The Building - the envelope

Building materials

The material input contributes greatly to the indoor air quality and energy consumption in a building. Insulation properties and heat storage capacity are both important energy factors, and clever dimensioning of new building constructions can also help:

• reduce energy consumption,
• even out fluctuations in temperature,
• prevent overheating in summer,
• shorten the warm-up period in a building.

Fig. 4: Temperature flow in a wall with conventional thermal insulation

Savings of up to 20 per cent in heating costs and an improvement in thermal comfort during the summer can be achieved when there is a well-balanced ratio between solar gains and the storage capacity of a building. Transparent insulation materials not only reduce thermal heat lost through conventional insulation within the envelope but also benefit from solar gains, thus leading directly to a reduction in heating costs.

Remarkable results can be found under the Austrian Program on Technologies for Sustainable Development in the subprogram "Building of Tomorrow":

• Wall Systems made of Renewable Resources
• Renewable Resources in the Building Sector

Heat insulation capacity

The heat insulation capacity of a building material is measured by its thermal conductivity (W/mK).

The thermal conductivity represents the amount of heat flow per second through 1m² of building material measuring 1m thickness with a 1 Kelvin difference in temperature between the inside and outside surfaces. The lower this value, the better the insulating effect of the building material. The U value (former K value) of a building material is gained by dividing thermal conductivity by the thickness of a building material.

Further information:

• See chapter "Insulation systems with higher levels of material thickness"
• The European Building Data Bank provides a Program for the Calculation of U values of building constructions
• Basic research on fixing elements: drawing up universal design guidelines for high resilience mechanical jointing techniques for insulation products
• Research work on outside sprayed-on cellulose insulation covered with plastering

Thermal storage and summer overheating protection

Heat insulation measures alter the heat storage capacity of building components. The storage mass does not have a strong influence on the heating energy demand in our Central European climate. Buildings should therefore have external insulation. Changes in the thermal storage capacity in buildings can occur when using internal insulation.

• The storage mass can store excess heat during periods of hot weather.
• A reduction of the storage mass will shorten the warm-up time.

The heat storage capacity of a building material is defined by its specific heat capacity c and expressed in storage mass per square metre of building component surface (kg/m²).

The effective storage mass depends on specific heat capacity, bulk density, limit depth and thermal conductivity of each building material. There is no further notable increase in heat storage capacity at the limit depth. This value amounts to approx. 20 cm with solid brick, hollow brick or light-weight concrete. The uppermost 5 to 10 cm of material thickness are important for a typical one-day period of heat storage. Further factors considerably contributing to the overall storage capacity are ceilings, floors, interior walls and furniture.

The room temperature in living spaces should be at least 3K but no more than 6K below the maximum external temperature on hot summer days and especially on humid days.

A simplified procedure published by the Austrian Standards Institute (ÖNORM B8110-3) can be used to calculate overheating levels. It takes into consideration the following factors, regarding the warm-up time of buildings in summer:

• size of transparent surfaces (glazings)
• effect of sun protection
• amount of natural ventilation in the interior space
• storage mass.

The higher the contribution of solar energy to interior space heating during winter, the more important it is to calculate it in advance.

Airtight constructions

Airtightness is becoming an important consideration in the construction of both new buildings and the thermal refurbishment of existing buildings. Current building standards state that roofs should be airtight, but this can rarely be realised in practice.

Measures to achieve airtightness (and moisture barrier) are always positioned on the warm side of the roof construction facing the interior, whereas measures to achieve windtightness are always positioned on the cold side of the construction. Non-airtight sections in the building envelope, like, for example, in the moisture barrier or window constructions result in

• large-scale thermal losses,
• the risk of building damage due to moisture build-up,
• a too dry indoor room quality in winter,
• less overheating protection in summer,
• poor sound insulation and
• uncontrolled air change.

