Windows, doors and solar control: glazing, frames, installation and shading
Window performance and installation
Certification requires a window U-value U_w ≤ 0.80 W/(m²K) and, critically, an installed U_wd ≤ 0.85 W/(m²K) including the installation thermal bridge. Good units combine triple glazing (U_g ≈ 0.5–0.6 W/(m²K)), insulated frames (U_f ≈ 0.7–0.9), and a warm-edge spacer with Ψ_g ≈ 0.03–0.04 W/(mK). Frame fraction and glazing perimeter matter as much as the centre-of-glass value. Install windows within the insulation layer, not at the structural reveal.
Solar control and shading
The g-value governs useful winter gains and unwanted summer gains. In cool climates, larger south glazing with high g and good U-values is beneficial; east and west glazing overheat. External shading, reveal depth and the shading factor must be entered in PHPP; internal blinds are far less effective.
Worked example: calculating U_w
A window is 1.2 m × 1.5 m (1.8 m²). Frame width 0.15 m gives glass 0.9 m × 1.2 m = 1.08 m², frame 0.72 m², glazing perimeter 2(0.9 + 1.2) = 4.2 m. With U_g = 0.55, U_f = 0.80, Ψ_g = 0.04: U_w = (1.08 × 0.55 + 0.72 × 0.80 + 4.2 × 0.04) / 1.8 = (0.594 + 0.576 + 0.168) / 1.8 = 1.338 / 1.8 = 0.74 W/(m²K) - it passes. Installation in an insulated reveal with Ψ_install ≈ 0.02 W/(mK) around 5.4 m perimeter adds about 0.02 × 5.4 / 1.8 = 0.06 W/(m²K), giving 0.80 - only just inside 0.85.
Traps
Quoting the manufacturer's U_w for a different size. Forgetting the spacer and installation Ψ. Treating the g-value as fixed when it varies with incidence angle and shading. Assuming internal blinds satisfy summer criteria.