Comparing Window U Values When Reviewing Quotations

Make sure you are comparing the same measurement

Our main guide to Windows and Doors explains what a U value is and why the whole window figure, not a glazing only figure, is the one that matters and the one used in Building Regulations and the SEAI Windows grant. This article does not repeat that explanation. It focuses on a narrower, practical problem, so that when you are holding two or more quotations, you know exactly what each figure represents. If your starting point is a single failed pane rather than a full replacement decision, our guide to replacing a failed glazing unit is the more relevant one to start with.

Confirm what is actually being quoted

Not every figure printed on a window quotation is the whole window U value. Some suppliers quote a centre pane or glass only figure, which describes the best case performance of the glazing alone and will always look better than the true whole window figure, since it leaves out the frame and spacer entirely. Before comparing any two numbers, confirm in writing from each supplier that the figure quoted is the whole window U value, covering the glazing, frame and spacer together, and ask for it to be restated clearly if the original quotation does not make this explicit.

Confirm the units and the standard being used

Whole window U values in Ireland are expressed in watts per square metre kelvin, and a lower figure means better performance, less heat lost through the window for a given difference between inside and outside temperature. Occasionally a figure may be presented differently, or calculated to a different reference standard, particularly where a supplier is using documentation prepared for a different market. If a quotation's figure looks unusually different from others you have received, ask directly which standard and units were used to calculate it, rather than assuming it is directly comparable.

Confirm the window size and configuration the figure applies to

A whole window U value is calculated for a specific window size and configuration, since the ratio of frame to glass changes with size, and a smaller window generally has a worse whole window U value than a larger window using the same glazing and frame system, simply because the frame makes up a larger proportion of a smaller opening. A figure quoted for a large picture window is not directly comparable to a figure for a small bathroom window, even from the same supplier using the same product range. Where possible, ask for the figure calculated for a reference size close to your own largest or most typical window, or at minimum confirm what reference size each supplier used.

A note on indicative ranges

General published ranges exist for how different glazing and frame combinations typically perform, for example that a well specified double glazed window with a modern frame will usually achieve a better whole window figure than an older single glazed frame, and that triple glazing can improve on double glazing further, particularly at the glazing itself. These ranges are useful for general orientation, but they are not a substitute for the specific whole window figure quoted for your own project, since the same general description, such as double glazing, covers a wide range of actual performance depending on the frame, spacer and gas fill used. Treat a general range as background context, and treat the specific figure on your own quotation, confirmed as a genuine whole window value, as the one that actually matters.

For what a U value is, the current Building Regulations backstop, and how it connects to SEAI grant eligibility, see our guide to Windows and Doors performance.