Thermal loss through air gaps

Insulation can drop to 4.8 times its initial value when there's an air gap measuring 1 mm in width and 1 m in length. Up to 800 g moisture can get into the roof construction through such an air gap per day and square metre - in comparison to only 0.5 g with an airtight moisture barrier.

General conditions:
Room temperature +20 ° C,
External temperature -10 ° C
Pressure difference 20 Pa = Wind velocity 2 - 3

Measuring values:
Without air gap: U value = 0.3 W/m2K,
With 1 mm air gap: U value = 1.44 W/m2K

Source: Thermal losses with a 1 mm air gap (Measurements carried out by the Fraunhofer Institute for Building Physics in Stuttgart)

Vapour diffusion

An important pre-requisite for the correct functioning of thermal insulation is to avoid humidity penetration. Vapour usually diffuses from the warm to the cold side of a building or from the side with a higher level of humidity to the side with the lower level of humidity. This means that the vapour diffuses from the interior to the exterior of the building envelope in winter. The resistance offered by a building component to the vapour being transported through the thermal insulation is shown in relation to the resistance in the air (=1) and regarded as the vapour-diffusion resistance indicator.

The diffuion resistance of a complete building component is defined by the equivalent diffused air space thickness, which shows the thickness of air spaces with the same diffusion resistance in m. The eqiuvalent diffused air space thickness is calculated by multiplying the thickness of the building components with the diffusion resistance.

Equivalent diffused air space thickness (sd) = Thickness of Building Components (in m) x Diffusion Resistance

Example:
Masonry 30 cm, Hollow Brick: sd = 0,3 x 10 = 3 m
PE-Foil 0.2 mm: sd = 0,0002 x 100.000 = 20 m

Grey energy

The energy consumption connected with the manufacture of these building materials is known as the "Grey Energy" of building materials. A more elaborate definition of grey energy also takes into consideration the energy consumption used in the transport, construction, demolition and disposal of building materials. The level of this type of energy consumption in low-energy houses is comparable to the amount of energy consumed throughout the whole period of occupancy.

Further information:

More data can, for example, be found in the IBO-Bauteikatalog - IBO Catalogue of Building Components at Österreichisches Institut für Baubiologie und -ökologie - the Austrian Institute for Building Biology and Ecology.

Windows / Daylighting

Windows and window frames usually represent the weakest point in an energy-efficient building (not even the most innovative window construction can compete with an insulated wall that meets the current building standards considering the U value). Correctly sized windows are, however, able to make optimal use of natural daylight and benefit from solar gains (thermal comfort in summer also has to be taken into consideration).

The dew-point temperature amounts to up to 9.3°C at an indoor temperature of 20 °C and relative humidity of 50 per cent. A temperature of 8.4°C is reached on the interior surface of double-insulating glazings at an exterior temperature of -10°C. Condensation thus forms on the whole window pane, and the result is that condensation forms in the corner sections of the window pane at external temperatures that do not even have to be extremely low due to the fact that the frame is one of the weakest points in the construction.

The glazing systems of passive houses usually have triple heat protection glazings, which are filled with an inert gas and have a special coating on the inside of the window pane. Net energy gains can be made in the heating period when using such south-facing, non-shaded glazing systems. The U value should thus at least amount to 0.8 W/m²K and the G value (overall transmission coefficient) 50 per cent.

Window frame

The U value of windows is based on the value in the centre of the window and does not take into consideration the window frame, a weak point in terms of energy. The U value will amount to about 15 to 20 per cent above the "normal" value when calculating the conventional aluminium window frame depending on the size of the window.

• a larger overlap of the window frame with the window pane will reduce this "thermal bridge".
• "Warm Edge"- thermo-edges made out of high-performance plastics, e.g. TS-Thermo Spacer, will help to minimize the problem of the frame. The additional expenses may be relatively low, but the effect is extremely high. The insulation of the windows is definitely improved and the formation of condensation is almost completely excluded.

Daylighting

Daylight systems make use of specular systems, prismatic elements and Venetian-blinds do not only divert and shade solar insolation, but also to optimise the use of natural light and lower the use of artificial light.

