Opaque building envelope

Hans Erhorn and Johann Reiss 9.1.1 Concept

Typically, 50 per cent to 75 per cent of the heat losses of conventional buildings results from transmission losses through the building envelope. These losses can be drastically reduced - for example, in Germany a 50 per cent reduction has been achieved since 1970.

This reduction has been halved again by high-performance houses. The transmission losses of a typical house (with 1.5 to 2.0 m2 of building envelope per m2 heated floor area) can be expected to be less than 0.3 W/K per m2 floor area. Air leakage and natural ventilation losses can amount to 0.4 W/m2K. These can be reduced to as low as 0.1 W/m2K by making the envelope air tight and adding an energy efficient ventilation system. The resulting total losses due to transmission and ventilation can then be as low as 0.3 and 0.5 W/K per m2 of heated floor area, as can be seen in Figure 9.1.1.

Source: Fraunhofer-Institut Bauphysik

Figure 9.1.1 Development of the mean U-values of building envelopes (including windows) of buildings in Germany over the last 35 years

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Wooden lightweight construction

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Limestone with composite thermal insulation 40 cm U = 0.18 W/m2K

Source: Fraunhofer Institut für Bauphysik, Stuttgart, www.ibp.fraunhofer.de

Figure 9.1.2 Comparison of different wall construction types achieving the same U-value by means of varying materials and dimensions: (a) wooden lightweight construction (30 cm; U = 0.18 W/m2K); (b) limestone with composite thermal insulation (40 cm; U = 0.18 W/m2K); (c) core insulation (45 cm; U = 0.18 W/m2K); porous bricks (52 cm; U = 0.18 W/m2K)

Also important is how much heating power is needed. The heating peak load of such high-performance houses range from 6 W/m2 in mild climates (design temperature 0°C) to 15 W/m2 in cold climates (design temperature -30°C). This amounts to only one third to one fourth of the heating power required by conventional new housing.

Numerous building envelope constructions achieve this impressive performance. Roofs radiate house heat to the cold winter sky, which can be up to 50 K colder than the ambient temperature. In the summer, they receive the strongest solar heating due to the high sun angles. This is why roofs are usually the most highly insulated envelope component.

External walls

Different wall constructions can achieve the same insulation value, but result in greatly varying thicknesses. Figure 9.1.2 shows constructions ranging from 33 cm to 52 cm.

Figure 9.1.3 shows an innovative, advanced development of the wooden lightweight construction (Kluttig et al, 1997). I-studs or I-joists (TJI) are used to reduce the wood ratio of the construction to

Gypsum plasterboard

Wood based panel

Timber frame construction, resp.

Mineral fibre WLG-035

Vapour barrier

Wood based panel

Wooden girder 34/340 mm, resp.

Mineral fibre WLG-035

Wlndproofing

Wood-wool building slab

Undercoat rendering

Cement plaster finish

Figure 9.1.3 Improved lightweight constructions: (a) energy-optimized construction of the external wall of a low energy house; (b) vertical section through the external wall of a high-performance house

Source: Kluttig et al (1997)

Figure 9.1.3 Improved lightweight constructions: (a) energy-optimized construction of the external wall of a low energy house; (b) vertical section through the external wall of a high-performance house

Source: Kluttig et al (1997)

improve the U-value still further. U-values of less than 0.1 W/m2K can be realized with such construc-Cellar ceilings/slabs

Over the year, the temperature difference between unheated cellars or the ground and heated space is only about 30 per cent to 80 per cent of the difference to the ambient air. This heat loss source can, however, reduce the effectiveness of the insulation of other parts of the building envelope with regard to heating energy demand.

A double-layered arrangement of the insulation has turned out to be an effective solution. The floor's acoustic insulation can be increased. Under-floor suspended plumbing and ducts can be insulated in this manner. A second insulation layer below the basement ceiling or slab on grade reduces the main transmission losses and thermal bridges from connecting walls or the foundation. Because of these thermal bridges, the U-values of the cellar or slab should not exceed 0.2 W/m2K.

The complete building envelope

Figure 9.1.4 presents the floor area-related heat loss of the different external building parts of a typical high-performance house. The losses are covered by the gains represented in the first column. Windows and ventilation cause more than 60 per cent of the losses (though windows also deliver passive solar gains). Once the opaque surfaces are well insulated, they lose little heat by comparison.

Figure 9.1.4 presents the floor area-related heat loss of the different external building parts of a typical high-performance house. The losses are covered by the gains represented in the first column. Windows and ventilation cause more than 60 per cent of the losses (though windows also deliver passive solar gains). Once the opaque surfaces are well insulated, they lose little heat by comparison.

