b'BACK TO BASICS BACK TO BASICS: WINDOW CONDENSATION By Alex McGowan, WSPI first along the sightline or just above it,A dead-air pocket partially insulates the am occasionally called upon to explain arcane building science fundamentalsdepending on the source. We expect conden- inside corner, preventing the room air from to technical people, so I thought Isation to show up first on window surfaces,heating this part of the window.might be able to save some time byas these assemblies exhibit much higher heatwriting it down in Elements magazine.loss than the rest of the building enclosure;4. Conduction through the spacer assembly My previous article (published inthe thermal resistance of a window is typically(and the window frame, if it is not an Spring/Summer 2020) talked about psychro- around R3 (RSI 0.5) versus R12-15 (RSI 2-2.6)insulating frame) reduces the surface metrics, and this time we will apply thosefor a wall. But why is there condensation attemperature in this region of the window.concepts to a common practical problem:that specific location?Four main mechanisms window condensation. are at work here (Figure 2): CASE STUDIESMECHANISMS BEHIND1. Convective flow inside the glazing unit Condensation can also occur on the frame, CONDENSATIONwhich is usually an indication of a highly under winter conditions, the outer glazingconductive frame or an installation problem. From the discussion about psychrometrics,cools the gas inside the IGU, whichFigure 3 (pg. 18) shows a case where the we know that condensation occurs whencreates convective flow that brings colderdesigner wanted to be sure that the area the temperature of a surface is below thetemperatures to the inner glazing at thebelow the windowsill was fully drained; dewpoint temperature of the air in thesightline of the sill. unfortunately, this meant that outside room. Simple enoughbut why does this2. Radiant heat transfer from the roomair could bypass the thermal break of the happen, and what can we do about it? Well,surfaces to the windowthis exchangewindow frame, which would have created the dewpoint temperature of the air in theis limited at the inside corner near thesignificant condensation problems. This room depends on dry-bulb temperaturesightline, because the window sees itselfwindow would have the condensation and humidity level, so if the room is warmas much as it sees the room. performance of a non-thermal metal window and humid, the dewpoint temperature willframe, and similar U-value performance. be higher than if the room were colder3.Convective flow by induced draft onFortunately, this was caught at the design and drier. The surface temperature of thethe room side of the windowas withstage, so the remedy didnt cost anything.window depends on the thermal perfor- Mechanism 1, the relatively cold glass of mance of the window frame and glazing, andthe interior window surface cools the roomFigure 4 (pg. 18) shows another case that to a certain extent the installation method. air next to it, creating air movement duehad already been installed when we were to buoyancy. But, as with Mechanism 2,called in. The windows were exhibiting Figure 1 shows a classic condensation patternthis convective flow is limited at the insidecondensation, but it wasnt clear why. The on a window glazing. This typically appearscorner near the sightline of the window.product had a reasonably good thermal FIGURE 1: TYPICAL U-SHAPED CONDENSATION PATTERN FIGURE 2: FOUR MECHANISMS CAUSING CONDENSATIONSPRING/SUMMER 2021 17'