4 Insulating Glazing Terminology

Definition of insulating glazing

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Insulating glazing has been around for about 60 years. The oldest patent on this subject actually originates from the year 1865.

The official definition of the term »insulating glass unit« is given in EN 1279-1:

“A multiple-glazed insulating glass unit is a mechanically stable and

durable unit consisting of at least two glass panes separated by one or more spacer bars and hermet-ically sealed at the edges.” There is no vacuum in the closed space between the panes, as is often incorrectly assumed, only dry air or a special gas.

4.0 Insulating glazing types 4.1 U value according to EN 673 4.2 Emissivity value

ε

according to EN 673 4.3 g value according to EN 410

4.4 Colour rendering index Raaccording to EN 410 4.5 Light transmittance

t

V according to EN 410 4.6 Energy absorption 4.7 b factor 4.8 Selectivity ratio S 4.9 Weighted sound reduction index RW 4.10 Double-glazing effect/ insulating-glazing effect 4.11 Interference effects

with insulating glazing 4.12 Anisotropy

4.13 Dew-point 4.14 Glass thickness

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4.0 Insulating glazing types

1. All-glass insulating glazing These products, such as “Gado” and “Sedo”, were manufactured by heating the edges of two glass panes to melting point, bending them and fusing them together. The space between the panes was then filled with dry air or gas, and the filling holes were subse-quently sealed.

2. Soldered insulating glazing In this system, e.g. “Thermo-pane”, two panes were copper-plated at the edges and soldered with a thin lead bar. The space between the panes usually did not contain any desiccants. It was flushed dry and the flushing holes were then soldered.

3. Insulating glazing with an organically bonded edge seal

Insulating glazing is produced with either single or dual edge seals. An insulating glass unit with a single edge seal has a perforated spacer frame made of aluminium or galvanised steel filled with a highly active absorbent (desiccant). The gap between the spacer frame and the pane edges is filled with permanently elastic sealant. Thermoplastic sealants such as “Hot Melt” are also used, mainly with smaller pane dimensions. With these hot-melt adhesives, the mechanical stability and density are reduced drastically as the tempera-ture increases.

For insulating glazing with dual edge seals, such as INTERPANE insulating glazing, perforated spac-ers filled with highly active absorb-ents (desiccant) are also used. The contact surface of the spacer frame is initially completely coated with a duroplastic sealant based on polyisobutyl (butyl). This inner pri-mary sealant is used essentially to prevent water vapour from pene-trating into the space between the panes and loss of the filling gas. Butyl has very low diffusion rates for water vapour and gas. As the second stage, the space outside the spacer frame is filled with per-manently elastic sealant up to the edges of the panes. Polysulphide polymer (thiocol) and poly-urethane are commonly used secondary sealants.

Siliconeis used as a sealant for glazing with an exposed edge seal, e.g. in overhead or structural glazing. However, silicone has a considerably higher diffusion rate for the filling gases generally used.

Warm edge and alternative edge seals

The German Energy-Saving Law rewards the reduction of thermal bridges. With “its”, INTERPANE presents an edge seal that mini-mises the heat losses at the edge of an insulating glass unit. Due to its lower thermal conductivity, a specially developed stainless steel spacer is used to replace the conventional aluminium spacer. The “its” system optimises the well-proven principle of the dual edge seal by combining it with a bent metal spacer.

In addition to stainless steel spacers, polymer spacers with metal diffusion barriers (such as Thermix, TGI) have also proven to be successful in reducing thermal bridges at the edges of insulating glass units.

As an alternative to the solutions mentioned above, other edge seal systems with similar thermal characteristics are also offered on the market. The TPS system (ThermoPlastic Spacer) is men-tioned as an example. With the TPS system, the desired distance between the panes is determined by a thermoplastic sealing com-pound rather than an aluminium spacer. At the same time, the sealing compound acts as the primary sealant for the space between the panes.

