Passipedia EN

Appendix A - Punctiform penetrations of thermally insulated exterior walls

Passivhaus Institut, Darmstadt

For estimating how high the additional heat loss due to fastening elements that penetrate the thermally insulated outer shell is, $\chi $-values for steel and high-grade steel anchors are provided in the following section. Two insulation thicknesses and two anchoring substrates are considered. The results are summarised Fig. A1. Here one can see the significant reduction in the $\chi $-value when high-grade steel is used instead of structural steel.

$\chi $-values for penetrations with structural steel and high-grade steel in masonry walls and reinforced concrete walls with an insulation thickness of 25 and 30 cm

In the case of structural steel which is more thermally conductive, the thermal conductivity of the anchoring substrate has a greater effect than the insulation thickness of the exterior wall. This applies in particular to the larger rod diameters. On the other hand, with stainless steel the thermal conductivity of the anchoring substrate has less influence than the insulation thickness. If thin anchor cross-sections (up to approx. 10 mm) are used, the influence of the insulation thickness and the anchoring substrate can be disregarded for both steel and high-grade steel elements. This is illustrated by the following example: a façade is attached using four steel anchors per m², each with a diameter of 6 mm. Table A1 shows the corresponding $\chi $-values and the impact on the U-value of the wall:

Exterior wall	Insulation thickness	U-value of the uninterrupted wall	$\chi $-value of each anchor	U + ΔU	Increase in the U-value
Masonry wall	30 cm	0.127 W/(m²K)	0.0035 W/K	0.141 W/(m²K)	11.1 %
Masonry wall	25 cm	0.151 W/(m²K)	0.0040 W/K	0.167 W/(m²K)	10.6 %
Reinforced concrete	30 cm	0.129 W/(m²K)	0.0037 W/K	0.143 W/(m²K)	11.5 %
Reinforced concrete	25 cm	0.153 W/(m²K)	0.0042 W/K	0.170 W/(m²K)	11.0 %

Table A1: Worsening of the U-value of the uninterrupted exterior wall due to ventilated rainscreen cladding with four steel anchors per m² each with a diameter of 6 mm

The results are presented in detail in the following illustrations. The boundary conditions of the calculations can be seen in Table A2:

Components	Thermal conductivity [W/(mK)]
Plaster	0.7
Thermal insulation	0.04
Sand-lime bricks	1.0
Reinforced concrete	2.3
Structural steel	50
High-grade steel	17

Table A2: Characteristic values of materials

Fig. A2: $\chi $-values for structural steel and high-grade steel penetrations in exterior walls consisting of sand-lime bricks with a thermal insulation thickness of 30 cm

Fig. A3: $\chi $-values for structural steel and high-grade steel penetrations in exterior walls consisting of reinforced concrete with a thermal insulation thickness of 30 cm

Fig. A4: $\chi $-values for structural steel and high-grade steel penetrations in exterior walls consisting of sand-lime bricks with a thermal insulation thickness of 25 cm

Fig. A5: $\chi $-values for structural steel and high-grade steel penetrations in exterior walls consisting of reinforced concrete with an insulation thickness of 25 cm

In addition to steel anchors and rods, brackets are also often used to attach façades or balconies. The extent to which the -values of round cross-sections can be transferred to angular cross-sections is analysed in another calculation (Fig. A6).

Fig. A6: position of a steel bracket on the exterior wall

In doing so, it is assumed that the cross-section of the steel rod and steel bracket penetrating the insulation remains the same (Fig. A7).

Fig. A7: Cross-section area of the steel rod and steel bracket

A relatively good correlation was seen for penetrating cross-sections. The longer and thinner the cross-section is, the greater the deviation from the $\chi $-value of the round cross-section. As a rough estimate, the $\chi $-value of the equivalent round cross-section can be increased by 50 % for unfavourable angular cross-sections.