Source: Fraunhofer Institut für Bauphysik, Stuttgart, www.ibp.fraunhofer.de

Figure 9.1.4 Presentation of the heat losses of different building parts of a highperformance house related to the floor area in moderate climates (Germany)

Investment costs for the different parts of the building envelope relative to insulation qualities are presented in Figure 9.1.5 (Erhorn et al, 2000). Achieving good insulation of the opaque envelope costs much less than is the case, for example, with windows. Generally, the roof has the lowest investment costs, followed by the cellar ceiling and the external wall. The rate of increase in costs relative to the improved insulation levels is very moderate. Increasing the insulation levels from conventional to high-performance may cost only €30 to €50/m2. Furthermore, some of these added costs can be offset because the heating plant and distribution system can be smaller.

Source: Fraunhofer Institut für Bauphysik, Stuttgart, www.ibp.fraunhofer.de

Figure 9.1.5 Comparison of investment costs and thermal insulation quality of the different parts of the building envelope in Germany (cost basis 2002)

Solution development into detail is important

The benefit of increased insulation can be reduced by as much as 25 per cent by thermal bridges. In the case of a 150 m2 single family house, the increase in heating energy demand can be up to 2000 kWh/a for a moderate climate.

Avoiding thermal bridges may save more energy at less cost than investments in a more expensive heating plant, added insulation or a solar system!

9.1.2 Variations of material

Thermal insulation materials insulate because they entrap still air. Indeed, the thermal conductivity of the insulation materials is close to that of a non-ventilated air space (X= 0.024 W/mK). Numerous insulation materials are available: mineral wool and fibreglass insulation are mineral materials; bulk insulation is mostly of volcanic origin. Polystyrene and polyurethane are synthetic materials produced by the chemical industry. Natural insulation materials include cork, wood, hemp fibres, cellulose, cotton and sheep wool. An overview of the properties and characteristics of different insulation materials is provided in Table 9.1.1 (Energiesparen im Altbau, 2000).

The thermal conductivity of an insulating material can be reduced in various ways. For example, an inert gas can be entrapped in the cellular structure of insulation. Another approach is to embed an infrared efficient substance, such as graphite, to reduce the radiation exchange across voids in the material structure. Extremely effective is the evacuation of the cell voids of a material. The conductivity of high-performance insulation materials can thus be reduced to less than 0.01 W/mK.

9.1.3 Example insulation systems

Graphite-embedded EPS (Neopor)

The thermal conductivity of insulation material is influenced by the skeleton structure of the foam; the lighter the foam, the higher the thermal conductivity (because of the higher air-gap ratio). Embedding graphite in expanded polystyrene (EPS) achieves a comparable insulation effect at a very low density. The radiant heat transfer across the cell pores is hindered, resulting in to up to a 20 per cent reduction in the conductivity, as shown in Figure 9.1.6. Compared to conventional EPS, less than half of the raw material is needed to achieve the same resulting insulation effect (see www.basf.de).

Table 9.1.1 Properties of insulation materials

Insulation Thermal material conductivity

Ingredients

Long-term performance

Recycling capability

Health aspects

Remarks

Cork

Coco

Wood-fibre boards

Cellulose

G.G45

G.G445

G.G45 G.G4G

Foam giass G.G4G-G.G55 Rock wooi G.G35-G.G4

Fibregiass G.G35-G.G4

Extruded 0.03-0.04 polystyrene

Expanded 0.035-0.04 polystyrene

Polyurethane 0.025-0.035 boards

Perlite 0.055-0.07

Bitumen

Ammonium sulphate (fire protection) Soft wood

Borax; boric acid

Quarry sand; recycling glass

With wetness fungal decay

Like massive wood

No mould; pest resistant

Ageing resistant; pest resistant Un-decayable; pest resistant

Un-decayable; pest resistant

Yes, biodegradable Yes

Durable; pest resistant

Not UV-resistant; moisture resistant; un-decayable 2

Reusabie; not recyciabie Rareiy reusabie or transferabie

Rareiy reusabie or transferabie

Possibiy Permanent carcinogenic eiastic; use oniy (Benzoapyrene) tar-iess products

Possibly Pay attention to containing PCB dust-poor

(printing ink); installation fine dust in high concentration critical

Reusabiiity unknown Partiy recyciabie

Durable

Possibly increased radio activity

Insulation material with KI > 40 or without harm-lessness certificate should not be used; Insulation material with KI > 40 or without harm-lessness certificate should not be used

If burning toxic gases If burning toxic gases

Non-recyclable

Incorrectly mounted status harmless

Incorrectly mounted status harmless

Source: Energiesparen im Aitbau (2GGG)

Source: www.basf.de

Figure 9.1.6 Comparison of the thermal conductivity of standard expanded polystyrene (EPS) and graphite embedded material

High-performance brick constructions

The brick industry has developed new bricks specifically for high-performance houses. One strategy is to optimize the hole configurations and reduce the conducting bridges in cross-section. Figure 9.1.7 shows two such constructions with conductivities below 0.09 W/mK.

