Two quotes can both say “double-pane low-E window” and still describe units with different energy performance. Ask each supplier for the label or rating sheet for the exact window being priced. Start with two figures that answer different questions about how the completed unit handles heat.
Read the U-factor first
U-factor measures heat transfer through the window assembly. A lower figure means less heat passes through under the rating conditions. A quote for a window with a U-factor of 0.25 therefore describes less heat transfer than one rated 0.30. Use whole-window figures for both products. Comparing a glass-only figure with a rating that includes the frame gives a misleading impression.
Then compare solar heat gain
Solar heat gain coefficient, usually shortened to SHGC, describes the fraction of incident solar energy that enters as heat. A lower value admits less solar heat. That may be desirable on an exposed elevation, while winter sun may be useful elsewhere. Ask the designer to explain the choice for your climate, shading and the direction the opening faces.
Put the model, operating style, U-factor, SHGC and price on the same line of each quote. Check the applicable climate zone using the ENERGY STAR window selection guide. If a supplier substitutes a different unit, request its figures before approving the change. This gives the broader goals in your sustainable home design a measurable place in the purchase decision.
