U-value optimisation of curtain walls - Part 2

In my last post I have described some basics of the Ucw-value calculation and defined the layout of a reference curtain wall. This time, the U- and Ψ-values ​​of each facade component will be determined in order to calculate the U-value of the complete curtain wall. Then I will try to answer the question of how far the Ucw-value can be reduced by means of thermal optimisation of each facade component.

Many system manufacturers of aluminium curtain walls offer a standard version of their profile systems with lower thermal insulation and a high-insulation version, often called "Passivhaus-tauglich" (i.e. meeting passive house standards) in German-speaking areas. The same can be said for glazing units: one can distinguish between standard double-glazing units and highly insulating triple-glazing units. I will therefore examine the Ucw-value of our reference curtain wall layout with "standard" components and then a version with thermally optimised facade components.

Ucw-value: "standard" version

Table A.1 in ISO 12631:2012 (1) provides a guideline for calculating the thermal transmittance of curtain walling. It describes how to obtain the necessary U-and Ψ-values of the individual facade components for calculating the overall Ucw-value. In most cases, there are two ways to determine these values: they ​​can be found in the corresponding tables of the standard, or they are determined by using calculation or measurement methods set in the standard (usually this is done by the product manufacturer).

The values given in the tables of the standard are usually on the safe side. The U- or Ψ-value of a particular product which has been calculated or measured by the manufacturer is usually lower and therefore better.

Frames

According to the above table A.1, the values ​​for frames, mullions and transoms Uf, Um and Ut can be determined by a numerical calculation method specified in 10077-2:2012, or they are measured according to EN 12412-2:2003. All major manufacturers of curtain wall systems provide the relevant U-values determined according to these standards.

The U-value for a particular mullion or transom depends on the thickness of the filling element and the internal depth of the profile. With a filling element of eg. 28 mm thickness (double-glazing unit) and an internal profile depth of 150 mm, one can assume Ut-and Um-values of about 2.1 W/(m²K) (2). This applies to standard systems without additional thermal insulation measures. The U-values ​​of a standard frame of an operable insert unit also vary depending on the manufacturer and product, with an average value of about 1.8 W/(m²K). Therefore, for our reference curtain wall, we assume the following values:

-> Ut/ Um: 2.1 W/(m²K)

-> Uf: 1.8 W/(m²K)

The values ​​of linear thermal transmittance Ψm,f and Ψt,f can be taken from table B.6, ISO 12631:2012, or they can be calculated according to ISO 10077-2:2012. Table B.6 provides five different junction types and assigns Ψm,f- and Ψt,f-values ​​to these types ranging from 0.05 to 0.11 W/(mK) to. In our case, we assume an average value of 0.07 W/(m²K).

-> Ψm,f; Ψt,f: 0.07 W/(mK)

Glazing

The values for Ug can be taken from ISO 10077-1:2006 or determined by calculation and measurement methods specified in EN 673:2011, EN 674:2011 and EN 675:2011. State of the art for double-glazing units is a Ug-value of about 1.1 W/(m²K).
-> Ug: 1.1 W/(m²K)

The values of linear thermal transmittance Ψf,g, Ψm,g and Ψt,g can be taken from tables B.1, B.2, B.3 and B.4 or calculated according to 10077-2:2012. The standard distinguishes between "normal" and "thermally improved" types of glazing spacer bars. Moreover, the table values ​​depend on the type of neighbouring profiles and of the glazing type. For normal spacer bars with low emissivity glass and aluminium mullions and transoms, Ψm,g- and Ψt,g-values are 0.11 W/(mK). For a metal frame with thermal break, the Ψf,g-value is also 0.11 W/(mK).

-> Ψm, g; Ψt, g; Ψf, g: 0.11 W/(mK)

Panels

The thermal transmittance of panels Up can be determined by calculation methods specified in ISO 6946:2007. Since the thickness of the insulating layer in the panel is crucial for the Up-value, we can for now neglect the inner and outer metal cladding as well as other components of the panel. With an insulation thickness of 140 mm and a thermal conductivity λ: 0.035 W/(mK) we reach a U-value of 0.24 W/(m²K).

-> Up: 0.24 W/(m²K)

The Ψp-values can be taken from table B.5 or established by calculations specified in 10077-2:2012. According to table B.5, Ψp-values depend on the thermal conductivity of the spacer, the panel type (Type 1: with air-filled space; Type 2: without air-filled space), as well as on the materials of the inner and outer panel cladding. Thus, panel type 2 with aluminium cladding on both sides and spacers with λ: 0.2 W/(mK) has a Ψp-value of 0.2 W/(mK).

-> Ψp: 0.2 W/(mK)

Determination of Ucw-value

All the necessary data are thus set for the calculation of the reference facade. The following two tables show the subtotals for ΣA×U and for ΣΨ×l.

