U-value optimisation of curtain walls - Part 1

Thermally insulated facades are an integral part of energy efficient building concepts. A thermal optimisation of all facade components is needed to meet the ever increasing requirements for facades. In this and the following posts I would like to point out some possibilities of thermal facade optimisation.

 

The specific reason to take on the topic is my current project, which is located in Central Europe. The specifications stipulate a U-value of less than 0.65 W/(m²K) for the curtain walls. Meeting this demand has proved to be quite difficult. In the following posts, I would therefore like to discuss in more detail the possibilities to influence the thermal performance of curtain walling. But first a few basics ...

Ucw-value: Two calculation methods

The decisive physical value of (winter) thermal insulation of facades is the thermal transmittance or U-value. ISO 12631: 2012-10 (1) specifies the procedure for calculating the thermal transmittance of curtain wall structures. The standard provides two calculation methods: the single assessment method and the component assessment method.

The single assessment method is based on detailed computer calculations of the heat transfer through the facade construction and is usually more complex than the component assessment method. In practise, the single assessment method is particularly useful in advanced design stages. It is well suited for special cases such as non-standard facade areas, local penetrations and individual facade component designs. The component assessment method, on the other hand, is very helpful during earlier design stages, because larger geometry or component changes can be incorporated in the calculations with relatively little effort. 

The U-value of curtain walling systems (short: Ucw) according to the component assessment method is calculated using the following complicated-looking but ultimately simple formula:

Ucw = (ΣAg×Ug + ΣAp×Up + ΣAf×Uf + ΣAm×Um + ΣAt×Ut + Σlf,g×Ψf,g + Σlm,g×Ψm,g + Σlt,g×Ψt,g + Σlp×Ψp + Σlm,f×Ψm,f + Σlt,f×Ψt,f) / Acw

where

• A is the area [m²]
• U is the thermal transmittance [W/(m²K)]
• l is the length [m]
• Ψ is the linear thermal transmittance due to the combined thermal effects [W/(mK)]

the subscripts mean:

• cw : curtain wall
• g : glazing
• p : panel
• f : frame
• m : mullion
• t : transom

The following figure illustrates the meaning of the different U- and Ψ-values:

U- and Ψ-values: schematic section through a curtain wall

According to the above formula, the U-values ​​of the individual components are multiplied by the respective facade surfaces and the Ψ-values ​​are multiplied by the corresponding lengths. Both products are then divided by the total facade area. The above formula can therefore be summarised as follows:

Ucw = (ΣA×U + ΣΨ×l) / Acw

Ultimately, the thermal transmittance values of the components are weighted according to their area percentage, with the Ψ-value additionally taking into account the thermal interaction between contacting components.

Essentially, there are therefore two groups of factors in the U-value calculation: physical factors and geometrical factors. In other words, there are technical aspects on the one hand and design aspects on the other hand (here, the interface between design and technology shows up once more, reflecting the title of this blog).

What is a "typical" curtain wall?

If the aim is to come to general conclusions about the possibilities to influence the thermal performance of curtain walls, it makes sense to choose a facade layout which is as general as possible. But what does such a facade layout look like?

Office buildings are a classical field of application for curtain walling. According to Eugene Kohn and Paul Katz, a typical planning module in the US is 1,5m (5 ft.), while in Europe and Asia it is 1,2 m (3' 11'') and 1,5 m (5' 0''). The typical floor to floor height of high rise office buildings in the US is between 13' 0'' and 13' 6'' (4.0 and 4.2 m), in Europe (Germany and France) it is 3.75 m (12' 4'') (2).

A general curtain wall layout - so to speak, the lowest common denominator of a global facade layout - could therefore look like this:

The horizontal division is also kept as simple as possible: there is an opaque area (here: 1.6 m for installation space + floor slab + parapet) and a glazed area (here: 2.4 m high). A transom and mullion width of 50 mm is assumed. Added to this is an operable insert unit with a frame width of 80 mm.

As seen above, the U-value of the curtain wall is determined in large parts by the surface area A of each facade component and the length l for contacting components. Therefore, it seems appropriate to have a closer look at these values ​​and their percentage of the total facade area. The following figure shows the areas of the each component of our reference facade in different colours.

The surface areas A and the percentage of the total facade are:

As expected, the glazing and panel areas Ag and Ap account with almost 90 % for the majority of the facade area. However, the facade profiles (Af, Am, At), despite the relatively slim profile widths, reach ​​almost 1.3 m² per facade field.

The following figure shows once more the reference facade. This time, the lengths l for contacting facade components are highlighted.

The lengths l and their percentage are:

It is remarkable that the lengths l total approximately 34 m despite the relatively simple division of the reference facade module with only few profiles. The area around the insert unit accounts for a large percentage of these lengths, since there are not only the contact areas between the frame and glass (lf, g) but also between mullion or transom and frame (lm, f and lt, f).

After the geometry of our reference facade has been defined, we need the U- and Ψ-values ​​of the individual facade components in order to calculate the Ucw-Value. More on this in the next post ...

References

(1) ISO 12631: 2012-10, Thermal performance of curtain walling - Calculation of thermal transmittance (ISO 12631: 2012)
(2) Cf. Kohn, A. Eugene; Katz, Paul: Building Type Basics for Office Buildings, New York 2002, p. 35 et sqq.