AI-generated cabin concept, used for illustration.
In this guide
Window ratings describe different things. U-factor concerns heat transfer through the window; solar heat gain describes the share of incoming solar energy admitted. Treat them as separate design inputs.
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A lower U-factor indicates lower heat transfer under the rating conditions. A lower solar heat gain coefficient indicates less admitted solar heat. That does not make the lowest number in every column the automatic best choice for every cabin.
Your Home: glazing explains these performance concepts. Be careful with international quotes: U-values may use different units, and glass-only figures should not be compared casually with whole-window ratings.
Ask the supplier to identify the rating system, units and exact product configuration. A brochure's headline performance may not describe the large custom unit in your quote.
Put the window back into the building
Discuss the glass area, orientation, external shading and local climate with the designer. The same product can have a different effect in a shaded bedroom and an exposed window wall.
Use a comparison sheet with one row per quoted opening. Record the actual dimensions, frame, glazing specification and applicable ratings. Leave missing information blank and request it rather than filling it from another product's page.
The decision should explain why the selected performance suits this opening. A generic “energy efficient” description is less helpful than a coordinated window schedule that the supplier and designer have both checked.