24-Bld-A7 Building Envelope Design · May 2017

Question 5 of 6

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Reference texts: Straube & Burnett, Building Science for Building Enclosures; ASHRAE Handbook — Fundamentals (Ch. 25 Thermal and Water Vapor Transmission Data, Ch. 26 Heat, Air, and Moisture Control in Building Assemblies); National Building Code of Canada (NBCC), Part 5 (Environmental Separation); ASTM C1472, Standard Guide for Calculating Movement and Other Effects When Establishing Sealant Joint Width; CMHC Best Practice Guides for Building Envelopes (brick veneer, shelf angles, movement joints). This is a closed-book paper; the exam instructs that only the first five questions as they appear in the answer book are marked, but all six questions are answered in full below as a complete study resource.

Question 5 (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given (Part A). Temperature index $I=50$ (a fixed property of the window under standardized test conditions $T_i=21°C$, $T_o=-18°C$). Actual service conditions: indoor $21°C$, 40% RH; outdoor Mean January temperature $-12°C$.

Find. The glass surface temperature under actual conditions, whether it falls below the indoor air's dew point, and a fix if condensation is predicted.

Approach. The temperature index is a normalized, geometry/construction-dependent property of the window and is taken as constant regardless of the actual boundary temperatures; use it (Eq. in the Appendix) to rescale the glass surface temperature to the new $T_i,T_o$, then compare to the dew point of the actual indoor air.

Rescale the glass surface temperature to actual conditions. Rearranging $I=\dfrac{T_s-T_o}{T_i-T_o}\times100$ for $T_s$ and substituting the actual $T_i=21°C$, $T_o=-12°C$: $$T_s = T_o + \frac{I}{100}(T_i-T_o) = -12+0.50(21-(-12)) = -12+16.5 = \boxed{4.5\,°C}$$
Dew point of the indoor air. Actual vapour pressure $p=0.40\times p_{sat}(21°C)=0.40\times2481\approx992\ \text{Pa}$. Solving $p_{sat}(T_{dp})=992\ \text{Pa}$ (Magnus approximation, matched to the Appendix table) gives $\boxed{T_{dp}\approx6.9\,°C}$.
Compare. $T_s(4.5°C) < T_{dp}(6.9°C)$, so the glass surface sits below the dew point of the room air — condensation will occur on the window surface at the Toronto Mean January design temperature.
Solutions. Either raise $T_s$ by specifying a window with a higher temperature index (warm-edge spacer, better frame thermal break, an additional pane or low-e/gas-fill upgrade, or a storm window) so that $T_s \ge T_{dp}$ at design conditions, or lower the indoor dew point by reducing indoor relative humidity (ventilation, dehumidification, reduced moisture generation) so that $T_{dp} \le T_s$ — in practice, a combination of a better-performing window and RH control below about 30–35% in cold weather is the standard remedy.

Quantity	Result
Glass surface temperature at Mean January temp.	4.5°C
Dew point of indoor air (21°C, 40% RH)	≈ 6.9°C
Condensation?	Yes (T_s < T_dp)

Part B. With the exterior air-barrier approach, the sealed exterior sheathing membrane itself is the air barrier — it must be lapped and sealed continuously to the window frame's nailing flange all the way around, including at the head, jambs and sill. The four control layers highlighted in Fig. 4:

Fig. 4 — Window/wall connection detail with the four control layers colour-traced for continuity: air barrier (red), vapour barrier (blue), water-resistive barrier (green), rainwater-shedding path (orange).

Air barrier (red) — the sealed exterior sheathing membrane, taped/sealed at the sheathing joints and lapped over the window's nailing flange with a compatible sealant or air-seal tape at head, jambs and sill; components: sheathing membrane, seam tape, window-flange sealant bead, backer rod/sealant at the rough-opening perimeter. Vapour barrier (blue) — on the warm side of the insulation (interior gypsum/poly or the window frame's own interior air-seal), continuous around the rough-opening reveal so interior humid air cannot reach the cold sheathing. Water-resistive barrier (green) — the same sheathing membrane functioning as WRB where it laps over the sill flashing (shingle-lap, so water on the WRB sheds onto the sill pan, never behind it); components: WRB sheet, sill pan flashing, head flashing lapped under the WRB above the window. Rainwater-shedding path (orange) — the exterior cladding surface and the pre-shaped sloped sill flashing, which collects any water that reaches the window perimeter and directs it back out over the cladding face via weep openings, never into the wall cavity.

How the window manages rainwater. The sloped, pre-formed metal sill flashing sits under the window frame and is turned up at the jambs (end dams) so any water that gets past the exterior sealant at the sill is caught and drained back out over the face of the wall below, rather than running down into the wall cavity; the WRB is lapped over the top surface of this sill flashing (never tucked under it) so that water draining down the WRB also lands on top of the flashing and is shed outward; and the head flashing above the window is lapped under the WRB (and over the window's head flange) so that water running down the wall face is shed over, not behind, the window head — the whole assembly works by consistent shingle-lapping (each layer lapping over the one below, upstream over downstream) rather than by any single sealant joint being relied on to keep water out.

Topic: Window condensation resistance (temperature index) and window/wall interface control-layer continuity.

Key relations: temperature index $I=(T_s-T_o)/(T_i-T_o)\times100$ (treated as a fixed window property, rescaled to actual service conditions); condensation criterion $T_s

Why this works: the temperature index isolates the window's own thermal performance from the specific test boundary conditions, so it can be legitimately reapplied at any other pair of indoor/outdoor temperatures to predict real-world condensation risk; window leakage in practice is overwhelmingly a lapping/sequencing error (WRB tucked under instead of over the sill flashing) rather than a sealant-quality failure.

Common pitfall: assuming a single higher-quality sealant bead can substitute for correct shingle-lap sequencing at the sill — without the lap order, any pinhole or joint failure sends water directly into the wall cavity with no drainage path back out.

Source: ASHRAE Fundamentals Ch. 15 (fenestration) & Ch. 26; Straube & Burnett, Building Science for Building Enclosures, Ch. 6; CSA A440/NAFS window standard (temperature index / condensation resistance rating).

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