U-values and R-values Explained

The Measure of Energy Transfer

The heat transfer coefficient, referred to as the U-value or U-factor, measures the amount of heat transferred through parts of a building. Simply put, that means measuring how much cold or heat moves from the outside of your home to the inside. Just like a house, each component of a window performs differently. Therefore, we talk about the frame value and glass value which combine to make the overall window value. Lower U-values mean more energy saved, lower monthly bills and a more comfortable and constant interior temperature. Please note we have chosen to provide the original metric values here. Both American standard and metric units can be found on the individual product pages. 

Vinyl Windows

Idealu 5000S

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
1.1 1.2 1.3 (1.30) 1.2 (1.23) 1.2 (1.21)
1.0 1.2 1.2 (1.24) 1.2 (1.16) 1.1 (1.14)
0.7 1.2 1.0 (1.03) 0.96 0.94
0.6 1.2 0.97 0.89 0.87

Idealu 8000S

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
1.1 1.0 1.2 (1.24) 1.2 (1.17) 1.1 (1.15)
1.0 1.0 1.2 (1.17) 1.1 (1.10) 1.1 (1.08)
0.7 1.0 0.97 0.90 0.88
0.6 1.0 0.90 0.83 0.81

Energeto 5000

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
1.1 1.0 1.2 (1.24) 1.2 (1.16) 1.1 (1.14)
1.0 1.0 1.2 (1.17) 1.1 (1.10) 1.1 (1.08)
0.7 1.0 0.97 0.90 0.88

Energeto 5000 ED

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
1.1 0.89 1.2 (1.20) 1.1 (1.13) 1.1 (1.11)
1.0 0.89 1.1 (1.13) 1.1 (1.06) 1.0 (1.04)
0.7 0.89 0.93 0.86 0.84
0.6 0.89 0.87 PHT 0.79 PHT 0.78

Energeto 8000

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
1.1 0.94 1.2 (1.22) 1.1 (1.15) 1.1 (1.13)
1.0 0.94 1.2 (1.15) 1.1 (1.08) 1.1 (1.06)
0.7 0.94 0.95 0.88 0.86
0.6 0.94 0.88 0.81 PHT 0.79

Energeto 8000 ED

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
0.7 0.79 0.90 0.83 0.81
0.6 0.79 0.83 PHT 0.76 PHT 0.74

Energeto 8000 P

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K]
0.7 0.92 0.80
0.6 0.92 0.72
0.5 0.92 0.68

 

1) Windows with a heat transfer coefficient of the glazing Ug < 1.9 W/m²K must always be specified with a standard measure of 1.23 m x 1.48 m (DIN EN 14351-1: Table E1. Footer "d").
2) Uw-Values < 1.0 W/m²K are declared with 2 decimals in accordance with DIN EN ISO 10077.
3) PHT: Passive House Suitable Uf-Value <= 1.2 W/m²K und Uw-Value <=0.8 W/m²K (see marking "PHT" in in the table).
4) The specified PSI-Values do not match the tables E.1/E.2 der DIN EN ISO 10077-1:2006, but are examples of actual calculation values of the Spacer Systems.
5) The Uw-Values in the brackets are not rounded and for information purposes only.

Wood Windows

Classic, Softline, Rustic with a construction Depth of 68 mm

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
Pine / Meranti / Larch Oak Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
  Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak
1.1 1.36 1.65 1.36 1.45 1.28 1.38
1.0 1.36 1.65 1.29 1.38 1.22 1.31
0.8 1.36 1.65 1.16 1.26 1.08 1.17
0.7 1.36 1.65 1.10 1.19 1.01 1.11

Classic, Softline, Rustic with a construction Depth of 78 mm

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
Pine / Meranti / Larch Oak Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
  Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak
1.1 1.24 1.52 1.32 1.41 1.25 1.34
1.0 1.24 1.52 1.25 1.34 1.18 1.27
0.8 1.24 1.52 1.13 1.22 1.04 1.13
0.7 1.24 1.52 1.06 1.15 0.97 1.06
0.6 1.24 1.52 0.99 1.08 0.91 0.99

 

1) 1) Windows with a heat transfer coefficient of the glazing Ug < 1.9 W/m²K must always be specified with a standard measure of 1.23 m x 1.48 m (DIN EN 14351-1: Table E1, Footer "d").
2) Uw-Values < 1.0 W/m²K are declared with 2 decimals in accordance with DIN EN ISO 10077.
3) PHT: Passive House Suitable Uf-Value <= 1.2 W/m²K und Uw-Value <=0.8 W/m²K (see marking "PHT" in in the table).
4) The specified PSI-Values do not match the tables E.1/E.2 der DIN EN ISO 10077-1:2006, but are examples of actual calculation values of the Spacer Systems.
5) The Uw-Values in the brackets are not rounded and for information purposes only.

