| Whole-Window U-Factor | Btu/h·ft²·°F or W/m²·K | The rate of heat transfer through the complete window, including the frame, glazing, and edge area. | Lower is better. Consider approximately 0.22–0.30 Btu/h·ft²·°F, or about 1.25–1.70 W/m²·K, for a high-performance aluminum window. | A lower U-factor generally reduces winter heat loss and heating demand. Always compare whole-window values rather than center-of-glass values alone. |
| Solar Heat Gain Coefficient | SHGC: 0–1 | The fraction of incident solar radiation admitted through the window as heat. | Choose by climate and orientation: approximately 0.25–0.40 for hot or cooling-dominated locations; approximately 0.35–0.55 may be useful in heating-dominated locations with beneficial winter sun. | Lower SHGC can reduce summer overheating and air-conditioning loads. A higher SHGC can admit more useful solar heat in cold climates. |
| Visible Transmittance | VT: 0–1 | The percentage of visible daylight transmitted through the glazing. | Approximately 0.45–0.70 for a balanced combination of daylight, solar control, and energy performance. | Higher VT improves daylight and may reduce electric-lighting demand, but heavily coated glass can reduce visible light. |
| Air Leakage | Usually reported as cfm/ft² at a specified pressure | The amount of air that passes through joints and seals in the window assembly. | Lower is better. Prefer a tested value at or below 0.20 cfm/ft² where available, subject to the applicable test standard and product type. | Lower air leakage improves comfort, reduces drafts, and limits uncontrolled heating or cooling losses. |
| Condensation Resistance | CR: 1–100 | A comparative measure of how well a window resists interior surface condensation under test conditions. | Higher is better. Select the highest practical rating, especially in cold or humid climates. | A higher CR value indicates warmer interior surfaces and a lower likelihood of condensation, although indoor humidity and installation also affect results. |
| Aluminum Frame Design | Thermally broken frame | An insulating separator reduces direct heat flow through the normally conductive aluminum frame. | Use a continuous, well-designed thermal break with insulated chambers where possible. | Thermal breaks reduce frame U-factor, interior surface temperature differences, condensation risk, and thermal bridging. |
| Glazing Configuration | Double or triple glazing | The number of glass panes and sealed insulating spaces in the unit. | Double glazing is often adequate in moderate climates; triple glazing is preferable where winter temperatures are severe or maximum comfort is required. | Additional panes generally improve insulation and sound reduction, but increase weight, thickness, and cost. |
| Low-E Coating | High-solar-gain, moderate-solar-gain, or low-solar-gain | A microscopically thin coating that reduces infrared heat transfer and can control solar radiation. | Match the coating to climate, façade direction, shading, and room use. | Low-E glass can improve insulation while controlling unwanted solar heat, but the wrong SHGC can increase heating or cooling demand. |
| Insulating Gas Fill | Argon or krypton | Inert gas placed between sealed glass panes to reduce convective and conductive heat transfer. | Argon is a practical choice for many double-glazed units; krypton can be useful for narrower gaps or high-performance triple glazing. | Gas fill can lower center-of-glass heat transfer, but the sealed unit, spacer, and installation quality determine long-term performance. |
| Warm-Edge Spacer | Low-conductivity spacer | The material separating the panes around the perimeter of an insulating glass unit. | Prefer a thermally improved spacer instead of a highly conductive metal spacer where condensation resistance is important. | A warmer edge can reduce perimeter heat loss and improve resistance to condensation near the glass frame. |
| Glass Orientation and Shading | North, south, east, or west exposure; exterior shading | The amount and timing of solar exposure received by the window. | Use lower SHGC and effective external shading for strongly exposed façades in hot climates. | Orientation can affect solar heat gain more than small differences between otherwise similar window products. |
| Window Operation | Fixed, casement, awning, sliding, or double-hung | The opening mechanism and the type of seals used around movable sections. | Prioritize low air leakage. Hinged units often provide a strong compression seal when correctly manufactured and installed. | Operation affects sealing, ventilation, cleaning, emergency egress, maintenance, and final energy performance. |
| Installation Quality | Continuous perimeter air and water sealing | How effectively the installed window is integrated with the wall, flashing, insulation, and interior finish. | Require a documented installation method with properly sealed joints, drainage paths, and compatible flashing materials. | Poor installation can create drafts, moisture intrusion, and thermal bridges even when the window itself has excellent laboratory ratings. |
| Certification and Label Information | Independent test and certification data | Verified performance values for the complete tested window assembly. | Compare independently verified whole-window ratings for the exact size, configuration, glazing, and operating style being purchased. | Ratings can vary by size and configuration. A verified label makes comparisons more reliable than marketing descriptions alone. |