Tower of strength

This was no exaggeration. Parkinson, managing director of consulting engineer Curtins, knew a gale had already ripped the cladding from similar 1960s blocks on the city outskirts. Immediate action was called for.

Bolting steel beams and brackets to the inside and outside of flats is the usual method for repairing brick cladding. But in the Liverpool blocks, many of the tenants were old and unwilling to be rehoused while the work was carried out. Steel would also have been expensive and, bolted to the blocks' exterior, would have done nothing to enhance their already austere appearance. In addition, some of them were soon to be demolished. An easier and cheaper solution was required.

Structural engineer Curtins was called in to survey the blocks. It came up with an unusual remedy, which was to glue the inner and outer walls together with structural polyurethane foam. This would minimise disruption to the tenants and require no additional steelwork inside or out, making it much less expensive. But although polyurethane foam was an established means of stabilising walls in low-rise buildings, less was known about its suitability for high-rises. Would it work?

Foam injection dummy-run

Parkinson was aware that a similar solution had been used on a high-rise in Edinburgh 10 years previously, but the client insisted on testing the foam on a block in Liverpool that was due for demolition, to ensure it was strong enough to do the job. It was, and in October 1996, a £20m programme of emergency works began on damaged and dangerous sections of cladding across 41 of the housing trust's blocks. Tarmac was the main contractor and MPI was specialist foam subcontractor. At the heart of the solution was rigid polyurethane foam – the same type used to insulate the trailers of refrigerated trucks. Two liquid components – isocyanate and resin – are mixed as the foam is injected with a hand-held gun through holes drilled into the wall's mortar courses at 1 m centres.

The wall cavity is flooded from the base up. The foam expands as it sets in the cavity, bonding the inner and outer walls together to form a single unit. Anchor bolts then secure this panel to the block's concrete structural frame.

Before the foam is injected into the cavity, any gaps in the wall's inner leaf must be filled. Usually, this is just a matter of squirting a sealant into gaps around the kitchen sink's waste pipe where it passes through the wall, and checking around window frames for gaps. "When we miss a gap, the foam squeezes through it and appears as an amorphous mass in the flat," says Don Darby of MPI.

Sealing each flat usually takes about two hours. It takes the installers another two hours to fill the cavity with foam, working from scaffolding outside, and the task is complete the next day. "It works out at about £20/m2 for the complete installation," says Darby.

In the Liverpool blocks, the foam has successfully stabilised the cladding. But on top of its structural properties, it has also provided much-needed thermal insulation to the poorly heated flats. And because the foam expands in the cavity, it fills cracks and prevents rain and draughts entering the flats.

So, the foam proved effective in emergency repairs, but could it be used to refurbish an entire block? This time, Rydecroft Block in Woolton, Liverpool, was used as a test. All the cavities in the block were filled, and, at the same time, a complete energy-saving package was fitted: double-glazing replaced the leaky metal windows; exposed balconies were enclosed; and efficient central heating systems and heat recovery ventilation systems were installed in each flat. The works were completed in mid-1998, for £17 699 a unit.

Raising temperatures, lowering bills

The trust will monitor the block over the next two years. Early indications are that the solution is performing well. Tenants' energy bills are down from £8.33 a week to £6.46 – a 25% reduction. Surprisingly, electricity bills have fallen more than gas heating bills. "We suspect that this is because shivering tenants had been using electric heaters to top up the old gas warm-air systems," says Tom Clay, director of development and planning at the housing trust.

A recent survey indicated that 94% of tenants are satisfied with the results. The trust now plans to use this solution on 16 more blocks, saving itself an estimated £8m.