The demand for guaranteed sup-plies of material, further thermal optimisation and automated pro-duction processes means that new types of edge seal systems can be expected in the future. The beginning of this trend can be seen, for example, in the use of reactive hot-melt adhesives and the development of a single-com-ponent TPE (ThermoPlastic Elastomer).

In addition to professional instal-lation, the long-term stability of the insulating glass unit is deter-mined by the quality of the edge seal. Three different types of insulating glazing can be differ-entiated according to the edge seal, whereby the first two are seldom manufactured nowa-days.

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2

3

welded soldered dual edge

4.1 U value according to EN 673

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New standards became binding with the German Energy-Saving Law of 2002. The well-known k value was replaced by the U value. This did not have any effect on the significance of the thermal trans-mittance for building science. The thermal transmittance (U value) specifies the heat flow per unit time through 1 m2_{of a building}

compo-nent for a temperature difference between the adjoining room and the outdoor air of 1 Kelvin. Thus, the lower the U value, the better is the thermal insulation. The units for specifying a U value are W/m2_K.

The U value is determined by calculation according to EN 673 or by measurement according to EN 674.

Under the same boundary condi-tions, the calculation and meas-urement procedures result in comparable U values.

c

value

The linear heat transmission coefficient (

c

value) describes the thermal bridge of a building component. Since introduction of the German Energy-Saving Law in 2002, this must be taken into consideration when the Uwvalue is

determined.

In a window, the thermal bridge is caused primarily by the interaction between the window frame, insu-lating glass unit and spacer. Thus, a

c

value cannot be defined for an insulating glass unit alone (see Chapters 3.3.1 and 7.2).

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4.2 Emissivity value

e

according to EN 673

The emissivity value

e

describes the radiative property of the surface of a body. With respect to the thermal insulation of insulating glazing, this means that the lower the emissivity value, the better the U value. In the past, the U values of glazing were always measured in a test rig – today, reliable calculation pro-cedures are available (EN 673). One of the values needed for the calculations is the

e

value. The emissivity value is deter-mined by measuring the infrared reflectance of a building com-ponent surface.

It is assumed that the angle of incidence is almost perpendi-cular to the considered surface and that radiation of different wavelengths is used for the measurement. The reflectance value R determined in this way is converted into the emissivity value using the formula

• Normal emissivity value

e

n according to EN 673 The determination of the normal emissivity value

e

naccording to

EN 673 is based on the meas-urement procedure described above, where 30 wavelengths between 5.5

m

m and 50

m

m are evaluated.

The mean value is determined from these individual results, with the spectral distribution of thermal radiation at + 10 °C being taken into account. The result is designated the “normal emissivity value

e

n”.

• Declared emissivity value

e

daccording to EN 1096 The declared value of the emissivity value

e

d is the

nominal value of the normal emissivity value specified by the manufacturer of the basic glass.

e

= 1 – R

As it is not possible, from a metrological point of view, to measure with an angle of incidence of 0°, measurements are generally made with an average angle of incidence of „10°.

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4.3 g value according to EN 410

The g value is the total solar energy transmittance (or solar heat gain coefficient) of glazing for solar radi-ation in the wavelength range be-tween 300 nm and 2500 nm. The value is significant for HVAC calcu-lations (heating, ventilation, air-con-ditioning) and is expressed in %. The g value is the sum of the direct solar energy transmittance teand

the secondary internal heat transfer factor qi describing

long-wave-length radiation and convection. g =

t

e+ qi

The rated value g for the total solar energy transmittance is determined according to the European standard EN 410. The total solar energy transmit-tance g0can be determined simply

according to DIN V 4108 Part 4 for 4 mm thick glass panes (for low-e insulating glazing) or 6 mm thick glass panes (for solar-control glaz-ing).

The g values must be corrected for thicker outdoor panes with a cor-rection factor c that depends on the coating type, in accordance with Table 12 of DIN V 4108 Part 4. This results in

g = g0•c

The rated value of the total solar energy transmittance is equal to the nominal value:

gBW= g

Total solar energy transmittance of iplus neutral E according to EN 410 – energy distribution – 100 % incident solar energy secondary external heat transfer factor qa = 11 % solar energy reflectance r = 29 %

Solar Heat Gain Coefficient g = 60 %

secondary internal heat transfer factor qi = 8 %

direct solar energy transmittance

t

e= 52 %

4.4 Colour rendering index R

a

according to EN 410

Colour rendering is very important

for physiological perception and for psychological and aesthetic reasons.

The colour effect in a room is influenced by the spectral distribution of the incident daylight. Consequently, the Ra,D value is used to describe colour rendering with daylight, firstly within the room and secondly outdoors, as viewed from in-doors through the window.

In a comparable manner, the Ra,R value characterises the colour rendering of the glazing for outdoor reflections. The colour rendering character-istics of glazing are specified by the general colour rendering index Raaccording to EN 410. The scale for Ragoes up to 100.

The optimal Ra value that can be achieved with glazing is 99.

The normal illuminant D 65 is used as the reference illuminant.

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4.5 Light transmittance

t

V

according to EN 410

The light transmittance

t

V

char-acterises the directly transmitted, visible radiation in the spectral range from 380 nm to 780 nm, weighted by the photopic spectral response of the human eye. The light transmittance is specified in % and is influenced, among other factors, by the thickness of the glass. Due to the varying iron oxide content of glass, slight fluctuations are possible.

● A single pane of float glass has a light transmittance of around 90 % in the visible spectral range.

● INTERPANE double glazing,

consisting of 2 float glass panes, has a light transmit-tance value of 82 %.

● iplus neutral E has a light trans-mittance value of 80 %.

The reference value of 100 % corresponds to a wall opening without any glazing.

The light transmittance of the glazing should be selected appro-priately for the building and the environment, in order to comply with DIN 5034 and the German regulation for workplaces. Alter-natively, the window area can be varied.

Spectral distribution of solar energy transmitted by iplus neutral E and conventional double glazing.

Transmitted energy with respect to the solar spectrum

iplus neutral E conv. double glazing

Light: 75 % 79 %

Thermal radiation: 29 % 66 % Total solar radiation: 52 % 73 % Spectral distribution of solar energy transmitted by iplus neutral E and conventional double glazing

* Energy distribution according to DIN EN 410 (Air Mass 1.0) UV 4 % light 55 % heat 41 % total solar radiation

100 %

solar spectrum photopic sensitivity of the eye conventional double glazing insulating glass unit with iplus neutral E coating

re

lative radiation intensity

re

lative photopic sensitivity of the eye

4.6 Energy absorption

4.7 b Factor

4.8 Selectivity S

4.9 Weighted sound reduction index R

w

In addition to transmission and reflection, absorption is the third process determining radiation transport through glass.

transmittance + reflectance + absorptance = 100 %

Radiated energy is converted by absorption into thermal energy. This causes the temperature of the absorbing glass pane to increase.

The »average transmittance factor b« is the decisive factor for calculating the cooling load. The b factor (also called the shading coefficient) is the ratio of the g value of the evaluated glazing unit to the g value of a

conventional double-glazed win-dow, according to VDI 2078 (July 1996).

g

glazing

b =

0.80

The ratio of light transmittance

t

Vto total solar energy

transmit-tance g is designated by the selectivity S.

t

V

S =

g

A high selectivity value indicates a favourable ratio. S = 1.8 is the physically feasible limit for neutrally coloured glazing prod-ucts.

An example of high-performance solar-control glazing (g value according to EN 410): ● ipasol neutral 68/34 68 S = = 1.84 37 ● ipasol neutral 50/25 50 S = = 1.85 27

The sound reduction index R of a building component depends on the frequency. The frequency range for building acoustics extends from 100 Hz to 3150 Hz. R designates the 10-fold loga-rithmic ratio of the acoustic

power P1incident on the

com-ponent to the acoustic power P2

reflected by this component. Due to this logarithmic scale, an improvement in the sound insulation of 10 dB halves the noise pollution.

