What is a good U-value for a dormer window?

HR++ glazing achieves a U-value of around 1.1 W/m²K; triple glazing is around 0.7 W/m²K. For an entire frame construction (Uw), a value of ≤ 1.65 W/m²K is often used for existing buildings and ≤ 1.4 W/m²K for new builds.

How does the U-value relate to the R-value (Rc)?

U is roughly the combined inverse of R: the lower the U, the higher the thermal resistance. The U-value is mainly used for windows and window frames (expressed in W/m²K, lower = better), the R-value (Rc) for built-up constructions like roofs and walls (in m²K/W, higher = better).

What do Ug, Uf, and Uw mean for window frames?

Ug is the U-value of the glazing, Uf of the window frame profile (frame), and Uw the combined U-value of the complete window. For practical performance, Uw is what counts, because it takes into account the glazing, the profile, and the spacer in between.

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The U-value expresses the thermal transmittance of a building component in W/m²K. The lower the U-value, the less heat is lost through the building component. The value is mainly used for window frames, glazing, and doors.

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Three U-values

Symbol	Meaning
Ug	U-value of the glazing package itself.
Uf	U-value of the window frame profile.
Uw	U-value of the complete window including frame and glazing.

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What is a good U-value for a dormer window?
HR++ glazing achieves a U-value of around 1.1 W/m²K; triple glazing is around 0.7 W/m²K. For an entire frame construction (Uw), a value of ≤ 1.65 W/m²K is often used for existing buildings and ≤ 1.4 W/m²K for new builds.
How does the U-value relate to the R-value (Rc)?
U is roughly the combined inverse of R: the lower the U, the higher the thermal resistance. The U-value is mainly used for windows and window frames (expressed in W/m²K, lower = better), the R-value (Rc) for built-up constructions like roofs and walls (in m²K/W, higher = better).
What do Ug, Uf, and Uw mean for window frames?
Ug is the U-value of the glazing, Uf of the window frame profile (frame), and Uw the combined U-value of the complete window. For practical performance, Uw is what counts, because it takes into account the glazing, the profile, and the spacer in between.