Compare the whole window
Glazing, frame, spacer and installation junction transfer heat differently. Ug refers to the glazing; Uw refers to a window with a defined geometry. Neither alone describes the installed junction. Compare quotations using the same dimensions, opening pattern and components. A small reference window and a large unit with a different frame proportion can have different overall performance even when the profile name is identical.
Solar gains need a seasonal view
Visible-light transmission and solar-energy transmission are different properties. Orientation, surrounding obstructions and shading change gains through the day and year. South-facing glazing may contribute useful winter gains while still requiring a summer assessment. Bedrooms also need consideration of blackout, noise and any safe opening arrangement included in the intended ventilation strategy.
The installation joint has several jobs
Supports carry weight, fixings restrain the unit, seals control air and water, and insulation limits heat transfer. Expanding foam cannot replace all these functions. Coordinate frame position with façade insulation, sills and thresholds. Drainage needs an actual route outside, while the internal airtight layer needs a continuous connection to the wall. These details should remain accessible for inspection before finishes cover them.
Check before ordering and concealment
Compare opening drawings with finished floor levels, reveals and shading devices. The quotation should identify performance for the ordered configuration and the included installation work. Inspect supports, connections and drainage before covering the joint, then check sash and hardware operation. If competing offers differ only by profile brand, request specifications for the complete window and its installation detail; those are the items that can be compared against the building's needs.