MATERIALS-2
Polythene finds many uses in building
GROWING use is being made to-
day of foamed polystyrene in the insulation of industrial and residen- tial buildings. This foam! con- sists. for all practical purposes. entirely of polystyrene and air, and has really exceptional properties to which it owes the great variety of ways in which it can be employed: as thermal and acoustical insulation in ceilings, walls, roofs and as the centre layer in sandwich construc tions of lightweight concrete and prefabricated building units.
The material is best obtained in the form of sheets of foam which may be applied to the inner or outer surface of walls. The foam may be bonded to the wall after it has been coated with special adhesives to raise the adhesive strength. or it may be nailed; but a more advan- tageous method is to sandwich. the in- sulating layer of foam between two layers of brick.
Easy to handle
Since the material is easy to handle because of its low density usually between 0.9 and 1.25 lb/cu ft and is easy to saw, it can be adapted on the site exactly to the local require. ments. It can be easily and cleanly laid at all times.
Moreover, the closed cellular structure of the polystyrene foam has the advan- tage of being able to stand up to re- latively high compression and relatively great impacts: for the air in the closed cellules is compressed by such stresses, and has а resilient effect. Sheets of polystyrene foam are mechanically less fragile than insulating materials whose fibres are easily pulled apart and which can fall to pieces if they are not care- fully handled.
An important factor in choosing an insulating material is its behaviour towards moisture which is present on the job. Polystyrene foams are used in build- ing because their high diffusion resistance and lack of capillary absorption combat the accumulation of moisture. special measures arc necessary where moisture diffusion comes from one side, say in a flat roof with a watertight top layer over moist rooms. These construc- tional measures ensure that the moisture
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content is kept low. The higher the coefficient of resistance to water vapour diffusion of the building and insulating walls, the lower is the danger of water- logging.
Although polystyrene feams, in spite of their low density, have a diffusion resistance much higher than most other materials, the material should not be regarded as a vapor barrier. In prac- tice, however, the foam considerably retards the diffusion of moisture.
Insulation
Steps must be taken to ensure that moisture from within the building is allowed to escape via ventilation grooves on the underside of the insulating layer. Perforated board and grooved insulating paneis serve well for this purpose. Ιτι this type of construction, sheets 2 to 6 cm thick with a density of 1.6 to 1.9 In ib/cu ít have given good results. many cases a proper vapour barrier can be applied to the underside of the foam. It is always necessary, if humid rooms have a flat roof above them, to provide a sufficiently thick layer of in- sulation so that the bottom layer of the roof does not cool too quickly even when there is heavy frost.
The fact that the solid material in polystyrene foam is mainly distributed in the form of thin films means that certain care should be taken over its thermoplastic properties. Care should be taken that the upper temperature limit of 70 to 75° C is not exceeded when the foam is exposed to the effects of sunshine. This may sometimes occur when the foam is laid under roofing felt, and there is no air movement whatso-
ever.
Thus when the sun's radiation im- pinges vertically on a surface. 67° C above air temperature has been measured over short periods. On a flat roof, a temperature of 56° C, on a slanting south-facing roof 62° C, and on a slant- ing eastor west-facing roof 55° C, above air temperature were measured. But they are, as mentioned above, only temperature maxima of short duration.
Light cover
In normal conditions of air movement the temperatures recorded are only half high. For safety's sake, no dark
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THE HONG KONG & FAR EAST BUILDER—VOLUME 16, NUMBER 4
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coloured roofing felt should be applied to the foam, because the heat absorp- tion depends greatly upon the colour of the surface, and white surfaces take up only half of the temperature above air temperature that dark surfaces do. Also suitable are constructions incorporating outer layers of roofing felt coated with sand ог gravel. In hot climates a different form such as Styroper H (made by the Badische Arilin-&- soda Fabrik A.G.) may also be employ- ed:
this foam is resistant to deforma- tion up to a temperature of 95° C.
Sheets of ordinary Styropor foam 0.6 to 1.0 cm thick laid beneath floating floors have proved an effective sound Το attenuating layer between storeys. ensure that no sound or heat bridges are formed, a sheet of tarred paper should be laid between the insulating foam and foam the floating floor, and strips of should be laid round the bottoms of the walls to insulate the floating floor from the walls.
For walls
class
Sheets of polystyrene foam containing perforations, pinholes in the surface, and milled or embossed grooves, have given first
results as acoustic panels. They are particularly effective if the fully perforated type las a layer of an
open celled insulating material behind it.
To simplify work with insulating sheets, panels are made of foam to which a layer of fibreboard or light- weight constructional units are bonded. These combined units are particularly effective because they incorporate the desirable properties of both materials. Such units may be transported without packaging. They are very rigid and extremely resistant to rough hanrling in transit. Moreover, these compined units act as good plaster carriers with- out any further treatment.
These lightweight units are suited for the finishing of attics; for incorporation in the external walls of rooms and of rooms which are seldom heated, such as kitchens and conference rooms; for ceilings beneath flat roofs, particularly industrial buildings and meeting halls; for employment beneath floating floors, particularly for ground floor flats, hall- ways, porches, etc.; and for insulating agricultural buildings.
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