Book - School of Science and Technology

36 The building in winterTable 2.4 Thermal transmittance (U value, W/m 2 K) for glazing and windows with framesGlazingWindows (glazing in frames)Wood Metal uPVCType Vertical Horizontal BareThermalBarrierSingle 5.6 7.1 4.5 5.7 5.4 4.7Double 2.9 3.6 2.7 3.4 3.1 3.0Triple 1.9 ± 2.1 2.7 2.3 2.4Double (low emissivity coating) 2.0 2.6 2.0 2.5 2.1 2.3Double (low emissivity coatingand argon fill) 1.7 2.2 1.9 2.4 2.0 2.2NoteValues quoted are for double glazing sealed at works; frame area 30% for wood and uPVC, 10% for metal; low emissivitycoating; ˆ 0:2 normal exposure.The final four columns of Table 2.4 show practical values of the transmittance coefficientfor real windows and differentiate between different materials and methods ofmounting. The variation from the notional values is due not only to the considerableproportion of the total area available taken up by the frame (often 20%), but also tobridging effects to the structure.It seems appropriate here to draw a clear distinction between double glazing and doublewindows. The principal difference is that double glazing mounts two sheets of glass, spacedand hermetically sealed in the works of the supplier in a single composite frame, whereasdouble windows normally consist of a secondary system of glazing, fitted quite independently,on the room side of an existing outer facade window. In consequence, since readyaccess to the space between the panes of double windows is necessary for good housekeeping,it follows that the air space between them will not be sealed, the resistance R a willbe reduced by about a quarter and the U values will be increased by about 0:5 W/m 2 K.Windows which are double glazed, if of good quality, may be expected to retain theirthermal characteristics whereas double windows, opened regularly for cleaning, are subjectto misuse and thus a falling performance. As a contra argument, in the refurbishmentof existing buildings, the capital cost of replacing windows and frames might be such thatadding an additional pane is the only practical alternative.Further, in terms of noise transfer from outside, double windows have an acousticadvantage arising from the wider air gap, provided that the reveals between the panes aretreated suitably. A perfectionist might choose to make use of an expensive compromiseand fit double glazing at the outer facade, to ensure the optimum thermal advantage,backing this up with acoustically treated reveals and a moveable inner pane!Wall cavity fillThe thermal resistance of an unventilated air gap, as stated earlier and demonstrated inTable 2.1, is 0:18 m 2 K/W for building surfaces having a high emissivity, i.e. the majority.However, if that cavity were to be filled with an insulating material having a resistance of,say, (1/0:035) (50/1000) ˆ 1:43 m 2 K/W, the U value of the structure would be greatlyimproved. Taking a conventional cavity construction having an outer skin of 105 mm brickand an inner skin of 100 mm lightweight concrete block, finished with 13 mm lightweightplaster, the thermal transmittance with an empty cavity would be 0:98 W/m 2 K, whereaswith insulation therein it would be 0:44 W/m 2 K, i.e. less than half the former figure.