The Ucw-value for the facade with standard components is thus:

Ucw = (ΣA×U + ΣΨ×l) / Acw

Ucw = (10.434 W/K + 4.534 W/K) / 12.00 m²

Ucw = 1.2 W/(m²K) (1,247)

Ucw-value: thermally optimised version

Frames

High-insulation curtain wall systems usually have an additional insulating body between the profile on the room side and the pressure plate on the external side. With a filling element of eg. 44 mm thickness (triple-glazing unit), Ut- and Um-values can get as low as about 0,8 W/(m²K) (due to the on-going development of all facade components, these and many other values mentioned here might be outdated soon again).

-> Ut; Um: 0.8 W/(m²K)

Additional insulation inserts and glass rebate insulation in frames of an operable insert unit can significantly reduce the Uf-value. Most manufacturers have high-insulation frames with a U-value of 1.0 to 1.2 W/(m²K).

-> Uf: 1.1 W/(m²K)

As described above, Ψm,f and Ψt,f can be taken from table B.6, ISO 12631:2012, or they can be calculated according to ISO 10077-2:2012. Values of about 0.025 W/(mK) are currently possible.

-> Ψm,f; Ψt,f: 0.025 W/(mK)

Glazing

Ug-values for a triple-glazing unit currently range between 0.7 and 0.5 W/(m²K), depending on the glass coatings and the gas used for the space between the glass panes.

Ψf,g, Ψm,g, Ψt,g can also be determined by calculations according to EN ISO 10077-2:2012. In Germany, the working group "Warme Kante" (warm edge) has published data sheets for thermally improved glass edges (spacers) of various manufacturers. With some products, values for a triple-glazing unit used in an operable insert unit can be reduced to 0.030 W/(mK). For our case we use slightly higher values:

-> Ψm,g; Ψt,g; Ψf,g: 0.04 W/(mK)

Panels

The thickness of opaque insulated facade panels is usually limited by the depth of the facade structure. If, for example, the mullion and transom profiles are 150 mm deep, the insulation thickness in the panel usually cannot exceed 150 mm. Another optimisation method would be the use of insulating materials with a lower thermal conductivity. But even here optimisation options are limited.

Vacuum insulation panels (VIPs) could be a promising alternative. The use of these panels has not yet become widely accepted. However, one can assume that this insulation method will soon become more popular especially for curtain wall systems. Particular advantages are insulated panels which have the same thickness like glazing units as well as a high degree of prefabrication due to  standardised curtain wall modules.

Vacuum insulation panels which have the same thickness like a triple-glazing unit currently have Up-values of 0,15 to 0,2 W/(m²K) and Ψp-values of about 0,02 W/(mK). Here, too, one can expect further improvements in the near future due to the continuous development of this relatively new panel type.

-> Up: 0.18  W/(m²K)
-> Ψp: 0.02 W/(mK)

Determination of the Ucw-value

Thus, we have all the necessary data to calculate the thermal transmittance of the optimised curtain wall. The following two tables show again the subtotals for ΣA×U and for ΣΨ×l.

The Ucw-value for the facade with standard components is thus:

Ucw = (ΣA×U + ΣΨ×l) / Acw
Ucw = (5.742W/K + 1.012 W/K) / 12.00 m²
Ucw = 0.6 W/(m²K)  (0.563)

Conclusion

Compared to the standard version of our reference curtain wall, the Ucw-value could be reduced by half (from 1.2 to 0.6 W/(m²K)) with the use of thermally optimised components. One can even assume that a Ucw-value of 0.5 W/(m²K) is quite possible by using optimum values for every single facade component.

It is striking that the overall U value of the curtain wall is similar to the U value of the glazing in both cases (standard and optimised). The Ug-value can therefore be taken as an indication for the Ucw-value for typical curtain wall layouts which are similar to the one illustrated here.

In addition, Ψ-values seem to leave more room for improvement than U-values. Comparing the U-values of the standard and the optimised version, values could be cut by half. The Ψ-values, however, could even be reduced to a quarter of the original value.

The individual values with the greatest potential for optimisation are on the one hand the Ut- and Uf-values (improvement from 2.1 to 0.8 W/(m²K)) and on the other hand the Ψp-value (tenfold improvement from 0.2 auf 0.02 W/(mK)).

As indicated above, the thermal optimisation of curtain wall components is only one side of Ucw-value optimisation. In my next post I will therefore be focusing on the other side, the geometrical optimisation of curtain walls.

References

(1) ISO 12631 (first edition 2012-10-01), Thermal performance of curtain walling - Calculation of thermal transmittance (Reference number: ISO 12631: 2012(E))
(2) current aluminium profile systems (February 2015) of brands Schüco, Wicona, Raico and Hueck were used to determine the U- and Ψ-values. Values ​​vary partly depending on manufacturer and product line. In this case, mean values ​​were taken as a basis for calculations.