| 1. Compare whole-window U-factor | Check the certified U-factor for the complete window, including frame, glazing, and spacers—not just the glass center. | Lower U-factor means less heat transfer. Compare products using the same rating method and units; in U.S. ratings, U-factor is commonly reported in Btu/(h·ft²·°F). | A thermal break reduces heat flow through the aluminum frame, but the whole-window rating gives a more useful picture of overall insulation. |
| 2. Match solar heat gain to your climate | Review the Solar Heat Gain Coefficient (SHGC), a value between 0 and 1 indicating how much solar heat enters through the window. | In cooling-dominated, sunny locations, a lower SHGC can help limit unwanted solar heat. In heating-dominated locations, a higher SHGC may be useful where winter sun is desirable. Consider orientation and exterior shading. | The right balance can reduce cooling or heating demand; one SHGC target is not suitable for every climate or façade. |
| 3. Specify glazing for the local weather | Ask about double- or triple-pane glazing, low-emissivity coatings, gas fill, and warm-edge spacers. | Double glazing is a common starting point; triple glazing may be worth considering in very cold climates or where improved interior comfort is needed. Confirm the complete assembly’s tested performance. | Glazing choices affect heat loss, solar gain, condensation risk, and comfort near the window. |
| 4. Check air leakage and water performance | Request tested air-leakage and water-penetration results for the window system, along with installation requirements. | Lower air leakage generally means fewer drafts. Compare ratings tested to the same standard and pressure conditions; performance depends on product size, configuration, and installation. | Even an insulated frame cannot compensate for leaky joints, poor seals, or incorrect installation. |
| 5. Consider condensation and frame details | Inspect the thermal-break design, frame-to-glass edge, drainage paths, seals, and available condensation-resistance information. | For cold or humid conditions, ask how the system manages interior surface temperatures and moisture. Maintain indoor humidity appropriate to outdoor temperatures. | Condensation depends on surface temperature, indoor humidity, and ventilation—not on the frame material alone. |
| 6. Choose acoustic performance for the noise source | Look for tested Sound Transmission Class (STC) and, where traffic or other low-frequency noise is important, Outdoor-Indoor Transmission Class (OITC) ratings. | Higher ratings indicate greater sound reduction under the applicable test method. Asymmetric glass thicknesses, laminated glass, and well-sealed frames may improve sound control; request ratings for the exact configuration. | Noise reduction depends on the whole window and installation. STC and OITC are not interchangeable, and neither guarantees a specific indoor sound level. |
| 7. Verify labels, sizing, and installation | Check independent performance documentation, the exact glazing and frame configuration, hardware, warranties, and installation instructions. | Use comparable certified ratings where available, such as NFRC ratings in the United States. Confirm opening dimensions, structural requirements, flashing, air sealing, and compatibility with the wall assembly. | Rated performance applies to a specified product configuration; careful installation is essential to achieving it in the building. |