Introduction

The first question that arises when planning buildings is the extent of the insulation thicknesses and therefore the costs when using industrially prefabricated sandwich elements following the introduction of the Order on energy-saving heat insulation and energy-saving systems engineering, known for short as the Energy-Saving Order (EnEV).

The EnEV is to be applied for buildings with:

1. normal internal temperatures > 19°C (§ 2 no 1 and 2)

2. with low internal temperatures < 19°C (§ 2 no 3).

Excepted are stables, operational buildings that are left open for long periods at a time, underground structures, under-glass installations for the cultivation of plants (nurseries), air halls, tents, etc.

Because of the very high heat insulation effectiveness of sandwich elements, especially with an insulation core made from polyurethane, there are no major changes compared with the earlier Heat Insulation Order. In economic constructions with temperatures < 19°C the insulation thicknesses remain unchanged.

As a rule, the heat insulation should only be increased slightly in buildings or parts of buildings that are heated to above 19°C for more than 4 months (with normal internal temperatures). Alternatively, it would also be conceivable to improve the heating system. The incorporation e.g. of a calorific value heating system has a beneficial effect on insulation, meaning that the insulation thickness of the sandwich elements has to be increased by a correspondingly smaller amount.

This emerged from a comparative study by the Forschungsinstitut für Wärmeschutz e.V. Munich of 22/08/2001. The report can be found in GALILEO basic info 7.2 under point 7.2.5 to 7.2.21. This study compared the amendments of the EnEV compared with the Heat Insulation Order from 1995.

Fig. 7.7.1a Infrared photograph in connection area of two different sandwich profiles without thermal bridges

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Fig. 7.7.1b Infrared photograph in corner area of a wall using sandwich construction also does not reveal any thermal bridges

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Fig. 7.7.1c Infrared photograph in inner area of a building using solid construction clearly shows the thermal bridges

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Fig. 7.7.1d Infrared photograph in outer area of a building using solid construction with significant thermal bridges in the façade

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Not only does the EnEV comprise heat insulation, it now also places requirements on the heating system, drinking-water supply, thermal bridges, building impermeability and summer heat insulation, as discussed in GALILEO basic info 7.6 Principles of heat insulation.

Thermal conductivity / heat insulation

Article 7.6 - Principles of heat insulation has already discussed thermal conductivity λ. In Germany a distinction is made between the measured values of thermal conductivity at different temperature and moisture, the nominal value of thermal conductivity and the reference value λBW in DIN 4108-4, table 1.

Compared to the nominal values, the reference values for thermal conductivity include allowances which take into account the influence of temperature, the practical quantity of moisture and the ageing of the material. In the case of insulating materials, the nominal value is declared by the manufacturer on the basis of measured values.

For sandwich elements the reference values λ (the coefficient “BW” for λ is generally omitted) are to be determined in each case by tests during the approval procedure. The result is assigned in accordance with DIN 4108-4, taking into account the allowances. This is currently thermal conductivity group WLG 025. The constant size of the very good arithmetic value of sandwich elements with a polyurethane insulation core of:

λ = 0.025 W/mK

is to be continually verified by external monitoring. To do this, samples are taken directly from the manufacturer’s ongoing production and examined by an independent institute. This ensures constant quality. When building cold stores and deep-freeze stores and on cold-insulation jobs, the slightly better measured values of thermal conductivity λ, which for a PUR insulation core are around 0.022 to 0.024 W/mK, can be used.

Fig. 7.7.3 GALILEO basic info 7.6 deals with the principles of heat insulation

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Fig. 7.7.4 Definition and unit of thermal conductivity λ Source: GALILEO basic info 7.6

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Comparison of thermal conductivity of different building materials

A comparison of sandwich elements with an insulation core made from polyurethane (PUR) with standard building materials reveals the small insulating thicknesses these elements require for the same heat insulation effectiveness.

A 60 mm thick wall with a heat conductivity resistance

R = 2.36 m² K / W

was assumed for the comparison.

The heat transfer resistances Rsi and Rse are not taken into account, because they are equal for all examples. The following figure shows a variety of building materials with the required thicknesses for the same heat insulation effectiveness.

The above examples clearly show the high heat insulation effectiveness of sandwich elements.