The weighted sound reduction index Rwaccording to EN 20140

Part 3, which is determined by measurements and comparison with the reference curve, is used in the acoustic characterisation of glazing. It is expressed in units of decibels (dB).

DIN 4109 (11.89) defines the following symbols:

Symbol Meaning

R’w weighted sound reduction index in dB with noise transmission via adjacent components

Rw weighted sound reduction in dB without noise transmission via adjacent components

R’w, res resultant weighted sound reduction index of entire component

Rw, P weighted sound reduction index measured in test rig

Rw, R weighted sound reduction index – calculated value –

Rw, B weighted sound reduction index – measured in building –

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Taking 3 mm single glazing as the

reference, the following applies:

g

glazing

b =

0.87

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● In order to take into account

the different frequency spec-tra of residential and spec-traffic noise, spectral adaptation values C andCtrhave been introduced in compliance with EN ISO 717-1 (see table).

Sound source

– domestic activities (conversation, music, radio, TV) – children’s games

– medium-speed and high-speed trains1₎

– motorway traffic > 80 km/h1) – nearby jet plane

– factories/workshops which predominantly emit medium and high-frequency noise1₎

– urban road traffic – low-speed trains1₎

– propeller plane – distant jet plane – disco music

– factories/workshops which predominantly emit low and medium-frequency noise

1_{) In several European countries, calculation procedures exist for road traffic noise and railway traffic}

noise which define octave band noise levels; these can be compared with spectra 1 and 2. C

(Spectrum no. 1)

Ctr

(Spectrum no. 2) Corresponding spectral adaptation value ●The spectral adaptation

val-ues C100-5000 andCtr 100-5000 also take into account the extended spectrum in the frequency range from 100 Hz to 5000 Hz.

4.10 Double-glazing effect/Insulating-glazing effect

4.11 Interference phenomena with insulating glazing

4.12 Anisotropy

The space between the panes of an insulating glass unit is a hermetically sealed volume, in which the universal gas laws apply. The panes are firmly fixed at the edges by the adhesive and thus act as membranes. The volume between the panes changes with all air pressure and temperature fluctuations, because the panes bend accord-ingly.

The bending is visible as stronger or weaker distortion of the reflections from the panes. This physically unavoidable

pheno-menon is called the double-glaz-ing or insulatdouble-glaz-ing-glazdouble-glaz-ing effect. This effect is actually a proof of quality for insulating glazing. It indicates that the space be-tween the panes is hermetically sealed.

The insulating-glazing effect depends particularly on the size and geometrical configuration of the panes, as well as on the width of the space between the panes and the glass thickness. See also details in Chapter 6.4.8, “Insulating glass units with small dimensions“.

The insulating-glazing effect is more pronounced in triple insu-lating glazing, as the spaces between these panes are added together, acting as a broad space, in other words: space 12 + space 12 = space 24!

The reason is that the middle pane usually remains undis-torted with air pressure or tem-perature fluctuations, so that the two outer panes deflect even more.

As the two surfaces of a float glass pane are extremely flat and excellently parallel, optical phenomena may be visible under certain lighting conditions. These are evident as rainbow-type spots, stripes and rings that change their position when pres-sure is applied to the pane. Interference phenomena are purely physical effects caused by refraction and superposition. They only occur in situations where two or more float glass panes are positioned behind each other.

As the magnitude of the pheno-menon depends on the local lighting conditions, the position of the pane and the incidence angle of the light, it only occurs rarely and only if several factors coincide. Interference phenome-na mainly occur under a certain viewing angle in reflection, sel-dom in transmission.

Thus, these interference pheno-mena are physical occurrences that can be interpreted as a hall-mark of excellent float glass quality.

