b'PSYCHROMETRICSbe. Most hygrothermal models are geared toTo assess the situation, I conducted a psychro- ventilation, with the various cupolas and RH, and most material propertiessuch asmetric survey, which meant using my trustydomes providing an exit path for stale air. moisture content, water-vapour permeancesling psychrometer (see sidebar) at each floorWith the OPEC oil crisis of the 1970s, a well-and sorption isothermsare measured andopen to the rotunda and in the interstitial spacemeaning attempt to reduce energy consump-characterized at various levels of RH. above the ceiling and below the great coppertion led to the closing of these ventilation dome that characterizes the building. Figure 4paths and the introduction of mechanical All of that said, RH is not particularly usefulillustrates the results of these measurements,heating, cooling and ventilation. The solution when we are assessing a building andplotted against the cross-section of the buildingwas to partially vent the dome space so the diagnosing problems.Thats because relativeto provide a sense of scale. humid air could escape, without drastically humidity is, well, relative. One might beMy point is that, had I relied on RH measure- increasing the heating and cooling loads.concerned about RH levels above 80 per cent ifments, I would not have been able to diagnoseOne more example of using absolute the air temperature is above 5C as a necessarythe problem.As Figure 4 shows, the RHhumidity and psychrometric measurement condition to support mould growth, but even(represented by the dotted line and the valuesin a diagnostic case, this time in a residential 90 per cent RH at lower temperatures is noton the right-hand y-axis) decreases the highersetting. Figure 5 shows the underside of necessarily a big deal. 40 per cent RH might beyou go in the building because stratification ina gabled attic space in a multi-residential a concern at an air temperature above 25C. the space causes the air temperature to increasebuilding in a rowhouse configuration. The The main problem with using RH is that itwith height. On the other hand, absolutestaining indicates a field-effect, moisture-could misdirect a diagnostician because RH is ashumidity (represented by the solid line and therelated problemin other words, its not sensitive to air temperature as it is to the actualvalues on the left-hand y-axis) increases. As thecaused by a specific leak or a localized source humidity level. On the other hand, the absolutewarm air rises, displaced by denser cooler airof water, but by large-scale humidity migra-humidity, or the humidity ratio (i.e., the verticalinfiltrating at ground level due to stack effect,tion. Clearly there is a problem with venting axis of the psychrometric chart) is almost directlyit carries humidity into the interstitial spacemoisture at the eaves, but the unusual aspect correlated to water vapour pressure and canbetween the rotunda ceiling and the dome.in this case was that the staining was only provide a good indicator of problem areas. Overnight, this large surface with high solarevident in this suite. The other units in the For example, I was called in to diagnose aemittance experiences radiant heat loss to therowhouse complex were in reasonably good problem with a large assembly building, wheresky, especially on clear nights. The resultingcondition for their age. the plaster ceiling of a rotunda was flaking andsurface cooling causes water vapour in theGenerally, this type of staining is either a case pieces of plaster were in danger of falling fiveinterstitial space to condense on the undersideof not enough venting for the moisture load storeys to the floor below (Figure 3a). Oneof the copper dome. This condensate thenor too much moisture load for the venting. In might expect that this would create a liabilityrains in the interstitial space, creating waterthat situation, you would start by calculating problem for a public building, so the ownersstains (Figure 3b) and softening the plaster. the code requirement for venting to see wanted a diagnosis of the problem and recom- Further investigation revealed that the originalwhere things stood. In this case, we knew that mendations for solutions. design of this building incorporated passivethere was something different about this unit FIGURE 4: PSYCHROMETRIC MEASUREMENT WITHIN THE ROTUNDA SPACE; DOTTED LINE REPRESENTS RH, SOLID LINE REPRESENTS ABSOLUTE HUMIDITY RATIO16 BCBEC ELEMENTSA BCBEC PUBLICATION'