Aluminum Clad Vinyl Windows

TwinSet 5000

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
1.1 1.2 1.3 (1.30) 1.2 (1.23) 1.2 (1.21)
1.0 1.2 1.2 (1.24) 1.2 (1.16) 1.1 (1.14)
0.7 1.2 1.0 (1.03) 0.96 0.94
0.6 1.2 0.97 0.89 0.87

TwinSet Energeto 5000

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
1.1 1.0 1.2 (1.24) 1.2 (1.16) 1.1 (1.14)
1.0 1.0 1.2 (1.17) 1.1 (1.10) 1.1 (1.08)
0.7 1.0 0.97 0.90 0.88
0.6 1.0 0.90 0.83 0.81

TwinSet Energeto 8000ED

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
1.1 0.83 1.2 (1.18) 1.1 (1.11) 1.1 (1.09)
1.0 0.83 1.1 (1.12) 1.0 (1.04) 1.0 (1.02)
0.7 0.83 0.91 0.84 0.82
0.6 0.83 0.85 PHT 0.77 PHT 0.75

TwinSet View

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
0.8 1.1 1.05 0.99 0.96
0.7 1.1 0.98 0.91 0.89
0.6 1.1 0.91 0.84 PHT 0.82
0.5 1.1 0.84 PHT 0.77 PHT 0.75

TwinSet Energeto View

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
  Insulating Glass-Spacer
e.g. aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Swisspacer V
Ψ (PSI) 0.032 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
0.8 1.0 1.02 0.96 0.93
0.7 1.0 0.95 0.89 0.86
0.6 1.0 0.88 0.81 PHT 0.79
0.5 1.0 0.81 PHT 0.74 PHT 0.72

 

1) Windows with a heat transfer coefficient of the glazing Ug < 1.9 W/m²K must always be specified with a standard measure of 1.23 m x 1.48 m (DIN EN 14351-1: Table E1. Footer "d").
2) Uw-Values < 1.0 W/m²K are declared with 2 decimals in accordance with DIN EN ISO 10077.
3) PHT: Passive House Suitable Uf-Value <= 1.2 W/m²K und Uw-Value <=0.8 W/m²K (see marking "PHT" in in the table).
4) The specified PSI-Values do not match the tables E.1/E.2 der DIN EN ISO 10077-1:2006, but are examples of actual calculation values of the Spacer Systems.
5) The Uw-Values in the brackets are not rounded and for information purposes only.

Aluminum Clad Wood Windows

Idealu 68

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
Pine / Meranti / Larch Oak Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
  Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak
1.1 1.28 1.55 1.37 1.46 1.27 1.36
1.0 1.28 1.55 1.30 1.39 1.20 1.29
0.8 1.28 1.55 1.16 1.25 1.07 1.16
0.7 1.28 1.55 1.09 1.18 1.00 1.09
0.6 1.28 1.55 1.03 1.12 0.93 1.02

Idealu 78

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
Pine / Meranti / Larch Oak Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
  Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak
1.1 1.20 1.46 1.34 1.43 1.24 1.33
1.0 1.20 1.46 1.27 1.36 1.18 1.18
0.8 1.20 1.46 1.13 1.22 1.04 1.13
0.7 1.20 1.46 1.06 1.15 0.97 1.06
0.6 1.20 1.46 1.00 1.09 0.91 0.99

Plano

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
Pine / Meranti / Larch Oak Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
  Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak
0.8 1.31 1.59 1.17 1.26 1.08 1.17
0.7 1.31 1.59 1.10 1.20 1.01 1.10
0.6 1.31 1.59 1.04 1.13 0.94 1.04

