Basic planning considerations
Planning a VCW
When designing a VCW, as in all construction projects, proper planning and execution determine the quality of the finished facade. Although damage to ventilated curtain walls is extremely rare compared with other systems, and is usually due to incorrect execution, careful planning that considers all the basic conditions applicable to the specific object is required.
In Germany, a range of regulations, which are continuously updated give planners and installation companies the necessary security in the areas of invitations to tender, execution and billing.
The following section deals with important influencing factors and planning principles in more detail.
Heat transmission calculation / Thermal bridge considerations
The German ordinances relating to heating systems and thermal insulation require the production of a thermal insulation certificate to record a building‘s energy efficiency. This takes into account not only the primary energy consumption required to heat the building but also the transfer of heat through the external wall.
In the case of VCWs with metal substructures, the energy loss through the external wall can be up to 30% higher due to the thermal bridge effect of the bracket construction and depends on the number of anchor points as well as the design and material used for the substructure (aluminium substructure / normal thermal insulation requirements). Here, the metal brackets act like „cooling fins“ breaking through the insulation and causing additional heat loss. These heat losses are taken into account using the thermal bridge coefficient.
The heat losses caused by the brackets can be reduced, e.g. by using alternative materials, reducing the contact surface of the brackets on the wall and re- ducing the average number of brackets used per m².
Increasing the thickness of insulation also significantly reduces heat losses due to thermal bridging of the substructures.
Principles of EnEV
The merger of the German ordinances relating to heating systems and thermal insulation into one joint ordinance extended the existing reporting framework in two ways: Firstly, by integrating the aspect of the heating technology into the energy equation, the new ordinance also takes account of losses arising during the generation, distribution, storage and transfer of heat. Consequently, the final energy transferred to the limits of the building is the relevant value rather than the useful energy available in the room.
Secondly, the energy demand is evaluated in terms of primary energy consumption by including the losses arising in the generation, transformation and transportation of the respective energy carrier. This is done using a primary energy factor in the building’s energy equation.
Independently of these, the maximum thermal transmittance values measured in [W/m²K] must be met by all components of the building envelope.
To a certain extent, this extended framework makes it possible to offset the performance of the building’s heating system against its structural thermal insulation in the overall energy equation. In other words, poor thermal insulation can be counterbalanced by an efficient heating system or vice versa. The key value required by EnEV for new buildings is the annual primary energy consumption compared to that of a reference building with the same geometry, dimensions and specified technical properties. In addition, the building must comply with a maximum value for transmission heat loss to the heat transmitting exterior surface area. This value is dependent on the type of building.
EnEV also sets requirements for protection against summer overheating and makes it possible to take into account the capture of solar heat.
Calculation method of EnEV
The decision as to whether and how proof must be provided in accordance with EnEV depends, among other things, on whether a new building is to be erected or an existing building altered.
For new buildings with a normal interior temperature (> 19 °C) proof of compliance with the maximum values for annual primary energy consumption stated in Appendix 1 Table 1 of EnEV as well as the specific transmission heat loss must be provided.
For new buildings with a low interior temperature (< 19 °C) or buildings with small volumes (< 100 m³) lesser requirements and a simplified compliance procedure apply. Within the framework of protection against summer overheating, proof of compliance with the key values for solar income energy must be provided.
In the area of insulating the building against summer heat, it is essential to document compliance with solar gain values.
For changes to existing buildings (old buildings) – depending on the scope of the measures – it is either necessary to comply with the required heat transmission coefficients (U-values) (component based procedure) or proof of compliance must be provided for the maximum annual primary energy consumption of the whole building (reporting procedure); these values may be up to 40 % above the threshold values for new buildings.
The requirements for new buildings apply to extensions of heated useful areas amounting to more than 50 m².
EnEV 2014
EnEV 2014 implements the standards set out in the German Energy Savings Act (EnEG 2013), which the German Federal government revised in 2013 in order to implement the European Energy Performance of Buildings Directive (2010) in Germany. Among other things, this directive requires the member states to introduce the low-energy standard for new buildings in accordance with the following timetable:
- public buildings from 2019 and
- all other buildings from 2021.
