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### Emanuel 6 40 trabajo energy efficiency and the demand]

1. 1. 120 ENERGY EFFICIENCY AND THE DEMAND FOR ENERGY SERVICES window, heat transfer also occurs through the frame and through the spacers that hold multiple glass layers apart.The resistance diagram for heat flow away from the edge of the window is shown in Figure 4.5.As conductive and radiative flows through air occur simultaneously over the entire window area, the corresponding heat transfer coefficients for a given layer are simply added, then the reciprocal is taken to get the resistance for that Iayer.The total resistances are added up, and the reciprocal is taken to get the overall centre-of-glass U-value. T, T, T, T, R,=1/h, R,=1/h„ Figure 4.5 Heat flow through a double-giazed window where Qis the volumetric flow rate (m3/s), g is the specific heat (J/kg/K), p is density, and the A Tin this case is the difference between the supply and return temperatures. For air, y cpa= 1004.5J/kg/K and e= 1.25kg/m3at 10°C and latm, while for water, g = g.= 4186J/kg/K and p = 10001(g/m3 at 4°C. From this it can be seen that, for a given volumetric flow rate and temperature drop, water delivers 3333 times as much heat as air. To keep a room at a given temperature, the rate of heat delivered by the heating system must equal the rate of heat loss. For a given flow of a given fluid (air or water), the rate of heating is proportional to the temperature drop of the fluid. A larger temperature drop will occur the warmer the air or water relative to the space being heated, the larger the radiator, or the slower the flow rate. If the building has a good thermal envelope, the rate of heat loss is small, so heat does not need to be supplied as rapidly by the heating system, and a lower supply temperature and/or a smaller flow rate is possible. However, most heating systems are rather inflexible, with a fixed supply temperature and a fixed flow rate, so part load is satisfied with intermittent on/off operation (and often overheating the building, requiring windows to be opened in winter). Energy used to move air or water Energy is required for fans or pumps to move air or water. The following discussion applies to both pumps and fans, moving water and air through pipes and ducts, respectively, but to keep the wording less onerous, the term Pump' will be taken to mean pumps or fans, the term Pipe' will be taken to mean pipes or ducts, and the term 'fluid' will be taken to mean air or liquid. When a fluid flows through a pipe, it encounters a resistance that leads to a drop in pressure (4.1) along the flow path. The pump supplies pressure to the fluid that exactly compensates for the pressure drop during the roundtrip circuit from the pump, through the pipes and back to the pump. The rate at which energy needs to be supplied to a fluid by the pump (i.e. the power input to the fluid, Pf/u) is equal to the pressure drop times the rate of flow. However, for turbulent flow, the pressure drop varíes with the flow velocity squared, so for a given pipe diameter, the pressure drop will vary with the flow rate squared. That is: P = Q a Q 3 fluid (4.7)
3. 3. 122 ENERGY EFFICIENCY AND THE DEMAND FOR ENERGY SERVICES 110 Throttle Valva 90 '''' Pumps Inlet Vane 100 — Fans Comparison of hydronic and air systems 80 70 60 15 O O- 1 pumps, motors, fans and VSDs are all greatest at or Glose to full load). m50OutleiDarrpper 40 30 - VSDs 20 1 0 10 20 30 40 50 60 70 80 90 100 %Peak Flow Note: The curve labelled 'cubic law' assumes that power a (flow)275. Source: Smith (1997) Figure 4.6 Variation of fan or pump power with flow for various ways of reducing the flow avoid oversizing the fans or pumps relative to the design flow (although, as noted aboye, the pipes can be oversized), so that they operate at as large a fraction of full load as possible. Although the VSD introduces a substantial efficiency loss at part load, the energy consumption at part load with a VSD is still less than without it because the pump is attempting to provide only the fluid power (Pb) that is needed. In summary, the keys to reducing the energy required to circulate air or water are: to oversize pipes or ducts (because the pressure drop decreases with pipe diameter to the fifth power), something that will be easier for pipes than for ducts due to space limitations; to lay out the heating and cooling elements so as to minimize the length of pipe or duct and to minimize the number of turns, and to use turning vanes in the bends of ducts; to design the heating and cooling system to require as little water flow or airflow as possible (because the power that must be supplied to the flow varíes with