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Pillow talk

technical & practice

Chelsea & Westminster Hospital and Centre Parcs have been for some time the best-known uk examples of inflated foil cushion roofing. More recently a string of projects has taken up the technology, with others in the pipeline such as the Eden Centre and two office projects for developer Land Securities. A recent workshop* explored this developing technology, focusing particularly on its performance relative to its main competitor, glass.

Typically cushions come as three layers - the two domed outer surfaces and a relatively flat surface halfway between. Cushions can be up to 3.5m wide, though are usually a bit less, by as long as you like, swaged into light subframes within a larger framework. Often the framing is of metal sections or extrusions, though, as Paul Romain of Buro Happold described, increasing use is being made of cable nets for support.

Cushions need not be rectangular. Triangular ones are common in domes, for example. Form-finding software allows cutting of pieces to shape from 1.55m foil rolls, subsequently heat-welded, to form the shapes required.

Foil cushion roofing is claimed to cost about half as much as high-performance glazing. And, depending on its framing, it could be as little as one- tenth of the weight.

Performance

The material - etfe foil is an extruded film with no evident grain. Untreated, it looks like clear polythene sheet, although it is much tougher (resistant to tearing). Foil is typically up to 200 microns thick; a three-layer cushion weighs less than 4kg/m2. Anticipated lifespan is put at 25-40 years, with warranties up to 15 years - of a similar order to double glazing.

Foil is largely transparent to uv and highly resistant to normal environmental pollution. Since etfe is non-stick, cleaning is suggested only at four- to-five-year intervals.

Inflation and breakdowns - Paired fans (one for back-up) pre-stress cushions by inflating them, typically to around 200Pa. This is enough to support 75kg/m2 of surface loading. The centre foil of a three-layer cushion may be perforated or not, so creating two separate volumes, depending on the supplier. Dehumidification of supply air is an option to reduce risk of algal growth. Fans can easily cope with small perforations, even a slit up to 500mm. Foil patching is possible. Cushions can be replaced working from the outside of the roof. Bird damage is a commonly expressed designer's concern, but this has not been a significant problem to date. Vandalism is largely avoided by keeping cushions at least 3m above floor level.

Energy - A 100W fan is normally enough to maintain inflation of 1000m2 of cushions, switching on automatically as needed, typically running for about 50 per cent of the time. The transparent material is highly transparent to daylight - about 94 per cent for a cushion compared with 76 per cent for double glazing. However, its curvature distorts rather than providing the picture-quality view of glass; this may matter less for a roof.

Horizontal average U-value for a three layer cushion is 1.95W/m2K. Cushions are highly transparent to uv, less so to infra-red. Some of the light and heat gain controls now found with glass are emerging for foils, such as surface printing, fritting, body tinting of the material and including layers with low-E coatings.

Fire - With a melting point of only 230degreesC, foils can be engineered to vent fires, though the source probably needs to be within two storeys for the smoke plume to be hot enough for melting. Flaming droplets are not a problem. Separately-framed conventional rooflights can be included for smoke or heat venting.

Acoustics - Cushions are transparent to sound, which could help reduce echoing, say in hard-surfaced atria or pool areas.

Structural - Cushions support normal wind and snow loads. For heavy snow, parallel stainless-steel wires can be set beneath the cushions. Elastic yield typically occurs at about 10 per cent strain, ultimate failure around 400 per cent. Tolerances can be taken up within the cushions. Cushions may well withstand impacts from falling objects and even explosions.

Contracting

There are two main cushion roof subcontractors: Vector Special Projects in the uk and Koch in Germany. In some jobs Happold has let a roof cushion contract separately from the supporting framing. It is possible to prefabricate framed panels, but to date site installation is the usual practice.

The main trends Happold sees for the future are larger cushions and less framing.

* Building Engineering Workshops in association with Buro Happold and Vector Special Projects. riba, March 1999

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