The article on K value and U value, g value, and SHGC value provides opinions on the ins and outs of K value and U value and suggestions for use, but does not quantitatively clarify the difference between K value and U value; the K value and U value of insulating glass, Who is higher and who is lower?

#### Background

Recently, many glass deep processing practitioners who imported insulating glass from China have consulted. I heard that the K value in China is 0.2 higher than the U value in Europe, and the window with a U value of 0.8 in Europe becomes 1.0 in China. Is this correct? Is there any data to support this claim? Jinan LIJIANG Glass once wrote an article about K value, U value, g value, and SHGC value. The conclusion is as follows: K value and U value are both heat transfer coefficients, the difference lies in the symbols and calculation boundaries, which are essentially domestic building energy conservation.

The contradiction between the standard system and the international building energy efficiency standard system. The two indicators involve standards and specifications such as architectural design, products, testing/calculation, and engineering applications, which can easily confuse, especially when using foreign testing and certification reports to communicate with domestic customers, these questions are often asked. Therefore, Jinan LIJIANG Glass researched and summarized different boundary conditions at home and abroad, simulated and compared the K/U value of typical insulating glass, and compiled it into a document. So, what is the difference in the K/U value of insulating glass?

Figure 1  The difference in the K/U value of insulating glass argon-gas filing 1

#### Calculation of boundary conditions

The calculation standard of K value for doors and windows/glass in China is JGJ/T151, the calculation standard for U value in the EU is EN673, ISO10292, EN6946, ISO 15099, and the calculation standard for U value in the United States is NFRC100.

It should be noted that the U value calculated according to the boundary conditions of the Chinese standard JGJ/T151 is actually the Chinese K value, so the Chinese K value is uniformly expressed below.

The boundary conditions for calculating the K/U value of glass and doors and windows in China and Europe are arranged as follows:

 Standard Season Indoor temperature ℃ Outdoor temperature℃ Internal surface heat transfer coefficientW/㎡*K(Indoor Air Velocity m/s) External surface heat transfer coefficientW/㎡*K(outdoor wind speed m/s) Solar radiation intensity（W/㎡） JGJ/T151 Winter 20 -20 3.6 16 0 Summer 25 30 2.5 16 500 EN673ISO102992 Winter 17.5 2.5 7.7 25 300 EN6946（PHI） Winter 20 -10 7.7 25 / ISO15099 Winter 20 0 3.6 20 300 Summer 25 30 2.5 8 500 NFRC100 Winter 21 -18 / 26 0 Summer 24 32 / 15 783 Note:1. Only the convective heat transfer part, the radiation heat transfer part is automatically calculated by the software;2. Contains two parts of convection heat transfer and radiation heat transfer;3. The heat transfer coefficient (convection part) of the inner surface should be determined according to the type of window frame, glass, etc.;4. Convective heat transfer coefficient and wind speed need to be considered at the same time.

We focus on winter K/U values, so only winter boundary conditions are considered. In terms of air temperature in winter, China and the United States are closer, but China and Europe are quite different; in terms of surface heat transfer coefficient (air velocity), there are certain differences between China and Europe.

This leads to different calculation results of the K/U value of insulating glass in China and Europe.

#### Analysis of the calculation results of glass K/U value

To clarify the difference in the K/U value of insulating glass, the typical glass structure is selected as follows:

(1) 5+12(16)A+5

(2) 5 Low-E+12(16)A+5

(3) 5 Low-E+12 (16)A+5+12(16)A+5

(4) 5 Low-E+12(16)Ar+5+12(16)Ar+5 Low-E

When the thickness of the spacer layer is 12 mm, the insulating glass window software K/U The calculation results are as follows:

 Glass configuration Chinese K value EU U value US U value JGJ/T151 EN673ISO10292 EN6946（PHI） ISO15099 NFRC100 5+12 A+5 2.681 2.824 2.761 2.876 2.719 5 Low-E+12 A+5 1.704 1.652 1.671 1.681 1.698 5 Low-E+12 A+5+12 A+5 1.274 1.279 1.273 1.296 1.268 5 Low-E+12 Ar+5+12 Ar+5 Low-E 0.751 0.750 0.747 0.757 0.746

When the thickness of the spacer layer is 16 mm, the K/U calculation results of the insulating glass window software are as follows:

 Glass configuration Chinese K value EU U value US U value JGJ/T151 EN673ISO10292 EN6946（PHI） ISO15099 NFRC100 5+16 A+5 2.675 2.705 2.701 2.765 2.711 5 Low-E+16 A+5 1.750 1.435 1.640 1.511 1.741 5 Low-E+16 A+5+16 A+5 1.255 1.111 1.199 1.147 1.243 5 Low-E+16 Ar+5+16 Ar+5 Low-E 0.737 0.622 0.677 0.639 0.724