flow rate to the third power); to utilize VSDs for efficient part-load operation; to avoid oversizing the pumps, motors or fans so that the system is operating on average as dose to full load as possible (because the efficiencies of As noted aboye, the heat that can be transferred for a given volumetric flow rate and temperature drop is over 3000 times greater for water than for air. However, the energy required to produce a given volumetric flow rate is also greater for water than for air. The relevant quantity in each case is the required pump or fan power divided by the rate of heat flow that is accomplished, that is, Equation (4.7) divided by Equation (4.6): E_ AP pcpAT (4.10) where A T is the difference between supply and return temperatures. Using the values of p and cp given aboye, and taking AP to be 1400Pa in an air system and 50,000Pa for an equivalent hydronic system (as computed by Niu et al, 2002, for a given building), it can be seen that heat transfer by water requires about 100 times less energy for a given A Tthan heat transfer by air, neglecting differences in pump, fan or motor efficiencies. However, if the temperature drop A Tin the air system is four times larger (i.e. 8K rather than 2K), the energy used to transfer a given amount of heat with water would be 25 times smaller than using air. This is still a substantial difference! 4.1.3 Sensible and latent heat Heat in air occurs in two forms: as sensible heat and as latent heat. Sensible heat refers to the heat that we can sense, as a warmer temperature. Latent heat refers to the heat that is released when water vapour condenses. The amount of moisture that air can hold depends on its temperature; the warmer the air, the more it can hold. The saturation vapour pressure is the water vapour pressure when the air is holding the maximum amount of water vapour possible, and it increases rapidly with increasing temperature. The relative humidity (RH) of an air parcel is the ratio of the actual vapour pressure to the saturation value, times 100 to give it as a percentage. The amount of moisture in air can also be represented by its mixing ratio, r (kg/kg), which is the
4. 4. ENERGYUSEINBUILDINGS123 ratio of mass of water vapour to mass of dry air. Quantitative relationships involving vapour pressure, mixing ratio and sensible and latent heat are given in Box 4.2. If an air parcel is cooled, the saturation vapour pressure decreases but the actual vapour pressure remains the same, so the relative humidity increases. Further cooling causes water vapour to condense, releasing latent heat that has to be removed along with sensible heat. Condensation begins when the relative humidity reaches 100 per cent, and the temperature at which this occurs is called the dewpoint temperature, Tap. If we allow iquid water in an air parcel to evaporate without adding heat from the surroundings, the temperature of the parcel will decrease (as heat energy is used to evaporate water) and the humidity of the air will increase. This will continue until the parcel becomes saturated, at which point no further evaporation or cooling will occur. The temperature at which this occurs is called the wetbulb temperature, T b, because it is given Box 4.2 Vapour pressure, sensible heat, latent heat and enthalpy The mixing ratio r (kg/kg, the ratio of mass of water vapour to mass of dry air) is related to the vapour pressure, eo, by the relation: r = 0.622ea Pa— e, (4.11) where 0.622 is the ratio of molecular weights for water vapour and air, and P is the total atmospheric pressure.The relative humidity of an air parcel is the ratio of the actual vapour pressure (or mixing ratio) to the saturation value, times 100 to give it as a percentage.That is: RH = el' 100% = 1100% e s rs (4.12) The latent heat content of an air parcel per unit mass (J/kg) of dry air is given by: L = Lcr (4.13) where is the latent heat of condensation — the amount of heat released when I kg of water vapour condenses. It depends weakly on temperature, with a value of 2.501 x 10 6J/kg at a temperature of 0°C. The sensible heat content of air per unit mass (J/kg) is given by: S= c T+ rc T Pa pwv (4.14) where T is in kelvin and cm, is the specific heat of air (I 004.5)/kg/K) — the amount of heat that must be added to warm I kg of air by I K — and cm, is the specific heat of water vapour (1860)/kg/K). The combination of sensible heat and latent heat per unit mass of dry air gives the specific enthalpy H of an air parcel, where: H= cPa T+ r (Lc + cpwv T) (4.15) In practice, it is more convenient to compute enthalpies relative to the enthalpy of an air parcel at a temperature of 0°C, that is, to use T in Celsius degrees rather than in kelvin.