Even if in accordance with the EnEV no additional insulation is required - and in buildings with normal internal temperatures only one additional insulation - in economic building, cf. FIW study, page 7.2.1 to 7.2.3, in GALILEO basic info 7.2, a higher heat insulation can obviously be selected at any time in the form of thicker sandwich elements.

An evaluation of economic efficiency must be carried out in each case to determine whether this is worthwhile.

 

Fig. 7.7.5 Different insulating thicknesses of different insulating and construction materials with the same heat insulation effectiveness. Because of the excessive wall thicknesses, the thicknesses in the “gas concrete”, “vertically perforated brick” and “normal concrete” examples have had to be depicted in abbreviated form.

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Thermal bridges

Weak points in the heat insulation of building shells are known as thermal bridges. They cause additional heat loss and in places lower surface temperatures in rooms, thereby reducing comfort levels. They also encourage condensation, which leads to the formation of mildew.

In principle sandwich elements with two metal cover shells and an insulation core made from polyurethane or mineral wool are practically free from thermal bridges, since the two metal shells are only joined via the heat insulation.

Similarly the longitudinal joints are constructed such that the outer and inner cover shells do not touch. This is true of both roof and wall elements.

To transfer this high-heat-insulated construction to the entire shell, careful planning of the joints and careful installation by a specialist firm are required.

Fig. 7.7.6 Practically thermal bridge-free roof and wall elements

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In heated or even higher-heated buildings, possibly also with increased humidity, the roof should not have any overhanging elements, to avoid a thermal bridge over the subshell.

If a larger roof overhang cannot be avoided, the heat flow can be interrupted by a “thermal cut” on the inner roof shell (see Fig. 7.7.7). This is made on site during installation. A static test must be carried out beforehand.

As a general rule, overhanging elements will create thermal bridges. This applies in particular to construction parts made from steel and reinforced concrete. Planners should therefore carefully consider whether overhangs are actually needed when designing a building.

If the external supporting members can be fastened e.g. to a reinforced concrete ring beam or the like, these problems do not exist (cf. GALILEO basic info 4.4 - House with sandwich elements in Vechta).

The following examples are designed to show the problems at various points. They do not, however, replace the planning and installation instructions of the individual manufacturers of sandwich elements (cf. Fig. 7.7.8, 7.7.9, 7.7.10.1 and 7.7.10.2).
 

Fig. 7.7.7 Sandwich roof, overhanging (roof element with thermal cut)

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Detail in Fig. 7.7.7

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Fig. 7.7.8 Solution with thermal bridge on subshell (eaves with sandwich roof and wall element)

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Fig. 7.7.9 Solution thermal bridge-optimised (eaves with sandwich roof and wall element)

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Fig. 7.7.10.1 Solution for low-heated rooms (verge flashing with sandwich roof and wall element)

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Fig. 7.7.10.2 Solution thermal bridge-optimised (verge flashing with sandwich roof and wall element)

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The same rules apply to wall elements as for the roof. For example, overhanging elements in the attic also constitute a thermal bridge.

Since a “thermal cut” is not normally possible for static reasons, rear insulation should at least be provided in the outer area. This applies in particular to overhanging elements (see Fig. 7.7.11).

 

Fig. 7.7.11 Solution with rear insulation of wall elements on parapets

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Influence of fastenings on thermal conduction

In practice the question of the influence of fastenings on thermal conduction in the case of screwed-through roof or wall elements crops up again and again. It can be said in this respect that this suspected high thermal conduction does not occur.

Firstly the fastenings are made from stainless steel, which has a thermal conductivity of only 17 W/mK compared with normal steel at 50 W/mK and secondly the sandwich elements are fastened to purlins or beams with only 2 to 3 screws per metre of purlin.

This gives e.g. for a wall with a gap between beams of 3.50 to 4.00 m just 0.6 to 0.8 fastenings in the span area and a maximum of 1.0 in the corner area per m² of surface.

According to the study carried out by the Forschungsinstitut für Wärmeschutz e.V. in Munich, the difference in heat transition coefficients U on the inside between the fastening and the surface of an 80 mm thick sandwich wall with a polyurethane insulation core, fastened to steel beam HEA 120 for:

•  0.6 screws per m² > U = 0.004 W/(m² K)
• 0.8 screws per m² > U = 0.005 W/(m² K)
• 1.0 screws per m² > U = 0.006 W/(m² K)

The difference in heat transition coefficient U between fastening and surface is therefore far less then 3% and can therefore be ignored in accordance with DIN EN ISO 6946.

