New York's iconic Empire State Building is set to have a green refurb - currently the ageing building is draughty and expensive to heat, with a huge carbon footprint. But the city’s tallest building could soon be one of its greenest. The owners of the 102-storey tower are hoping to cut its carbon emissions by 38% and shave £2.8m off its £6.7m annual energy bill. There are several areas involved in the upgrade, each of which contributes between 2% and 8% of the overall saving. The windows are one of the biggest challenges, and are due to be finished soon. There are 6,514 windows altogether. The building was constructed with single-glazed timber sashes, which were last replaced about 17 years ago, with double-glazed, aluminium sash units. Physically, they are in good shape; but the same cannot be said for their thermal performance.

A key part of the Empire State Building project is to build the business case for eco retrofits and find the best balance between the financial investment and carbon reduction. Ripping out the windows and replacing them with new, high-performance units was an option, but it would have been expensive and disruptive. Plus, the old windows and frames still had plenty of life left in them.

So the design team hit on the idea of “re-manufacturing” them. This offered a cost-effective way of improving the centre pane U-value and cutting solar gain by increasing the solar heat gain coefficient. Together with improving the seals around the windows, it was calculated that 5% could be shaved off energy use and the payback period would lie within the 15-year target.

The upgrading process involves taking the sashes out of their frames and replacing them with a temporary window, a procedure which takes a matter of minutes and can be done out of office hours. They are then taken to the workshop where they are disassembled and the glass cleaned and inspected for chips or cracks. About 97% of the glass is being reused.

A new super-insulating glass unit is then created by taking one of the original panes of clear uncoated glass, laying a warm edge spacer on to it and then a suspended low-emissivity film which acts to cut down the solar gain and the load on the air-conditioning system. Another edge spacer is added before the outer pane of glass is laid on top, creating what is effectively a triple-glazed unit of the same thickness and weight as the original double-glazing. The assembly is then baked in an oven for about 90 minutes which shrinks the film, making it taut, and turning it translucent. The last step is to inject a mix of krypton and argon into the two voids to improve the U-value. Before it is reinstalled, new sealing strips are applied to cut down on infiltration.

The procedure allows the original glass to be recycled and frames reused, and takes the windows from a U-value of 0.5W/m2K to a U-value of 0.125W/m2K, while halving solar gain. Ultimately it should save 1,150 tonnes of CO2 a year and knock more than £250,000 off the building’s annual energy bill. With 43% of all office space in New York City built before 1945, it should be an example others will follow.

The various elements of the entire refurbishment will be

1. Window Light Retrofit: Refurbishment of approximately 6,500 thermopane glass windows, using existing glass and sashes to create triple-glazed insulated panels with new components that dramatically reduce both summer heat load and winter heat loss.

2. Radiator Insulation Retrofit: Added insulation behind radiators to reduce heat loss and more efficiently heat the building perimeter.

3. Tenant Lighting, Daylighting and Plug Upgrades: Introduction of improved lighting designs, daylighting controls, and plug load occupancy sensors in common areas and tenant spaces to reduce electricity costs and cooling loads.

4. Air Handler Replacements: Replacement of air handling units with variable frequency drive fans to allow increased energy efficiency in operation while improving comfort for individual tenants.

5. Chiller Plant Retrofit: Reuse of existing chiller shells while removing and replacing “guts” to improve chiller efficiency and controllability, including the introduction of variable frequency drives.

6. Whole-Building Control System Upgrade: Upgrade of existing building control system to optimize HVAC operation as well as provide more detailed sub-metering information.

7. Ventilation Control Upgrade: Introduction of demand control ventilation in occupied spaces to improve air quality and reduce energy required to condition outside air.

8. Tenant Energy Management Systems: Introduction of individualized, web-based power usage systems for each tenant to allow more efficient management of power usage.