glazed window walls and luxuriate in daylight, transparency and rooftop panoramas of the city. But the big innovation is the outer curtain wall of continuous glass louvres. This is an independent weatherscreen, separated from the building envelope of double glazing by a 2 m wide buffer zone. The buffer zone serves as a habitable external balcony space, and is protected by the louvres from the wind – it can be 30 mph faster at the top of the building than the bottom – and from the hostile traffic below.

The louvred glass wall, which is made by Glastec, offers views outside and the capacity to be mechanically adjusted to any angle. With the louvres' angles set by the occupants, Simpson sees the whole facade becoming animated and shimmering in an architecturally controlled manner.

At eaves-level, the external weatherscreen switches from glass louvres to clear frameless glazing that folds over to the sloping roof plane. As well as providing better protection from severe winds at this height, the frameless glazing provides a crisp transparent eaves detail, like a crystal casket.

The top floors beneath the sloping roof are occupied by penthouses with spectacular double-height living rooms.

The tick flourishes on the rooftop add a couple more amenities to the penthouses – clerestory windows that channel daylight into the living rooms and balconies.

Finally, the building's crisp outline comes partly thanks to a sophisticated secret component. No cleaning cradles are to be seen trundling along rails at eaves level. Instead they are supported on telescopic cranes that are hidden behind the eaves but can stretch up, over and down to reach all areas of the facades.

No 1 Deansgate is an all-too-rare example of speculative housing designed as pure architecture. Unsurprisingly, Crosby Homes' project manager David Teague recalls that when the housebuilder was first presented with Simpson's scheme, it elicited "a fair degree of nervousness in everyone". What gave them particular concern was the generous semi-external buffer zones, which soaked up 31 m2 of floor space; the natural inclination of a housebuilder would have been to add this to the internal space.

"This buffer zone was not balcony space, because you would just get blown away by the wind," Teague explains. "The view we came to was that it was habitable space, as it is surprising how well the rainwater is kept off it. So we reckoned this floor area had a reduced value."

In the event, the punters needed little persuasion to accept Simpson's revolutionary design. All 84 flats, except for the penthouses, were sold by February this year, within a couple of weeks of the show flat opening. The asking price for a two-bedroom, 83 m2 flat was between £200,000 and £350,000 – double the going rate for comparable apartments elsewhere in Manchester. "The rate of sales was faster than anything else in this sector," comments a satisfied Teague.