If the fastenings are in concrete, the minor influence is reduced and with wooden substructures is no longer detectable.

Progress of surface heat flux

The progress of the surface heat flux can be followed in Fig. 7.7.12. No changes occur on the underside of the beam and at the screw tip.

 

Fig. 7.7.12 Progress of surface heat flux

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Summer heat insulation

Summer heat insulation should guarantee comfortable temperatures in indoor areas and ensure that cooling can largely be dispensed with, especially since this uses 3 to 4 times as much energy as heating does in winter.

Man’s feeling of comfort is influenced not only by the room temperature, but also by the ratio of relative humidity to temperature. Thus high humidity, especially with high temperatures, is felt to be uncomfortable.

In buildings where the internal temperatures are kept low, <19°C, such as workshops and storerooms, verification of summer heat insulation is not required.

For buildings with an internal temperature >19°C the EnEV requires verification if the proportion of window area exceeds 30% of the entire building shell. The requirements of DIN 4108-2:2003-4 must be satisfied at all times, irrespective of the EnEV.

Summer heat insulation is largely dependent on the overall energy transmittance of the transparent outer components (windows, fixed glazing and skylights), the shading they offer, their proportion of the outer shell and the inclination of the windows and skylights.

The north-south orientation of the building, the colour of the shell (dark colours absorb more heat), the ventilation of the rooms, the thermal capacity, including internal components and the heat-conducting properties of the non-transparent outer shell also play a part. Large window areas without sunshades can lead to overheating.

Summer heat insulation can largely be influenced by correct planning to the extent that cooling can be dispensed with in buildings with a normal use.

To maintain a good indoor climate it is also recommended that the building shell be properly insulated:

• The planning of compact buildings with as few outer surfaces as possible.
• The planning of adequate ventilation options, so that the warm indoor air can be exchanged for cool outdoor air in the 2nd half of the night.
• The installation of heat-storing components in the interior as solid components in floor and partitions, to delay any increase in temperature.
• Keep window areas as small as possible and where necessary provide a rear-ventilated sunshade on the outside.

 

Fig. 7.7.13 Link between air temperature and relative humidity for people’s feeling of comfort (after W. Bender, Lexikon der Ziegel, Bauverlag Wiesbaden)

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In accordance with DIN 4108-2:2003-4, the following limits for internal temperatures may not be exceeded by more than 10% of the period of stay (in residential buildings per 24 hrs and in office buildings per 10 hrs).

The summer outdoor climate in Germany was divided into the three following summer regions (see map p. 7.7.11).

Fig. 7.7.14 Because of man’s adaptation to the prevailing external climate, it was decided to vary the internal temperature limit. If the same requirements were placed on the summer climate in all regions as in the cool-in-summer region, no sufficient window sizes could be allowed for daylighting in the warmer regions.

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Map of Germany with summer climate regions for evidence of summer heat insulation see Fig. 7.7.15 on page 7.7.11.

Quality

There is no standard for actual sandwich constructions. An “allgemeine bauaufsichtliche Zulassung” (national technical approval) must therefore be applied for from the Deutsche Institut für Bautechnik (German Institute of Civil Engineering) for each element.

The constant quality of the elements is guaranteed through constant self-monitoring as well as external monitoring by an independent institute.

The licence covers heat insulation, static concerns with material qualities, flammability and more. Both the building owner and worker therefore receive checked prefabricated components of consistently high quality, which is guaranteed by the Ü-mark (conformity mark).

Summary

In conclusion it can be said that the stricter requirements of the Energy-Saving Order EnEV, together with the currently applicable DIN standards, have only a minor effect on sandwich elements, with no appreciable increase in the cost of using these elements.

Fig. 7.7.15 Summer climate regions that apply for evidence of summer heat insulation

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Alongside heat insulation and the avoidance of thermal bridges, the impermeability of the building shell also plays an important role according to the EnEV. One of our next articles (GALILEO basic info 7.9) will address the subject of building impermeability.