ECO Idealu

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
Pine / Meranti / Larch Oak Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
  Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak
0.8 0.97 1.10 1.05 1.10 0.96 1.01
0.7 0.97 1.10 0.99 1.03 0.90 0.94
0.6 0.97 1.10 0.92 0.97 0.83 0.87

ECO Plano

Ug Glass Uf Frame Uw Window
DIN EN 673
ΔT (15°)
Pine / Meranti / Larch Oak Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Aluminum
Ψ (PSI) 0.07 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
Insulating Glass-Spacer
e.g. Warm Edge PVC
Ψ (PSI) 0.04 W/m²K
DIN EN ISO 10077-1
1) 2) 3) 4) 5)
[W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K] [W/m2K]
  Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak Pine / Meranti / Larch Oak
0.8 0.92 1.01 1.04 1.07 0.95 0.98
0.7 0.92 1.01 0.97 1.00 0.88 0.91
0.6 0.92 1.01 0.91 0.94 0.81 0.84

 

1) Windows with a heat transfer coefficient of the glazing Ug < 1.9 W/m²K must always be specified with a standard measure of 1.23 m x 1.48 m (DIN EN 14351-1: Table E1, Footer "d").
2) Uw-Values < 1.0 W/m²K are declared with 2 decimals in accordance with DIN EN ISO 10077.
3) PHT: Passive House Suitable Uf-Value <= 1.2 W/m²K und Uw-Value <=0.8 W/m²K (see marking "PHT" in in the table).
4) The specified PSI-Values do not match the tables E.1/E.2 der DIN EN ISO 10077-1:2006, but are examples of actual calculation values of the Spacer Systems.
5) The Uw-Values in the brackets are not rounded and for information purposes only.

What is the U-value (U-factor)?

In plain English, the U-factor is how much warmth an object transfers from its warm side to its cold side. It is given in W/m²K (Watts per m² Kelvin). The lower the value, the better the insulation.

A U-factor of 0.6 W/m²K is one of the best possible today and increases from there. Triple glazing for example, typically produces U-Factors between 0.4 and 0.8 W/m²K.

The following terms are used in the overall U-value formula:

  • Uf describes the heat transmittance coefficient of the frame
  • Ug indicates the temperature loss through the glass plate
  • ψg defines the energy loss through the edge compound (warm edge)
  • Uw describes the total U-factor of the overall window 

Heat Gain and Loss

Heat flow is unavoidable when two objects with different temperatures come into contact. Think of wrapping your cold hands around a hot coffee mug. Windows, doors and skylights gain and lose heat in three different ways:

  • Direct conduction through the glass surface or frame
  • Heat radiating in and out of a house via interior objects like people, furniture and walls
  • Air leaks

Solar radiation plays an important role in heating interior spaces. Sunlight enters via the glass panes and is absorbed by things like carpets, curtains and furniture. This stored heat is later slowly released, warming up the air in the room.

Special glazing ensures that long wave radiation (i.e. the heat) does not leave by reflecting it back inside. This helps maintain a consistent temperature and prevent the need for additional heating. However, this has no effect on visible light which can pass back and forth unobstructed.

Heat Insulation R and G-Values

The difference between single glazing (i.e. one pane) windows and modern double and triple is so large that single glazing is rare today and even forbidden in many building codes. Switching to two and three panes of glass can reduce heat loss by 20% or more and dramatically improves a home's insulation.

What is the G-Value ?

The G-value measures how much solar heat (infrared radiation) is allowed in through a specific part of a building. Therefore, a low number indicates that a window allows a very small percentage of solar heat to enter. This may be desirable in hot climates but not in cooler ones. In winter, with less sunlight, the U-value will actually be higher than the G-value. When sufficient sunlight is available, both will be high. In this case, however, the higher and somewhat worse U-value is actually better.

What is the R-value?

The R-value is the measure of thermal resistance. This means the ability of energy to move from hot areas to cold areas via materials such as insulation, walls, windows and doors. The higher the R-value is, the better that material prevents heat transfer. A material's R-value is based on its resistance to conduction as well as its thickness. Considering the example of camping. Sleeping directly on cold ground will quickly make you cold. However, sleeping on a foam pad makes it more difficult (due to the higher R-value) for the cold to move from the ground to your body.

U, R and G Value Summary:

In short, for insulation in windows, high R-values are good. Low U-values are good and a balanced G value is the best for most climates.

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