U-Values Explained for Secondary Glazing
A U-value is the rate of heat flow through a square metre of construction for each degree of temperature difference. Lower is better. The trap is that the same window can be quoted three different ways, and the flattering one is usually the one you are shown.
Centre-pane, whole-window and installed
Centre-pane ignores the frame and the edge effects entirely, so it always reads best. Whole-window includes the frame and the perimeter, which are worse than the glass, so the number rises. Installed performance also picks up the reveal and any air leakage. When comparing quotes, make sure you are comparing the same one — and ask which it is.
- Centre-pane: glass only, most flattering
- Whole-window: includes frame and edge
- Installed: the number your heating bill responds to
How secondary glazing gets to 1.868
A single-glazed sash sits around 5.4 W/m²K. Add a sealed secondary panel and the trapped air layer alone takes it to roughly 2.6–2.8. Specify soft-coat Low-E on the secondary pane and the radiant component collapses, bringing the assembly to about 1.868 W/m²K — comfortably inside the retrofit target for existing dwellings and better than a great many 1990s sealed units still in service.
Where the rest of your heat is going
Windows are usually 10–20% of a period house's fabric loss. Loft insulation, suspended-floor draughts and an uninsulated solid wall are typically bigger. We will tell you where your window sits in that hierarchy rather than pretend glazing alone will transform your bills.
Frequently asked questions
For a retrofit to an existing dwelling, the relevant Part L target for replacement glazing is met by a Low-E secondary specification. Where the original window is protected, secondary glazing is generally the accepted route.
Ready for a free, no-obligation quote?
South Yorkshire's specialists in secondary glazing — survey, measure, manufacture and install.