Solar-Control Glass Versus External Shading in a South Living Room
A south-facing living room in a 150 m² passive-house dwelling has 9 m² of triple glazing with U_g = 0.60 W/(m²K), U_f = 0.95 W/(m²K), g = 0.52, and an installed Ψ_install of 0.03 W/(mK). The room is 32 m² and has no external shading. PHPP shows heating demand 13.8 kWh/(m²a) but summer overheating at 12% of hours above 25°C, above the 10% criterion. The client dislikes external blinds and asks for solar-control glass with g = 0.28 instead. The window supplier says the lower-g coating adds 8% to glazing cost and will reduce winter solar gains. The designer must keep the project within Classic certification and avoid increasing the heating demand above 15 kWh/(m²a). The climate has 1,100 kWh/(m²a) incident solar on the south facade and 70 kKh heating degree hours.
1. Quantify the winter solar-gain penalty of changing from g = 0.52 to g = 0.28, and explain why this is risky for the heating demand.
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The glazing area is 9 m² and incident solar on the south facade is 1,100 kWh/(m²a), so incident energy is 9 × 1,100 = 9,900 kWh/a. Reducing g from 0.52 to 0.28 reduces transmitted solar by 0.24 × 9,900 = 2,376 kWh/a. Not all of that is useful; allowing 50–70% useful fraction gives roughly 1,190–1,660 kWh/a. Spread over 150 m² TFA, that is about 8–11 kWh/(m²a). The current heating demand is 13.8 kWh/(m²a), so even the lower end of that range would likely push the project above the 15 kWh/(m²a) limit. The exact PHPP result depends on utilization factor and internal gains, but the margin is only 1.2 kWh/(m²a), so a change of this size is not a safe trade. The lower-g coating is therefore not a harmless summer fix; it attacks the winter balance that the Passive House design depends on.
2. Recommend a shading strategy that satisfies the summer criterion without sacrificing winter gains.
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Keep the higher-solar-gain glazing and add external movable shading, preferably Venetian blinds or a retractable fabric blind with a verified reduction of about 75–80% for the relevant summer angles. External shading is more effective than internal blinds because it intercepts radiation before it enters the glazing. The control strategy matters: the device should close automatically when solar gains and room temperature rise, and it must be modelled with a realistic, not always-closed, shading factor. A fixed overhang can help at midday but is less effective at low morning and evening sun angles, so it should be combined with movable shading if used. If the PHPP still exceeds 10%, reduce the glazing area slightly, improve night ventilation or adjust the shading factor, rather than jumping to g = 0.28. The certifier will want the summer model to use the actual device, control and operation assumption, and the design team must commission and label the controls so the as-built performance matches the model.
3. The window is to be installed with Ψ_install = 0.03 W/(mK). What checks should the designer require at the interface and at commissioning?
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A Ψ_install of 0.03 W/(mK) is only credible if the frame is set into the insulation layer and the installation detail is built as drawn. The designer should require a junction detail showing the window frame, the airtight tape or membrane, the sill, the reveal insulation and the weather seal, and a site check before the reveals are closed. The frame U-value of 0.95 W/(m²K) and the spacer must match the certified component data; substituting a cheaper frame or spacer invalidates the PHPP entry. The shading device should be commissioned with its sensor and manual override, and the occupant should receive simple instructions, because a blind that is never used cannot deliver the summer performance. Finally, check that the lower-g glazing was not substituted without a revised PHPP: a glass change of that magnitude affects both heating demand and overheating, and the certificate must reflect the installed product.