5. 5. 124 ENERGY EFFICIENCY AND THE DEMAND FOR ENERGY SERVICES by the temperature of a thermometer bulb with a wet doth over it, in equilibrium with a steady airflow. The initial air temperature is referred to as the drybulb temperature, T, because it is the temperature that a dry thermometer measures. The wetbulb temperature is the lowest temperature to which an air parcel can be cooled by evaporating water into the air parcel, or to which water can be cooled through pardal evaporation. The lower the relative humidity, the greater the difference between T and Twb — that is, the greater the cooling potential through evaporation. The relationships between temperature, mixing ratio, dewpoint and wetbulb temperatures and relative humidity are given in a specially constructed diagram called a psychrometric chart, which is shown in simplified form in Figure 4.7. Temperature and mixing ratio are given as the vertical and horizontal fines, respectively. The leftmost concave upward fine gives the variation of saturation mixing ratio (r) with temperature; as noted aboye, es and hence r, increases rapidly with increasing temperature. Any point on this fine corresponds to 100 per cent relative humidity. The temperature at which a given mixing ratio fine intersects this curve gives the dewpoint temperature. The other concave upward fines correspond to various other RHs. When water evaporates into air, the mixing ratio of the air increases but the air is cooled; Chis is represented by the fines sloping upward to the left, which are fines of constant wetbulb temperature. Evaporation can continue only until the air has become saturated, which is where a given wetbulb fine intersects the saturation mixing ratio line. The temperature at this point is the wetbulb temperature. Thus, given the mixing ratio r and temperature Tof an air parcel: 60%RH • 40%RH 1 00'4,RH 30 28 % 26 24 22 SI ID. 20 ,92 18 Wetbulb Temperature Dewpoint Temperature Drybulb (actu temperature Unes of constant enthalpy 16EE 14 o 12 es 10 c8 6 b rs 4E - z o' ' 5 10 , • -- " • - - • 15 20 25 30 35 40 Temperature CC) Source: Computed by author. A more detailed version is found in ASHRAE (2001, Chapter 6) Figure 4.7 The psychrometric char4 showing wetbulb temperature (dashed fines sloping upward to the left) and relative humidity (concave upward fines) as a fimction of drybulb temperature (vertical lines) and humidityratio (horizontal fines) 1
7. 7. 126 ENERGY EFFICIENCY AND THE DEMAND FOR ENERGY SERVICES 41 50 5.9 95 (V) 58 77 85 3 2 • •••• 3 0 28 1) • • 7126á s "".0` • • • • Ct. t o 22 .g 18 c 16 ■ ffl• -.11 • • •-• • • E Ú 20 o ■ LL 82 e 0 o E 24 ■ 8 5  • ■ 79 rt ■ =m- •-••• •••~- ■ .• 72 . 090% acceptability limits o o•••• • • • • •••". o_ 75 E 80% acceptahility limits a a a E 68 3 64 c 6 1 = 14 - 35 40 10 15 20 25 30 Mean Monthly Outdoor Air Temperature ( ° C) Source: Brager and de Dear (2000) Figure 4.8 A standardfir acceptable indoor temperatures that takes into account psychological adaptation to different outdoor temperatures overwhelmingly preferred to a temperature of 26°C with a fixed air speed of 0.2m/s (de Dear and Brager, 2002). In Thailand, Busch (1992) found that the maximum temperature accepted by 80 per cent of survey respondents is about 28°C in air-conditioned offices and 31°C in naturally ventilated offices. The higher temperature setting for air-conditioning that is permitted under an adaptive standard would reduce the thermal shock often encountered in moving between indoor and outdoor spaces on hot days. Increasing the thermostat from 24 to 28°C in summer will reduce annual cooling energy use by more than a factor of three for a typical office building in Zurich and by more than a factor of two in Rome (Jaboyedoff et al, 2004), and by a factor of two to three if the thermostat setting is increased from 23 to 27°C for night-time air conditioning of bedrooms in apartments in Hong Kong (Lin and Deng, 2004). The percentage savings in air conditioning energy use, however, will diminish the greater the cooling load of a building, and hence the warmer the outdoor temperature. 4.2 Thermal envelope and the role of building shape, form, orientation and size The term thermal envelope refers to the shell of the building as a barrier to the loss of interior heat or to the penetration of unwanted outside heat into the building.
