It’s what’s inside the glass that saves you money

Yes. Filling the sealed cavity with an inert gas such as argon slows the heat that moves across the gap compared with plain air. Krypton performs a little better in very slim cavities. It’s one of several factors — the low-E coating and the spacer matter just as much — but the gas fill is a genuine, measurable part of a unit’s efficiency.

The spacer is the bar around the perimeter that holds the two panes apart and seals the gas in. Older aluminium spacers conduct heat quickly, leaving a cold border where condensation forms. A warm-edge spacer uses low-conductivity materials, keeping the edge of the glass closer to room temperature so it stays clearer and loses less heat.

A low-emissivity (low-E) coating is a microscopically thin metal-oxide layer on the glass that reflects long-wave heat back into the room while still letting daylight through. It’s usually the single biggest lever on a unit’s U-value, which is why a coated unit can out-perform a thicker uncoated one.

It depends on what you have now, your home and your energy use, so any figure is an indicative range rather than a promise. The Energy Saving Trust and manufacturers publish typical annual savings ranges for replacing older glazing with modern energy-efficient units — treat those as a guide, and a home survey will give you a realistic picture for your property.

Payback varies widely with your starting point, energy prices and the units you choose, so it’s best read as a range of years rather than a fixed number. Our double glazing payback period guide walks through how it’s calculated using attributed Energy Saving Trust and manufacturer data.

The quotes are aimed at homeowners. If you rent or don’t own the property you can still request information, but an installer will need the homeowner’s go-ahead before any work is agreed.