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 firsttime 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 nonresidential buildings with an internal temperature of min. 19°C 
Zones of nonresidential 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 41086: 200306 Appendix E. For the calculation of otherwise opaque components, DIN EN ISO 6946: 200804 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: 200707.
Calculation of the thermal transmittance value of an external wall with a VCW
The installation of a VCW with a twopart 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 (except for substructures with no thermal bridges) and must be taken into account when calculating the thermal transmittance value using the socalled thermal bridge loss coefficient χ.
The point thermal bridge loss coefficient χ [W/K] of a bracket used in the VCW substructure is productspecific 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 productspecific 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.