8. 8. ENERGY USE IN BUILDINGS 4.2.1 Insulation The thermal properties of insulation, doors and windows are rated using three different parameters: the U-Value (W/m2/K), equal to the rate of heat flow per unit atea and per degree of insideto-outside temperature difference; the RSI-Value (W/m2/K)-1, a term used in Canada and here to denote the resistance to heat flow and equal to 1/ U when U is expressed in metric units (SI denotes Systéme International); the R-Value (Btu/ft2/hr/°F)-1, which in Canada and the US is a resistance to heat flow equal to 1/ Uwhen U is expressed in British units, but outside North America means the metric resistance. To convert from RSI-values to North American Rvalues, multiply by 5.678. The U-value of a layer of insulation is equal to its thermal conductivity (having units of W/m/K) divided by the thickness of the layer (see Equation (4.2a)). The R-value, being equal to the reciprocal of the U-value, 1.0 Walls at R12 (RSI 2.1, U=0.47 W/m2fK) 0.9 0.8 Relatwe Eleat Loss 0.7 Walls at R20 (RSI 3.52,U=0.28 Wirn2/19 0.6 Roof at R32 (RSI 5.6, U=0.18 VV/m2/K) 0.5 0.4 0.3 0.2 Advanced House- 0.1 0.0 Walls (R40, RSI 7.0) Roof (R60, RSI 10,6)  10 20 30 40 50 6C R-Value 24 RSI-1/51Lie Figure 4.9 Heat flow versus R-value for the range of R-values encountered in insulated walls and ceilings, relative to the heat flow at R12 127 increases in direct proportion to the thickness of the insulation. However, there are diminishing returns to adding ever more insulation because the heat flow varíes with 1/R This is illustrated by Figure 4.9, which shows how the relative heat loss for a given temperature difference varíes with the R- or RSI-value for the range of values that characterize walls and roofs. Insulation thickness is directly proportional to the insulation R-value, while heat loss is directly proportional to the U-value. Conventional insulation Conventional insulation will be taken here to mean one of the following: fibreglass insulation batts; mineral fibre batts; blown-in, loose cellulose; sprayed adhesive cellulose fibre; other wood-based fibre products; solid foam panels; blown-on foam insulation. Table 4.2 lists the thermal conductivity and density for these and for two unconventional kinds of insulation (discussed later), along with the thickness of insulation required to achieve a U-value of 0.1W/m2/K. The most highly insulated houses in the world have wall U-values of 0.1-0.2W/m2/K (R28—R60) and roof U-values of 0.10-0.15W/m2/K (R40—R60), which is two to three times better than required in most cold-dimate countries. Except when insulated with foam insulation, such houses normally require more wood, but high levels of insulation can be combined with advanced framing systems so at to minimize the amount of wood required and the amount of wood waste generated (Baczek et al, 2002). Optimized wood framing can reduce the cost of framing by 40 per cent and reduce the generation of wood waste by 15 per cent compared to conventional (non-optimized) framing. As seen from Table 4.2, solid foam insulation has the lowest thermal conductivity of any conventional insulation and so provides the greatest insulation value for a given thickness. It is widely used in commercial buildings, as it can be directly glued to the outside of concrete or masonry walls. There are several different kinds of solid foam insulation, all of which are manufactured from chemical products produced from
9. 9. 128 ENERGY EFFICIENCY AND THE DEMAND FOR ENERGY SERVICES Table 4.2 Thermal conductivity and density of different insulating materials, and thickness requiredfor a U-value of 0.1W/m2/K Insulating material Fibreglass Mineral fibre Cellulose Flax, hemp Wood fibre Kenof XPS Spray-on PUF Solid PUF Aerogel Vacuum Conductivity (W/m/K) Density (kg/m3) Thickness (cm) for U = 0.1W/m2/K 0.042 20 42 0.037 0.04 0.04 0.038 0.038 0.030-0.034 0.030-0.035 0.022-0.024 0.012 40 32 30 40 40 32 32 32 100 37 40 40 38 38 34 35 24 12 0.004 4 Note: XPS = extruded polystyrene; PUF = polyurethane foam. For XPS and PUF, the lower thermal conductivity pertains