scope:

Current facades and window sizes call for detailed considerations. The temperature development in buildings in summer is essentially influenced by the energetic characteristics of their facades. Whereas previously a simple calculation of the external cooling loads, based on reduction factors (see VDI 2078:1996), was sufficient in almost any case, more precise calculations will often be required today. This trend has been taken into account in this guideline.

The new guideline VDI 6007 Part 2 aims to offer the planner a powerful tool for calculating energetic characteristics, especially for the total energy transmittance which is decisive for summertime cooling loads. The energetic behaviour of contemporary transparent facades becomes comprehensible, and pertinent energetic characteristic values can be tabulated for VDI 2078.

For convective as well as radiant loads to be considered properly, a pertinent distinction has to be made. This means that it is important to separately determine the components of the total energy transmittance, i.e.

• radiation transmittance and
• secondary heat transfer factor,

as well as any additional room load due to ventilation as is the case with internal sunshading. The radiation transmittance characterises the shortwavelength radiation transmitted through the window into the room. Like radiation absorption in the individual layers of the window, it can initially be determined without considering heat transport. However, absorbed radiation then acts as a heat source driving thermal transport processes within the window and, consequently, secondary heat transfer to the room. Such a calculation offers the advantage that the secondary heat transfer can be distinguished into

•  radiant heat,
•  convection and
•  ventilation.

The equations given allow to calculate facades with up to five solid layers, e.g.,

•  glass panes
•  textiles and/or
•  lamellar sunshading,

with non-ventilated or ventilated interstices. Regarding lamellar sunshading, energetic characteristics for the entire arrangement (comparable to a solid layer) are initially determined based on the lamella material (see Figure 1).

The equations for five solid layers allow to cover the combinations

•  double or triple glazing
•  internal/external sunshading (screen or lamellae) and, using an additional pane,
•  double-skin facades and
•  extract-air facades.

The method is not intrinsically limited to five solid layers but is applicable to any number of layers.Pertinent tabulated values (standard values) for six selected glass combinations are given for VDI 2078, taking into account, in addition to clear glass, the widely-used coated glasses

• thermal insulation glass,
• neutral solar control glass and
• reflective solar control glass.

Furthermore, indicative values for three different types of double-skin facades are tabulated for use in VDI 2078.

The solution algorithm specified in this guideline represents an optimum in terms of objective treatment and reproducibility. In principle, however, further effects can be taken into consideration; this would result in a significantly more complex algorithm, impeding objective treatment and reproducibility, see also [1].

Finally, it must be noted that complex facade systems will require adequate engineering assessment in any case, giving heed to the potentials and limitations of the calculation procedure.