Comparing Window U Values When Reviewing Quotations
You already have more than one quotation with a U value printed on it. This article is about making sure you are actually comparing the same thing before you compare the numbers.
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.
Check what your windows and doors need
Answer a few practical questions in our free Window and Door Check to see what to establish before you request quotations.
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.
Before comparing U value figures between quotations
- Confirm each figure is the whole window U value, not a centre pane or glass only figure
- Confirm the units and reference standard used are consistent between quotations
- Confirm the window size and configuration each figure was calculated for
- Check the figure against the current Building Regulations backstop and SEAI grant requirement described on our main guide, if grant eligibility matters to your project
- Ask for the figure in writing as part of the formal quotation, not only quoted verbally
Indicative whole window U values by glazing type
The table below gives rough, whole window (Uw) ranges for common types of window, to help you judge whether a quoted figure is in a sensible range. They are indicative only. The actual figure for any window depends on the glazing, the frame material and profile, the spacer bar, any gas fill and coating, and the size of the window, so two windows in the same row can perform quite differently.
| Window type | Indicative Uw | Basis |
|---|---|---|
| Older single glazing | About 4.8 (timber or uPVC frame) to 5.7 (metal frame) | SEAI DEAP default values for single glazed windows |
| Older standard double glazing, air filled, no low emissivity coating | About 2.7 to 3.1 in timber or uPVC frames, higher in metal frames | SEAI DEAP default values, depending on the gap between the panes |
| Modern high performance double glazing, with a low emissivity coating, argon fill and warm edge spacer | About 1.2 to 1.4 | 1.4 is the Irish Building Regulations backstop for replacement windows and the SEAI grant requirement |
| Triple glazing | About 0.7 to 1.1 | Passive House Institute certified windows must achieve 0.80 or lower; many standard triple glazed products sit nearer 1.0 |
Sources: default window U values in the SEAI Dwelling Energy Assessment Procedure (DEAP) manual (opens in a new tab), Irish Building Regulations Technical Guidance Document L, and the Passive House Institute component criteria.
Use the ranges for orientation, not for choosing between quotations. A figure well outside its row, or a figure that looks unusually good for the glazing described, is a reason to check whether you have been given a glass only (Ug) or centre pane figure rather than the whole window value, as described above. The number to compare between installers is the whole window Uw stated in writing for your own windows.
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.