to insulation made using an HFC blowing agent, while the higher thermal conductivity pertains to use of a non-halocarbon blowing agent. See subsection 4.10.6 for the significance of the choice of blowing agent. Source: Harvey (2006, 2007); Ardente et al (2008) oil or natural gas and require a substantial amount of energy to produce (as discussed in subsection 4.10.5). They also require some gaseous blowing or expanding agent in order to make the liquid raw materials expand into a foam as they are heated during the manufacturing process. At first, CFCs were used as the expanding agent in many foams, then these were replaced with HCFCs, which in turn are being replaced with HFCs. HFCs have no adverse affect on the stratospheric ozone layer (unlike CFCs and HCFCs) and might be promoted as 'green', but the most popular ones are powerful GHGs. As discussed in subsection 4.10.6, leakage of HFC blowing agents from the foam insulation during its lifespan can cancel the dimatic benefit of needing less energy for heating and thereby emitting less CO2. A number of foam insulation products are available that use water, pentane or CO, as blowing agents, the leakage of which has negligible dimatic effects. Cellulose insulation is recyded newsprint, treated with borate to provide fire and insect resistance. Insulation batts can be made of fibreglass or mineral fibres. Insulation batts and blown-in cellulose are usually applied between wood studs. As wood is a relatively poor insulator, the studs constitute thermal `bridges'. The bridges can be eliminated and the insulation level between studs increased by adding a layer of solid foam (or wood fibre) insulation on the outside of the stud wall, crossing over the studs. Conversely, thermal bridges can be minimized through the use of engineered wood I-beams. These are illustrated in Figure 4.10 and have the advantage that they are stronger than solid studs with the same outside dimensions, can be manufactured from wood waste or from small but fast-growing tree species, and the exact lengths needed for a given job can be ordered from the factory, thereby eliminating wood waste from the construction site. Sloppy workmanship (leaving gaps in the insulation or compressing the insulation to fit around wiring, plumbing and electrical boxes rather than cutting it to size) can seriously undermine the benefit of high levels of insulation, and this seems to be a widespread problem. An advantage of blown-in cellulose insulation is that it will fill the various irregularly shaped gaps in the wall structure, thereby providing a more continuous insulation barrier. There are a number of insulation materials available that are made from various wood fibres or from hemp, flax or straw, with thermal conductivities comparable to that of fibreglass insulation or mineral fibre (0.04W/m/K). Vacuum insulation panels Vacuum insulation panels (VIPs) have received considerable attention since the 1990s as they provide the possibility of high thermal resistance at low
10. 10. ENERGY USE IN BUILDINGS 129 No r ma l i n s u Ta ti o n In thickness I nter i or Sobd toarn Insulation• wall bo ar d Cavity for retracted Vac uur n o f f e r n a l b l l nd i nsulati on panel iriple-glazed Retracted window externa' b l in d Source: Author Figure 4.11 Use of a vacuum insulation panel to maintain high thermal resistance behind the housing for retractable external window shading blinds because the floor height inside and outside can be made equal while providing stringent (RSI>5) resistance to heat loss on the outside part of the roof. Another niche Source: The Engineered Wood Association (www.apawood.com) applicationofVIPs isnext tothehousingfor externalwindowsshadingblinds,as illustratedinFigure 4.11. Figure 4.10 Engineered I-beams that can be used as OneconcernwithVIPsisthepossibilitythatsuchpanelscouldbedamagedduringtheconstructionprocess.Forthisreason,prefabricatedassembliesprotectingtheVIP columns, beams andfloor joists arepreferred.Prefabricated concretewall slabsareavailableintheSwissand German marketswithatotal