b'FIGURE 7: METAL SILL PAN BYPASSES THERMAL BREAK FIGURE 8: REVISED MODEL MATCHES ON-SITE CONDITIONSThis measure was predicted in theSOMETIMES WINDOWbe installed at a place in the room where it modelling to shift the surface tempera- CONDENSATION IS NOTcan achieve even that flow rate.ture of the window frame from 12 C toABOUT THE WINDOWSFlexible ductwork that is installed with approximately 17.5 C, which was almost exactly observed when the work wasWe returned to the site and, sure enough,many twists and turns, so that even if the fan completed, much to the developersthere was condensate on the windows we hadwere of sufficient capacity, it wouldnt have amazement (oh, ye of little faith, where- remediated. The window surface temperaturea hope of overcoming the resistance in the fore didst thou doubt?). Incidentally,still measured a reasonably good value, so I usedductwork. We prefer hard ducting, sloped along the way I noted one of mymy trusty sling psychrometer to measure thetoward the exterior to drain condensate, personal pet peeves about electroniccondition of the room air. As Table 1 shows, thewith joints and seams taped (and NOT with equipment, as the infra-red thermom- humidity levels continued to drop the longer Iduct tape!).eter I was using kept reading lower andwas there, so I surmised that the humidity had Size of the residential unitresidential lower values as the work progressed,been even higher before I arrived. Evidently, the when in fact the work was intended toproblem was not the windows, but the ventila- spaces are trending smaller, leaving less of increase the temperature. Eventuallytion system that was not removing humiditya buffer between the source of the moisture the temperature readings were lowerbefore it could condense on the windows. and the windows. So, the lag time between than the outdoor temperature, which ishigh levels of moisture in bathrooms or not in keeping with the laws of thermo- This can have a variety of causes, including: kitchens and the formation of condensation dynamics. So while the contractor wasis shorter, and condensation shows up onBathroom exhaust fans that arent operated working on the window assembly, I wentwindows sooner.for a walk to get a new batteryand,long enough to clear the moisture generated by long showers or bathsthe fan should be magically, the laws of thermodynamicson before the shower starts to establish theEXTERIOR CONDENSATIONstarted behaving again. necessary air flow and should run afterwardAnother phenomenon that will become quite So the window frame was operating atfor long enough to make a difference (itcommon is that condensation will show up about 17.5 C, which is reasonably goodshould be controlled by a humidistat). on the outside of high-performance windows. performance for a winter day in Victoria, Bathroom exhaust fans that dont have theThe first time I saw this was in 1994, when I was asked to provide a legal opinion on the but we were called back to the site whencapacity to remove enough moist air quicklymatter. A homeowner in Alberta had purchased the same windows started exhibitingenoughthe Contractor Special is typi- windows that were advertised as being immune condensation again.cally rated for 50 cfm (23 l/s) and might notto condensation, and they found that these windows (triple-glazed with two low-e coating, Table 1: Measured Interior Conditions argon-gas fill, warm-edge spacers and wood frames) were exhibiting condensation on the TIME OFROOM AIRRELATIVEDEW-POINTexterior surface of the glass, similar to the photo in Figure 11 (pg. 20).MEASUREMENT TEMPERATURE HUMIDITY TEMPERATUREI explained to my client (the window supplier) 09:35 23C 73% 17.8C that the windows were so well-insulated that the exterior surface of the glass was very close to the outdoor ambient temperature. Radiant heat 10:20 22C 64% 15.2C transfer from the window to a clear sky would drop the surface temperature by 3-5 C, which 10:40 21.5C 61% 13.5C could be below the dewpoint temperature of the outdoor air, resulting in condensation.SPRING/SUMMER 2021 19'