The EU directive defines a "nearly zero energy building" as one with a defined, very high overall level of energy efficiency.
Its energy requirements should be either nearly zero or very low and primarily supplied using energy from renewable sources – including renewable energy generated on-site or nearby.
EnEV 2014 replaced the previous regulation introduced in 2009 and has introduced more stringent standards. In addition, EnEV 2014 requires a further improvement of around 20 percent in the thermal insulation of building envelopes from 1 January 2016. The benchmark for this standard is the specific transmission heat loss over the enclosing surface (H´t) of the new building measured in (W/m²*K). From 2016, this must be reduced to 80 percent of the previous applicable value.
Permitted thermal transmittance values for external walls in accordance with EnEV
For ventilated curtain walls, compliance with the specified thermal transmittance value for the insulated external wall, in practice, principally affects the design of the thermal insulation and the substructure.
EnEV 2014, Appendix 3 (Table 1) specifies the maximum thermal transmittance coefficient for the various building elements which can transmit heat.
For thermally insulated external walls with VCWs, the applicable thermal transmittance values are stated under 1 "External walls".
However, from 2016, it is expected that the thermal transmittance values required for new buildings in practice will be significantly lower than the maximum values stated in EnEV 2014 as the permitted transmission heat loss for the entire building envelope will be reduced by 20%.
Maximum values for the thermal transmittance coefficient with first-time installation, replacement and renewal of components (Source: EnEV 2014 /Appendix 03 No. 7. Table - Requirements)
| Line | Component | Measure according to | Residential buildings and zones of non-residential buildings with an internal temperature of min. 19°C | Zones of non-residential buildings with an internal temperature greater than 12°C and lower than 19°C |
|---|---|---|---|---|
| Maximum values for the thermal | transmittance coefficient Umax1 | |||
| 1 | 2 | 3 | 4 | 5 |
| 1 | External walls | Nr. 1 sentence 1 and 2 | 0,24 W/(m²·K) | 0,35 W/(m²·K) |
| 2 a | Windows, French windows | Nr. 2 a and b | 1,3 W/(m²·K) 2) | 1,9 W/(m²·K) 2) |
| 2 b | Roof windows | Nr. 2 a and b | 1,4 W/(m²·K) 2) | 1,9 W/(m²·K) 2) |
| 2 c | Glazing | Nr. 2 c | 1,1 W/(m²·K) 3) | no requirement |
| 2 d | Curtain walls | Nr. 6 sentence 1 | 1,5 W/(m²·K) 4) | 1,9 W/(m²·K) 4) |
| 2 e | Glass roofs | Nr. 2 a and c | 2,0 W/(m²·K) 3) | 2,7 W/(m²·K) 3) |
| 2 f | French windows with tilting, folding, sliding or lifting mechanisms | Nr. 2 a | 1,6 W/(m²·K) 2) | 1,9 W/(m²·K) 2) |
| 3 a | Windows, French windows, roof windows with special glazing | Nr. 2 a and b | 2,0 W/(m²·K) 2) | 2,8 W/(m²·K) 2) |
| 3 b | Special glazing | Nr. 2 c | 1,6 W/(m²·K) 3) | no requirements |
| 3 c | Curtain walls with special glazing | Nr. 6 sentence 2 | 2,3 W/(m²·K) 4) | 3,0 W/(m²·K) 4) |
| 4 a | Roof areas including gables, walls adjoining unheated attics (incl. jamb walls), ceilings of uppermost storeys | Nr. 4 sentence 1 and 2 a, c and d | 0,24 W/(m²·K) | 0,35 W/(m²·K) |
| 4 b | Roof areas with waterproofing | Nr. 4 sentence 2 b | 0,20 W/(m²·K) | 0,35 W/(m²·K) |
| 5 a | Walls against earth or unheated rooms (with the exception of attics) as well as floors above earth or unheated rooms | Nr. 5 sentence 1 and 2 a and c | 0,30 W/(m²·K) | no requirement |
| 5 b | Floor constructions | Nr. 5 sentence 2 b | 0,50 W/(m²·K) | no requirement |
| 5 c | Floors above outdoor air | Nr. 5 sentence 1 and 2 a and c | 0,24 W/(m²·K) | 0,35 W/(m²·K) |
1) The heat transmittance coefficient of the component taking into account the new and existing component layers; for the calculation of the component in accordance with lines 5 a and b, the applicable standard is DIN V 4108-6: 2003-06 Appendix E. For the calculation of otherwise opaque components, DIN EN ISO 6946: 2008-04 must be used.