Interference phenomena in insulating glass units with panes of the same thickness

These are iridescent effects that can occur in thermally treated panes (thermally toughened glass / heat-strengthened glass). Thermally toughened glass and heat-strengthened glass are pro-duced in special thermal ses. These manufacturing

proces-ses generate stress zones in the glass that lead to birefringence under polarised light. If the heat-treated glass is observed under certain light conditions, polarisa-tion fields are visible as patterns. This effect is characteristic of ther-mally toughened glass and

heat-strengthened glass and is caused by physical factors.

Natural daylight contains varying proportions of polarised light, depending on the weather or time of day.

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4.13 Dew-point

The dew-point is the air tempera-ture at which the relative humidity reaches a value of 100 %. If the air temperature decreases with unchanged moisture content, condensation occurs.

Dew-point temperatures can occur at various positions: a) Dew-point in the space

between the panes of an insulating glass unit A new insulating glass unit should have a dew-point in the space be-tween the panes of < – 60 °C. This temperature, which is deter-mined according to EN 1279, is an important quality characteristic and ensures a long lifetime of the insulating glass unit.

b) Dew-point of the indoor pane surface

Condensation can form on the indoor surface of an insulating glass unit under the following conditions:

● Hot air cools suddenly on a cold pane surface (warm air can absorb more water vapour than cold air, as is widely known).

● Moisture is added to relatively cold air. This occurs very fre-quently in kitchens, bathrooms, laundries and bedrooms. In these areas, an annoying con-densation film can form within a short time as the moisture condenses on the cold pane surface.

The tendency to condensation can be considerably reduced by the use of thermally insulating glazing (low-e), such as iplus neutral E, as the indoor surface temperature of the pane is increased due to the improved U value. This can be seen clearly in the dew-point dia-gram.

A high level of water vapour can be prevented by appropriate ventila-tion (Chapter 3.4).

c) Dew-point of the outdoor pane surface

In specific cases, condensation can also occur on the outdoor pane surface of insulating glaz-ing. It occurs in the early morn-ing if the outside air contains a high level of moisture. In the early morning, the temper-ature of the outdoor pane can fall below the dew-point. The

reason for this is that the out-door pane of an insulating glass unit cools off appreciably at night due to the high level of thermal insulation, which means that the indoor temperature hardly affects the outdoor pane. If the temperature of the outdoor air then rises more quickly than that of the panes, condensation can occur.

Dew-point diagram with an example

From the dew-point diagram, the outdoor temperature can be determined at which condensation occurs on the indoor pane surface (= dew-point).

Plotted example: iplus neutral E, U value 1.1 W/m2K, room temperature + 21 °C, relative humidity 50 %. Result: Condensation does not occur on the indoor surface of iplus neutral E until the outdoor temperature falls to - 48.2 °C.

room air temperature outdoor air temperature

relative humidity [W/m2K] U = 1.1 U = 1.4 U = 1.6 U = 1.8 U = 3.0 U = 5.8

outdoor air temperature

– 50.0 °C–48.2 °C

However, the condensation disap-pears again quickly with the first rays of the sun.

The formation of condensation, both on the indoor and the outdoor surfaces, is due to physical and climatic factors.

d) Dew-point at thermal bridges

In practice, thermal bridges are constantly encountered that result from the materials used and/or geometric configurations deter-mined by structural considerations. Near these thermal bridges, higher

heat flows occur, which have a lower surface temperature in com-parison to undisturbed elements. Under appropriate climatic condi-tions, condensation can form on these cooler surfaces.

4.14 Glass thickness determination

The glass thickness is determined according to German regulations (DIBt) (see Chapter 7.7.1). Proof of suitability for glazing with thicknesses deviating from these specifications must be

provided in consultation with the responsible building inspection authorities.

The maximum dimensions given in this manual represent the technical production limits.

The customer ordering our prod-ucts is responsible for ensuring that the glass thickness is dimensioned correctly according to the applicable technical regu-lations.

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