It can be seen that the K value of insulating glass in China ≈ the U value in the United States, and there is a certain difference between the K value of insulating glass in China and the U value in the EU:

5+12(16)A+5: China K value < EU U value

5 Low-E+12(16)A+5: China K value > EU U value

5 Low-E+12 A+5+12 A+5: China K value ≈ EU U value

5 Low-E+12 Ar+5+12 Ar+5 Low-E: China K value ≈ EU U value

5 Low-E+16 A+5+16 A+5: China K value > EU U value

5 Low-E+16 Ar+5+16 Ar+5 Low-E: China K value > EU U value

It can be seen that the K value of insulating glass in China is close to the U value in the United States; the difference between the K value in China and the U value in the EU is related to the glass configuration, the thickness of the spacer layer, and the type of gas, which needs to be analyzed in detail.

Jinan LIJIANG Glass also calculated the K/U value of typical ultra-low energy building insulating glass, and the results are as follows:

 Glass configuration K value U value JGJ/T151 EN673 ISO15099 NFRC100 5 Low-E+18 Ar+5+18 Ar+5 Low-E 0.75 0.56 0.62 0.75 5 Low-E+16 Ar+5+16 Ar+5 Low-E 0.74 0.61 0.64 0.73 5 Low-E+15 Ar+5+15 Ar+5 Low-E 0.73 0.64 0.66 0.73 5 Low-E+14 Ar+5+14 Ar+5 Low-E 0.73 0.67 0.69 0.73 5 Low-E+12 Ar+5+12 Ar+5 Low-E 0.75 0.75 0.76 0.76 5 Low-E+9 Ar+5+9 Ar+5 Low-E 0.89 0.93 0.93 0.90 5 Low-E+18 A+5+18 A+5 Low-E 0.91 0.72 0.77 0.91 5 Low-E+16 A+5+16 A+5 Low-E 0.89 0.78 0.81 0.89 5 Low-E+15 A+5+15 A+5 Low-E 0.89 0.82 0.83 0.89 5 Low-E+14 A+5+14 A+5 Low-E 0.89 0.86 0.87 0.89 5 Low-E+12 A+5+12 A+5 Low-E 0.95 0.97 0.97 0.95 5 Low-E+9 A+5+9 A+5 Low-E 1.14 1.19 1.18 1.15

Similarly, the K value of insulating glass in China is close to the U value in the United States; the difference between the K value in China and the U value in the EU is mainly related to the thickness of the spacer layer and the type of filling gas, which requires specific configuration and specific analysis.

In the commonly used glass configuration of doors and windows in ultra-low energy buildings, the Chinese K value of 5 Low-E+16 Ar+5+16 Ar+5 Low-E is higher than the EU U value, while the Chinese K value of 5 Low-E+12 Ar+5+12 Ar+5 Low-E It is the same as the EU U value.

#### In conclusion

K value and U value are both heat transfer coefficients, which are used to characterize the thermal insulation performance of doors, windows, and curtain walls, and to measure the heat transfer intensity caused by air temperature difference; the two are mainly different in calculation boundary conditions except for different symbols. At present, the calculation standard of K value for doors and windows/glass in China is JGJ/T151, the calculation standard for U value in the EU is EN673, ISO10292, EN6946, ISO 15099, and the calculation standard for U value in the United States is NFRC100. The Chinese K value of insulating glass is close to the U value of the United States; the difference between the Chinese K value and the EU U value is related to the glass configuration, the thickness of the spacer layer, the gas type, etc., and needs to be analyzed in detail.In the commonly used glass configuration of doors and windows in ultra-low energy buildings, the Chinese K value of 5 Low-E+16 Ar+5+16 Ar+5 Low-E is higher than the EU U value, while the Chinese K value of 5 Low-E+12 Ar+5+12 Ar+5 Low-E It is the same as the EU U value.

This article focuses on the difference between the thermal calculation boundary conditions of glass/windows and doors in China and Europe and the K/U value difference of typical insulating glass. We will also analyze the K/U value difference between the window frame and the whole window later.

References:

[1] JGJ/T 151-2008. Specification for thermal calculation of building doors, windows, and glass curtain walls [S]

[2] EN 673:2011 Glass in building - Determination of thermal transmittance(U value) - Calculation method

[3] ISO 10292:1994 Glass in building - Calculation of steady-state U values (thermal transmittance) of multiple glazing

[4] EN ISO 6946:2017 Building components and building elements - thermal resistance and thermal transmittance - Calculation method

[5] ISO 15099-2003 Thermal performance of windows, doors, and shading devices - Detailed calculations

[6]NFRC 100-2017 Procedure for Determining Fenestration Product U-factors