Window U-value explained for installers and homeowners (2026)

"Triple glazed", "Uw 0.8", "A++ rated" — thermal terms appear on every modern window quote. But what does U-value actually mean for your customer, and why does it matter for subsidies, energy labels and how installers are evaluated? Plain-English guide.

What is U-value?

U-value (W/m²K, watts per square metre per kelvin) measures how much heat passes through a surface per square metre for every degree of temperature difference between inside and outside. Lower U-value = better insulation.

• Ug — glass-only U-value
• Uf — frame-only U-value
• Uw — whole window U-value (glass + frame + edge effect)

Uw is the number that matters for customers — it's how the installed window actually performs in the wall.

Reference values 2026

Glazing type	Ug (glass)	Typical Uw (whole window)	EPC / energy label impact
Single glazing	5.8 W/m²K	~5.5	EPC F–G
Standard double	2.9	~2.5	EPC D
Low-E double (argon)	1.1	~1.4 – 1.6	EPC B–C
Low-E double (warm edge)	0.7	~0.9 – 1.2	EPC A – A+
Triple glazing	0.5 – 0.6	~0.7 – 1.0	EPC A+ – A+++

Why U-value belongs on the quote — not just the brochure

1. EPC / energy label updates — homeowners need the Uw value per window position for a new energy performance certificate after renovation. If it's on your quote, they don't need to call you later.
2. Government grants and ECO4 schemes — retrofit grant schemes often require documented U-values for the installed products. Missing from the invoice = grant may not be paid.
3. Competitive comparison — a customer with three quotes uses U-values to compare apples to apples. Your quote that clearly shows Uw 1.0 vs a competitor's vague "double-glazed" wins on transparency.

How to present U-values on a quote — practical format

Per line item:

• Glazing: Low-E double (4-15-4 argon, warm edge spacer)
• Ug: 1.0 W/m²K
• Whole window Uw: 1.3 W/m²K (frame: uPVC 70mm)

If the main quote is too crowded, a technical spec sheet at the back works well. Customer sees total price upfront; energy assessor sees full specs at the back.

How Kozynofferte handles U-values

For each glazing option in the product configurator, you store the Ug and typical Uw range. When you configure a window in a quote, the values appear automatically in the technical spec block.{" "} See how it works.

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U-value question for a specific project?{" "} Email me.

— Richard Bajnath, Kozynofferte