wallthickness(indudinginteriorfinish) of 27cm and anaverageU-valueof 0.15W/m2/K (Binz and Steinke, 2005). An exampleis showninFigure 4.12. VIPs are also availableinprefabricated wood-frame roof and wall units and in VIP doors, and have beenused inretrofitapplications(asdiscussed insubsection4.12.1). thickness. The core of the panel consists of a microporous material (such as fumed silica) under a soft vacuum (10 4 atmospheres), which yields a thermal conductivity eight to ten times less than that of cellulose (0.004-0.005W/m/K versus 0.04W/m/K). The evacuated core is wrapped in an airtight envelope that Aerogel forms a thermal bridge, the extent of which depends on the thickness and thermal conductivity of the envelope. For 1 x lm, 3cm thick panels, Wakili et al (2004) measured an overall RSI (including thermal bridges) of 6.2 (U-value = 0.16W/m2/K; R35). Conventional insulation with the same RSI-value would be 20-25cm Aerogel insulation panelsconsist of silica(SiO2) granules. Theyare highlyporous and thus have adensityof onlyabout 100kg/m3 (compared to 2200kg/m2 forglass, whichisalso thick. Even if the vacuum is lost, the thermal resistance in the core of a VIP is still twice that of cellulose (Nussbaumer et al, 2006). silica).Thethermal conductivityofoneproductis0.013W/m/K —aboutthreetimesthatofvacuum insulationbutonethirdthatofcellulose. About 10,000m2 of VIPs, covered by a protective layer, waterbarrier and concreteplate, had been installed by 2004 on terrace roofsin Switzerland (Simmler and Brunner,2005).VIPsarepreferredinsuchapplications
11. 11. 130 ENERGY EFFICIENCY AND THE DEMAND FOR ENERGY SERVICES ti capping stone lath anchor polyurethane foam vacuum insulation vapour stop concrete ca 27 cm Source: Binz and Steinke (2005) 74 ,. , :u' . - Figure 4.12 On the lefi is a cross-section of a prefilbricated wall unit with a VI1? having a total thickness of 27cm and a U-value of 0.15W/m2/K On the right is a photographic example 4.2.2 Windows Windows are intended to permit light to enter into a building and to provide a view to the outside, but offer substantially less resistance to the loss of heat than insulated walls. Minimizing this heat loss is important in cold dimates. At the same time, windows permit solar energy to enter a building. This can be an asset in winter (reducing the heating requirements) but a problem in summer (increasing the cooling requirements). Heat flow through windows A single-glazed window has a centre-of-glass U-value (that is, exduding the frame and the area next to the frame) of about 5W/m2/K. Thus, when the indoor temperature is 20°C and the outdoor temperature is — 20°C, heat is lost through the window at a rate of 200W per m2 of window ama. Compared to a single-glazed window, heat loss can be reduced by: Adding extra layers of glass (glazing). The thermal resistance of glass is essentially zero. In a single-glazed window, the resistance to heat flow arises from the thin motionless layer of air on either side of the glass, as motionless air is a relatively good insulator. In double- and triple-glazed windows, additional layers of motionless air are created, increasing the thermal resistance. Double and triple glazing alone will reduce the U-value to about 2.5 and 1.65W/m2/K, respectively. Using alternative gases between the window glazings. Gases with a heavier molecular weight than air have a lower molecular thermal conductivity than air. Argon has a thermal conductivity a third less than that of air, while krypton and xenon have thermal conductivities of just over one third and just over one fifth that of air, respectively. In a double-glazed window, argon and krypton will reduce the U-value from 2.5W/m2/K to 2.4 and 2/K, respectively. Argon- and krypton-filled 2.3W/m windows are commercially available in many countries, while xenon-filled windows have been used in Germany. Using low-emissivity coatings. The loss of heat to the outside by emission of infrared radiation can be reduced if the emissivity of the glazing surface is
