Windows Are the Most Thermally Vulnerable Element in the Building Envelope - Selection Decisions Have Major Energy Implications
Windows are the weakest point in the building envelope's thermal resistance. The best walls achieve R-30 to R-50 with continuous insulation; code-minimum windows typically achieve R-4 to R-5 (U-0.25). This 6-10x difference in thermal resistance means that window area has a disproportionate impact on building energy performance and heating/cooling loads. Window type selection - operational type, frame material, glazing configuration, and performance specifications - is the architect's decision, made during design development and documented in the window schedule as part of the construction documents. The ARE PPD tests window performance metrics and the ARE PDD tests how windows are specified and documented in construction documents.
Operational Window Types
Fixed: No operable elements; excellent weather tightness and thermal performance because there are no operable seals that can wear. Used where ventilation is not needed or is provided mechanically. Casement: Hinged on the side; opens with a crank; provides excellent natural ventilation (full opening area is available); weathers well when latched; most common operable type in residential and institutional buildings. Double-hung: Two sashes, both operable vertically; lower half ventilation from inside; upper half ventilation from outside; traditional residential type; can be cleaned from inside (tilt-in feature). Awning: Hinged at top, opens outward at bottom; can be left open during rain (water runs off the tilted glass); used for high windows and basement windows. Hopper: Hinged at bottom, opens inward at top; common for basement windows and commercial applications. Sliding: One or two sashes slide horizontally; only half the opening area is available for ventilation at any time; lower thermal performance than casement because sliding sashes have less airtight seals.
Glazing Performance Metrics
U-value: Thermal transmittance (Btu/hr·ft²·°F); lower = better insulation. Single pane: U ≈ 1.0; double pane: U ≈ 0.30-0.50; double pane with Low-E coating and argon fill: U ≈ 0.25-0.35; triple pane: U ≈ 0.10-0.20. ASHRAE 90.1 sets maximum U-values by climate zone; the architect's window spec must meet or exceed the minimum. Solar Heat Gain Coefficient (SHGC): Fraction of solar radiation that enters the building as heat; lower = less solar heat gain; important for cooling-dominated climates. Visible Transmittance (VT): Fraction of visible light that passes through; higher = more daylight. There is typically a tradeoff between low SHGC (tinted or reflective glass) and high VT; high-performance Low-E coatings can achieve both better than tinted glass.
Key Exam Points
- Fixed windows: best thermal and weather performance; no ventilation.
- Casement: excellent ventilation and weather tightness; standard institutional operable type.
- U-value: lower = better; triple pane U ≈ 0.10–0.20; double pane standard U ≈ 0.30–0.50.
- SHGC: lower = less solar heat; important for hot climates and east/west exposures.
- VT: higher = more daylight; Low-E coatings allow high VT with low SHGC (better than tinted glass).
AREprep's PPD and PDD building envelope content covers window types, glazing performance metrics, ASHRAE 90.1 fenestration requirements, and window specification and scheduling in construction documents - giving ARE candidates the window performance and documentation knowledge the exams test across both the sustainable design performance and construction documents production content areas.
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