High-performance joinery (Uw, Sw)

High-performance windows combine a low Uw value (thermal transmittance of the entire window, frame + glazing, in W/m²·K) and a Sw value (solar factor) adapted to the orientation and intended use. Simply put: a low Uw value means minimal heat loss, while a well-chosen Sw value ensures good solar gain in winter without overheating in summer. In addition to these two indicators, factors such as TL (light transmission), the Ψ (psi) value of the glazing/spacer (warm edge), air permeability (AEV rating), and installation quality (thermal breaks and window frame treatment) often make a significant difference in comfort and energy efficiency.

High-performance joinery: operational definition

Uw (window):

The lower the value, the better (e.g., 1.3 → acceptable; ≤ 1.0 → very efficient; triple glazing down to ~0.8). It depends on the frame (wood/aluminum/PVC/mixed), the glazing (double or triple low-emissivity, argon/crypton gas), the spacer (warm edge spacer), and the dimensions.

Sw (solar factor):

Percentage of solar energy transmitted to the interior (0 to 1). High solar heat gain benefits the south-facing facade in winter; moderate/low solar heat gain limits overheating on the west-facing facade (and sometimes on the south-facing facade depending on external shading). This should be compared with natural light (TL).

Other parameters:

Air tightness (seals, hardware), thermal bridges in the frame (insulation strips, flashings), acoustics (laminated/asymmetric glazing), security (laminated), external solar protection (BSO, blinds).

In RE2020, a good Uw + Sw combination improves Bbio and DH (summer comfort). During renovations, careful attention must be paid to installation: continuity of insulation, sealing, supports and thresholds to prevent air leaks and cold spots.

Advantages, limitations and points to consider regarding high-performance joinery

Interests

• Less heat loss (low Uw) → reduced heating and “warmer” walls in winter.
• Summer comfort is better maintained by combining suitable SW + external protections.
• Visual comfort & IAQ: Sufficient light transmission, limited air currents, less condensation.
• Improved acoustics with laminated/asymmetrical glazing.
• Enhanced overall heritage value and watertightness.

Boundaries

• Higher cost (high-performance frames, triple glazing, hardware).
• Weight & size (triple glazing) → installation constraints/hardware.
• Sw too low = less winter sun (heating needs ↑ if poorly chosen).
• Risk of disappointment if the installation is not up to standard (thermal bridges, leaks).

Points to consider

• Orientations: South (rather high SW with external BSO), West (moderate/low SW), North (priority low Uw, TL).
• Frame choice: mixed wood/aluminum (durability + break), high-performance PVC, aluminum with serious thermal break.
• Glazing: low emissivity, suitable gas, warm edge, differentiated thicknesses if acoustic.
• Installation: continuous insulation, flashing, sealing membranes, compliant fixings, rain flaps.
• External protections: BSO/controlled blinds (BMS) on West/South; interior blinds useful for glare but not much against heat.
• Maintenance: adjusting hardware, seals, water drainage.

Anecdote — “Glass windows that have calmed the Corniche in Marseille”

In Marseille, west-facing glass-fronted offices were becoming unbearably hot in the afternoons. The targeted replacement combined aluminum frames with thermal break, low-emissivity double glazing with a warm edge spacer, a Uw value of approximately 1.2, and a lower Sw value on the west side, complemented by external adjustable sunshades controlled by the building management system. The result the following summer: lower temperature peaks, significantly reduced glare, and less heating in winter thanks to the improved Uw value on the other facades. The site manager summed it up: "We kept the sea view, but not the heat."