Innovative daylight systems have also been designed for various different uses and help:

• transport daylight into greater depths of the rooms.
• bring more daylight into rooms in cloudy climatic zones.
• use more daylight in extremely hot and sunny climatic zones where sun protection is required.
• increase the use of daylight in buildings where the solar insolation cannot directly enter the building.
• transport daylight into rooms without windows.

Fig. 5: Optimised use of natural daylight with the help of sun protection screens in the "Design-Center" in Linz (Photo: Bartenbach)

Further information:

• Research Forum 3/2000
• IEA Solar Heating & Cooling Programme - Task 21 Daylighting
• Bartenbach Lichtlabor

Thermal bridges and the airtightness of the building envelope

Thermal bridges are areas in the external building envelope with a notably increased heat flow. Geometric thermal bridges are created due to the building shape, whereas constructive thermal bridges are created out of special constructions within the external building envelope. The importance of heat losses due to thermal bridges increases with the better insulation of the building. These areas are also at more risk of getting damp.

Thermal bridges usually occur where an exterior wall meets the uppermost ceiling of a building, at the window flannings (window head, side parts, window sill) or at the connection between an exterior wall and a ceiling (especially with overhanging balconies).

Quality control

The comparison between the actual and calculated energy requirements for heating provides rather late and unreliable results on the incorrect use of energy systems in buildings. The following methods can be used to localise weak spots in energy systems, which are either created by the incorrect use of energy systems or bad planning in the construction of the building:

• Thermal Imaging: Thermal imaging registers the distribution of the surface temperature in a building and evaluates the thermal properties (thermal bridges, tightness) without coming into contact with the surface area.
• "Blower Door" tests the airtightness of the building envelope by using tests based on the differential pressure method.

Thermal imaging

Thermal imaging is a measuring technique that changes the invisible thermal infra-red radiation an object sends out into a visible diagram - known as a thermal imaging diagram. A thermal imaging diagram of a building construction thus registers the distribution of the surface temperature in a building and evaluates the thermal properties (thermal bridges, tightness) without coming into contact with the surface area. Thermal imaging tests should be carried out before renovation work or building extensions and provide information about weak points and critical areas as well as presenting new material on the history of the construction. Measuring techniques can be carried out in combination with the blower-door-test. These mainly show non-airtight sections in light-weight building constructions, roof extensions, window and door casings. Ideal conditions for carrying out these measurements are provided at night-time and by an external temperature of under 5°C (bearing in mind that there should not have been major fluctuations in temperature prior to measuring).

The following factors have to be taken into consideration when evaluating a thermal imaging diagram: Fluctuations in temperature, solar insolation, different capacities of material emission, wind velocity, thermal reflections.

Fig. 6: Thermal imaging picture of a building construction in need of thermal refurbishment (Source: Grazer Energieagentur)
Thermal imaging pictures show the critical areas - window flannings and lintels. Red, yellow and green till light blue are all colours that signify high thermal loss.

Further information

arsenal research is one of many companies to provide thermal imaging measuring techniques.

Blower-Door-Test

Airtightness is state-of-the-art in the technology of today (Austrian Standards: ON B 8110 Part 1). The airtightness of the building envelope can be tested by carrying out the blower-door-test, according to ISO 9972. This test is carried out prior to the installment of interior panelling, as improvements to the vapour barrier could otherwise not be made.

Air in non-airtight sections of roof constructions flows from the interior to the exterior of the building envelope in the winter due to the fact that the warm air rises. The normally humid indoor air temperature cools off quickly in these non-airtight sections. Condensation then forms and is deposited in the neighbouring building component (convection) - and damage to the building structure is inevitable.

Further information

The following companies provide blower-door-tests:

• arsenal research, Vienna
• Fa. Isocell, Neumarkt/Wallersee
• Fa. Hofer&Weratschnig, Wels

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