Reading a double glazing spec sheet
A double glazing specification is where two quotes are really won and lost. Prices only compare fairly when the products behind them match, and the spec sheet is where those details live. Once you can read a few key lines — the glass, the U-value, the frame profile and the hardware — you can tell a well-built window from a thin one, and a genuine bargain from a false economy. Here is how to decode it.
U-values and energy ratings
The U-value measures how much heat passes through the window — the lower the number, the better the insulation. You may also see a Window Energy Rating band from A++ down to E. According to the Energy Saving Trust, replacing older single glazing with modern energy-efficient windows can typically reduce heat lost through the glass, though the actual figure depends on your home. Treat these as comparison points between quotes rather than a promise of specific savings, and make sure each quote states them.
The glass unit
A sealed double glazed unit is two panes separated by a spacer bar and filled, often with argon gas, to improve insulation. The spec may mention low-emissivity (low-E) coatings, the cavity width, and whether the glass is toughened or laminated for safety in certain positions. If two quotes differ on glass, they are not comparing like with like. To understand the options, it helps to compare glazing types before you compare firms.
Comparing specifications? We match you with vetted installers so you can request the same spec from each and compare properly — free and with no obligation.
The frame profile
The frame is more than a colour choice. Look for the material (uPVC, aluminium or timber), the number of chambers in a uPVC profile, and whether the frame is internally reinforced. These affect strength, insulation and longevity. For a rounded view of how frame materials stack up, see materials ranked and reviewed. Our guide to national vs local firms is worth a look too, since different firms favour different profile systems.
Hardware and security
Check the locking system, hinges and handles. Multi-point locking and accreditation to a recognised security standard are worth noting, especially on ground-floor and accessible windows. Good hardware rarely shows up in the headline price but makes a real difference to how a window feels and how secure it is.
Turning the spec into a fair comparison
Once you can read the spec sheet, put the key lines from each quote into a simple table so differences jump out. Our guide to how to compare double glazing quotes shows exactly how, and pairing the spec with proper accreditation checks gives you the full picture. If you would like a gentle primer first, walk through the quote process step by step, and read how to vet an installer first.
Requesting and comparing quotes is free and no-obligation, so you can take the time to read every specification properly. Comparing while installers still have Q3 fitting slots in your area keeps your dates open while you decide.