Book - School of Science and Technology

42 The building in winterFrom Table 2.6, select insulation as, say:50 mm of extruded polystyrene ˆ 2:0m 2 K/Wor 40 mm of polyurethane (board) ˆ 2:11 m 2 K/Wor 40 mm of phenolic foam (board) ˆ 2:22 m 2 K/WCondensationIf the temperature at the internal surface of any element of the building structure fallsbelow the dewpoint temperature of the air within the space, condensation of the watervapour in the air will take place on that surface. The problem is likely to occur as aresult of dense occupancy, of the use of flueless heaters as shown in Table 4.2, and ofdomestic moisture-producing activities such as cooking, bathing, and clothes or dishwashing. In commercial or industrial premises, further hazards exist as a result of steamor vapour producing activities and of the need for a humid atmosphere to suit certainprocesses.In the case of windows, the problem may be ameliorated by either the introduction ofdouble or triple glazing or by provision of a warm air convective current to `sweep' theglass area. Where the building element is solid, however, such as a wall, a ceiling or afloor, the surface may be absorbent to a greater or lesser degree and thus condensationwill be less visible although it will still exist. A concrete floor, suspended above a spaceopen to the outside air, for example an office floor above an open car park, may be subjectto condensation on the floor surface, even when insulation of apparently adequate qualityand thickness has been incorporated in the structure.Similarly, in multi-storey blocks of flats, conduction from the edge of an exposedbalcony has been known to cause problems where the balcony and the slab forming theceiling of the flat below have been constructed as a single unit. This type of structuralbridging, epitomised by concrete or metal mullions and ribs formed across what shouldhave been a weather barrier, without allowance for discontinuity in the thermal sense, hasled to many cases of condensation on surfaces within the building.The present drive for energy conservation and the consequent introduction of higherstandards of structural insulation have led to problems arising from condensation actuallywithin the structure. Perimeters used for the more traditional forms of buildingconstruction were homogeneous and to a large degree impermeable and had a widemargin of safety inherent in their character. More modern building structures with therequired better level of insulation are, in effect, laminates of diverse internal and externalfinishes with layers of insulation etc. sandwiched between. Inevitably, some of the outerlayers outside the insulation remain colder and the risk of intermediate or interstitialcondensation then arises. A brief treatment of this aspect of the subject follows.Interstitial condensationThe majority of the materials used in building construction and many insulating materialswill allow the movement of water vapour through them by diffusion. If a higher vapourpressure exists on one side of the material than on the other, then movement of moisturewill take place, subject only to the vapour resistance offered. Table 2.7 provides values ofvapour resistivity for a limited range of building and insulating materials.