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Replacing glass units and getting quotes

When a sealed unit has failed — misted, cracked or simply past its best — you often do not need a whole new window. Because the replacement glass unit is a separate component from the frame, it can usually be swapped on its own. This page explains when that is the right approach, how the unit is measured, and how to compare quotes so you know you are paying for the specification you want.

Unit only, or whole window?

If the frame is sound and only the glass has gone — the classic misted or blown unit — replacing the sealed unit alone is usually the sensible, lower-cost route. You would look at a full window replacement instead when the frame is rotten, warped or draughty, when the hardware has failed, or when you want to change the style or material of the window entirely. An installer will advise which makes sense once they have seen it.

How a replacement unit is measured

A replacement is made to match the exact sight size and the overall unit thickness of the frame's glazing rebate, so the new unit fits where the old one sat. The installer records the width, height and the build — for example 4-16-4 — and confirms whether safety glass is needed in that position. This is why the glass thickness and the frame both constrain the specification; the new unit has to physically fit as well as perform.

Specify the upgrade while you are at it

Replacing a unit is a good moment to improve the specification. A new unit can be ordered with a better Low-E coating, an argon fill and a warm-edge spacer, so the replacement outperforms the one that failed. If you are replacing glass in period or box windows, it is worth looking at double-glazed sash window options too, since slim sealed units are now available for many traditional frames.

Comparing quotes fairly

Make sure each quote states the same things: the unit size, the pane build, the gas fill, the spacer type and the safety glass where required. That way you are comparing like with like rather than a bare price. Look for an installer registered with a competent-person scheme such as FENSA or CERTASS for replacement work, and ask about insurance-backed guarantees and deposit protection. The U-value tells you how the new unit will perform once it is in.

What happens next

Once you request quotes we match your enquiry with vetted local installers who cover your area. They arrange a survey, measure the units, confirm the specification and give you a written, no-obligation quote — so nothing is agreed until you have seen the detail in black and white.