Another strategy is to produce the brick in an extrusion process. This leads to a new foam material with conductivity below 0.04 W/mK. A monolithic brick in this construction can have a U-value below 0.1 W/m2K (www.wienerberger.de).

Source: Fraunhofer Institut für Bauphysik

Figure 9.1.7 (a) Optimized hole configurations; and (b) reduced brick piers in high-performance brick stones

Source: Fraunhofer Institut für Bauphysik

Figure 9.1.7 (a) Optimized hole configurations; and (b) reduced brick piers in high-performance brick stones

High-performance plaster systems

The plaster industry has also improved the properties of plaster layers to be suitable for high-performance houses. One strategy is to integrate glass bubbles within the plaster mix. This application is known from the transparent insulation industry; but the advantages of the transparent plaster alone on high-efficiency brick constructions had not been studied. Figure 9.1.8 shows an application on brick stones. Absorption of solar radiation and convective heat transfer to the outside air no longer occur at the same layer. This improves the gains from both direct and diffuse solar radiation. The measured heat losses from such a wall are 15 per cent to 25 per cent lower than a conventional plaster system (www.sto.de).

Source: Fraunhofer Institut für Bauphysik

Figure 9.1.8 Application of glass-bubble plaster compared to conventional plaster on brick

Source: Fraunhofer Institut für Bauphysik

Figure 9.1.9 Application of an infrared-coated plaster system with lotus effects

Source: Fraunhofer Institut für Bauphysik

Figure 9.1.8 Application of glass-bubble plaster compared to conventional plaster on brick

Source: Fraunhofer Institut für Bauphysik

Figure 9.1.9 Application of an infrared-coated plaster system with lotus effects

For plaster on the interior face of walls, phase-change materials (PCMs) can be integrated within the structure. In one example, micro-encapsulated wax droplets are mixed into the plaster, as shown in Figure 9.1.10. As a result, the thermal capacity of the wall is increased. This reduces temperature peaks, decreasing the risk of overheating in summer. It can also increase the usability of passive solar gains in winter. This technology is still under development (see www.ise.fhg.de/english/ press/pi_2001/pi05_2001.html).

Source: www.ise.fhg.de/english/press/ pi_200Vpi05_2001.html

Figure 9.1.10 Principle scheme of micro-encapsulated phase-change material (PCM) in plasters

Polyurethane (PU) insulation systems

Polyurethane (PU) is a hard-covered plastic foam cell structure derived by using an expanding agent. The thermal conductivity varies between 0.025 and 0.035 W/mK. The material is heat resistant up to 90°C. Typical applications are roofs, ceilings or interior walls (see www.ivpu.de).

Table 9.1.2 Typical applications of polyurethane insulation systems

Fields of Step Step application roof, roof, above below rafters rafters

Flat Terrace/ Gradient Ceilings/ Attic roof parking roof walls deck

Floor Internal Inner Industrial insulation ceilings buildings

PU products with Special paper WLG 035

Mineral fleece WLG 030

Aluminium lamination WLG 025

Composite film WLG 025

Composite film WLG 030

Non-laminated boards WLG 030

Composite elements WLG 030 Formed components In-situ foam

i different coatings □ □

Vacuum insulation systems

Evacuated building elements enclosing a vacuum are now on the market. As a result, the thermal conductivity is reduced by more than factor 11 compared to conventional materials, as illustrated in

Source: www.vip-bau.ch

Figure 9.1.11 Vacuum insulation material panel compared to mineral wool panel of the same thermal resistance

Figure 9.1.11. This allows much slimmer constructions for high-performance houses. One construction uses an evacuated silica gel covered by a high-performance aluminium foil. The gas pressure in the construction is approximately 1 mbar; the leakage rate is predicted to be below 2 mbar per year. For protection during transport and at the building site, the units are glued into a polystyrene panel. A disadvantage of such a construction is the limitation of sizes available and the fact that the panels can not be cut to fit at the building site. Another factor limitation, to date, has been the fact that such panels cost ten times more than conventional insulation. Within the framework of International Energy Agency (IEA) Energy Conservation in Buildings and Community Systems (ECBCS) Annex 39, the quality of vacuum insulation panels (VIPs) and possible applications were investigated (see www.ecbcs.org).

Source: www.marmorit.de

Source: Fraunhofer Institut für Bauphysik

Figure 9.1.12 Exemplary wall-ceiling joint with minimized thermal bridge effect and optimized air tightness

Figure 9.1.12 Exemplary wall-ceiling joint with minimized thermal bridge effect and optimized air tightness

Source: www.marmorit.de

Figure 9.1.13 Daylight wedges in insulation systems allowing a better use of daylight: (left) window in a wall with 'light-wedges'; (above) horizontal section of the window in the wall

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