14. 14. ENERGY USE IN BUILDINGS shoulder of the mullion, thereby reducing the otherwise substantial heat loss through the mullion, so the overall U-value is dependent on the details of the construction. The spandrel portion can achieve U-values of 0.40W/m2/K. In high-performance curtain walls, the spandrel will contain additional insulation on the inside. Alternatively, vacuum insulation panels are available for the spandrel from the German company Okalux (www. Okalux. com) , with U-valu es of 0.080.16W/m2/K. These U-values are a factor of three to eight smaller than permitted for the spandrel under the ASHRAE 90.1-2004 building code. 3 Perimeter Heating Needed m 2 1 o c1 Perimeter Heating Not Needed o -30 -0 2 -10 o 133 C Source: Geoff McDonell, Omicron Consulting, Vancouver Figure 4.13 Window U-values below which perimeter heating is not neededfor a 2m high window, as a fimction of the winter design temperature 4.2.3 Curtain walls in commercial buildings A curtain wall is a wall on the exterior of a building that canjes no roof or floor loads. It commonly consists entirely or largely of glass and other materials supported by a metal framework, although precast concrete panels have also been used. The frame is referred to as a mullion, while the opaque panels between the glazed portions are referred to as the spandrel. Mullions can be provided with or without thermal breaks. Mullion U-values range from 16.8W/m2/K for a double-glazed curtain wall with an aluminium frame without a thermal break, to 4.3W/m2/K for a triple-glazed curtain wall using recently available insulated framing systems. The mullion can easily account for 10 per cent of the total wall area, so the large mullion U-values are a significant factor in the overall U-value. Double-glazed and triple-glazed curtain walls are commercially available, with overall U-values (induding the frame) for the 7500 Kawneer series as low as 1.8W/m2/K and 0.8W/m2/K, respectively, for large units (see wwwkawneer.com). The triple-glazed units represent a 70 per cent reduction in heat loss compared to the glazing value of 2.6W/m2/K that is permitted under the ASHRAE 90.1-2004 commercial building code in moderately cold dimates. These units entail thermal breaks and insulation covering the 4.2.4 Air leakage When air inside a building is heated aboye that of the outside air, a pressure variation is established such that the interior air pressure in the upper part of the building is slightly greater than the outside air pressure, while the interior pressure in the lower part of the building is less than the outside pressure. As a result, cold inside air is sucked into the lower part of the building through various tracks and openings in the walls and windows, and warm air exits through the upper part of the building. This temperature-induced air exchange is referred to as the stack effect. Up to 40 per cent of the heating requirement for houses in cold dimates is to heat the outside air that continually replaces the inside air. The rate of air exchange will be greatest when it is coldest outside, which is the very time that a given rate of air exchange will cause the greatest heat loss. In hot humid dimates, air leakage can be a significant source of indoor humidity. In residential construction, careful application of a continuous impermeable barrier can reduce rates of air leakage by a factor of five to ten compared to standard practice. This is not a matter of advanced technology, but rather a matter of attention to detall and the enforcement of careful workmanship during construction. In buildings with very low air leakage, a mechanical ventilation system is required that circulates fresh outdoor air through the building and then exhausts. Up to 95 per cent of the available heat in the warm exhaust air can be transferred to the cold incoming air in winter using a heat exchanger. Air leakage is more difficult to measure and control in commercial buildings but can nevertheless be greatly reduced through simple measures that are part of a higher overall quality of construction. In office buildings in the central and northern US, these measures can reduce