2) Rated value of the thermal transmittance coefficient of the window; the rated value of the thermal transmittance coefficient of the window can be found in the technical specifications for the product or must be defined in accordance with the energy specifications for construction products stated in the state building codes. In particular, these include energy specifications from European technical assessments as well as energy specifications stated in Building Regulation List A Part 1 (Bauregelliste A Teil 1) and those based on the definitions stated in general approvals granted by building authorities.
3) The rated value of the thermal transmittance coefficient of the glazing; please apply footnote 2 accordingly.
4) The thermal transmittance coefficient of the curtain wall must be calculated in accordance with DIN EN 13947: 2007-07.
Calculation of the thermal transmittance value of an external wall with a VCW
The installation of a VCW with a two-part adjustable substructure requires the thermal insulation laid over the full surface of the external wall (undisturbed wall) to be penetrated at various points by brackets.
These points reduce the effectiveness of the thermal insulation and must be taken into account when calculating the thermal transmittance value using the so-called thermal bridge loss coefficient χ.
The point thermal bridge loss coefficient χ [W/K] of a bracket used in the VCW substructure is product-specific and primarily dependent on:
- the design and type of material used in the bracket
- the design and type of material used in the bracket
- the thickness and type of material used in the external wall
The thermal bridge loss coefficients can be found in the technical documentation provided by the supplier of the substructure.
In general, it is possible to reduce the energy losses caused by brackets, e.g. by using alternative materials with lower thermal conductivity, reducing the contact surface of the brackets against the wall and minimising the average number of brackets used per m².
Calculation:
First the thermal transmittance value of the undisturbed wall must be calculated taking into account the thickness and type of material of the external wall and thermal insulation measured in [W/m²K].
Then the type and number of brackets (points at which the thermal insulation is penetrated) per m² must be calculated and multiplied by the product-specific point thermal bridge loss coefficient χ [W/K].
The total thermal transmittance value [W/m²K] of an external wall with a VCW is calculated by adding the thermal transmittance value of the undisturbed wall to the sum of the point thermal bridge loss coefficients.
Wind load
Wind load is one of the factors caused by climatic conditions, which has a variable effect on buildings. It results from the pressure distribution around a structure, which is subject to a wind flow.
It generally acts as an area load perpendicular to the contact surface and is primarily a combination of pressure and suction. The slowing of the air current creates an overpressure on the frontal surfaces exposed to the wind. In the areas of the roof and side surfaces, the air current dissipates at the edges of the building creating an underpressure (suction) at these locations. An underpressure is also generated by the wake vortex on the lee side of the building.
Influencing factors:
Locations:
The key factors influencing the extent of the wind load are those of the location with the local wind climate and the topography. The wind climate is recorded in the Eurocode 1 or DIN 1055-4 standards using a wind zone map, which provides a time-weighted average wind speed for various geographic regions. The topography and nature of the site surrounding the building location are provided in the standards through the terrain categories.
Building geometry:
Additional influencing factors arise from the geometry of the building or component. Wind speed at ground level is practically zero and increases with increasing distance from the ground, i.e. with the height of the building. As well as the height of the building, the geometric form influences the intensity of the forces of pressure and suction. This is taken into account using aerodynamic coefficients.
Wind force:
The resulting wind force on a building or component is calculated as the product of the speed pressure, aerodynamic force coefficients and building surface areas.
Anchoring base
As well as the basic conditions stated above, the anchoring base available for anchoring the ventilated curtain wall, i.e. the material used in the construction of the supporting external wall, is also a key planning consideration.
The selection of the type and number of anchors required depends on the load capacity and condition of the external wall. The lower the load capacity of the anchoring base, the more anchors and therefore brackets must be used in the substructure.
The following diagram provides examples of a selection of commonly used anchoring base materials: