Insulated Glass Terminology
A series of factors and physical regularities define the characteristics of insulating glass as it is used in heat and solar protection applications.
To achieve thermal insulation properties, several float glass panes must be combined with at least one low-e-coating on an insulating glass unit.
Two or more panes of the same size are aligned with each other at a defined distance and glued together. The resulting hermetically sealed interspace will be filled with especially high thermal insulating inert gas. No vacuum is generated, as laymen often assume.
The width of the pane interspace depends on the inert gas that is used. Argon is used most often, krypton more rarely. To reach its optimum thermal insulation efficiency, argon needs an interspace of 15 – 18 mm; krypton needs only 10 – 12 mm for better insulating results. The interspace is usually filled to 90 % capacity. Krypton is many times more expensive than argon since it ismuch more rare.
The spacer that permanently separates the panes has some influence on the insulating performance, and thus on the dew-point at the edge of the glazing (see > chapter 3.6). For the past several decades, aluminium spacers have been the industry standard. TodayNowadays, they are being replaced by systems that have lower heat conductivity.
The insulating panes are glued together using the dual-barrier system, in which a spacer is used to keep the two panes separated, and a continuous string of butyl adhesive is applied around the edges of the spacer to keep both panes of glass glued together. The space that is created is filled with a desiccant that keeps the interspace permanently dry.
|Innere Abdichtung||Inside seal|
|Äußere Abdichtung||Outside seal|
|unsichtbare Wärmedämmschicht||Invisible thermal barrier coating|
|Trockenmittel (Molekularsieb)||Desiccant (molecular sieve)|
During the gluing process, it is important that the coated side of the pane of float glass faces the interspace and that the adhesive is applied to this side. Some types of coatings have to be mechanically removed first before the adhesive can be properly applied. Removing the coating first before the adhesive is applied increases the bonding strength and protection against corrosion. The functional layer is now hermetically sealed and permanently protected. The butyl adhesive sealant, also called the inner sealant level, prevents the water vapour from forming and the inert gas from escaping. After the two panes of glass are bonded together, a gas-pressure press is used to withdraw some of the air from between the panes and replace it with a defined amount of inert gas. Before that process occurs, however, the insulating pane receives its second sealant and adhesive level by filling in the hollow between the installed spacers and the outer edges of the panes. The material most-often used is polysulfide or polyurethane.
Instead of these adhesive materials, a UV-resistant silicone is used in special installations that have exposed insulating glass edges. The insulating panes that have a UV-resistant edge seal are filled often with air, since the gas diffusion density is lower for silicone. However, to a lesser extent, using this silicone also reduces the insulating glazing’s U value (see > chapter 3.5).
3.3 Thermo-technical function
Three factors define heat transmission: heat radiation, heat conductivity and convective flow.
The electromagnetic long-wave thermal radiation that every entity emits due to its temperature transfers thermal energy without transmitting the entity or medium itself.
Heat conductivity is the heat flow within a medium because of temperature discrepancy. In this case, the energy always flows in the direction of the lower temperature.
Convective flow is a flow of gas particles in the interspace that is due to the difference in temperature between the inner and outer panes of insulating glass. The particles drop on the colder surface and rise on the warmer one. Consequently, the gas circulates, thus creating a heat flow from warm to cold.
Insulating glass consisting of just two uncoated panes of float glass where air fills the interspace loses about 2/3 of the heat that room would otherwise have due to the radiation loss between the two panes, and loses 1/3 due to heat conductivity and heat convection to the outside air.
The result is an extreme difference in temperature between the inner pane and a massive loss of heat during the cold seasons due to the heat transfer from the inner pane to the outer pane.
Typically one side on today’s insulation glass is coated with a low-e film. These coatings with emissivities up to 0.01 (1 %) are capable of reflecting 99 % of the incoming long-wave-heat radiation, so that radiation loss is completely eliminated.
This is an improvement of approx. 66 % as compared with traditional insulating glass. Heat conductivity and convective flow are not affected by low-e-coating. This heat conductivity can, however, be reduced by using an inert gas like argon. Inert gases have significantly lower heat conductivity than air, thereby reducing the heat flowing through the insulating glass system. Depending on the fill gas, the convective flow in the insulating glass requires a minimum amount of spacing when there is a defined pane distance, for example, for air: approx. 16 mm; for argon: 15 – 16 mm; and 10 – 12 mm for krypton.
3.4 Edge Seal
Conclusions made thus far refer to the centre area that is between the panes without any influences from the insulating glass edges.
Until very recently, the majority of insulating glass has been produced using aluminium spacers. Increased requirements have created thermo-technically improved alternatives that are gaining ground in insulating glass production.
3.4.1 Stainless steel
Extremely thin stainless steel profiles possessing considerably reduced heat conductivity as compared with aluminium are the most frequent alternative. They are similar to aluminium, however, in terms of their mechanical stability and diffusion capability.
3.4.2 Metal / plastic combination
Another option is plastic spacers that offer excellent thermal insulation capability but do not have a sufficient gas diffusion density to guarantee the life cycle for insulating glass. Therefore, combinations of plastic have been developed that have gas-impermeable stainless steel or aluminium foils.
3.4.3 Thermoplastic systems (TPS)
A hot extruded, special plastic substance, which is placed between two panes during production and which guarantees the required mechanical strength as well as gas diffusion density after cooling down replaces the conventional metal. The desiccant is part of this substance.
There is a wide range of disposable alternatives nowadaystoday that provides more or less important reductions of the y value, the unit of the heat transport in the boundary zone, when they are directly compared against each other (see > chapter 3.5.3).
3.5 U value – heat transmittance coefficient
This value states the heat loss through a component. It indicates how much heat passes through 1 m² of component when there is a temperature difference of 1 K between the two adjacent sides – for example, between a room and an outside wall. The smaller this value is, the better the heat insulation.
Please note that the European U values are different from the American values. This must be taken into consideration when making international comparisons.
3.5.1 Ug value
The Ug value is the heat transfer coefficient for glazing. It can be determined or calculated according to defined standards. Four factors determine this value: the emissivity of the coating, which is determined and published by the producer of the float glass, the distance of the panes, filling type and the fill rate when using inert gases.
(To find the rated value for real-life usage, you have to consider national aggregates – DIN 41408-4 applies for Germany)
188.8.131.52 Ug value for inclined glass surfaces
The Ug value that is most often defined and published refers to glazing that is vertically (90°) installed. Installation with inclination changes the convection in the interspace and decreases the Ug value. The bigger the glass surface inclination, the faster the circulation in the interspace and the bigger the heat flow from the inner to the outer pane. This can reduce the Ug value by up to 0.6 W/m²K for double insulating glass.
3.5.2 Uf value
The Uf value is the heat conductivity coefficient of the frame, the nominal value of which can be determined by three different ways:
– measuring according to EN ISO 12412-2,
– calculating acc. to EN ISO 10077-2,
– using the EN ISO 10077-1 definition, appendix D.
The nominal value plus the national aggregates determine the rated value for the real-life usage.
3.5.3 Y value
The value (Psi value) is the linear thermal bridge loss coefficient for a component. Regarding windows, it describes the interaction of insulating glass, dimensions, spacer and frame material, and defines the component’s thermal bridges. The insulating glass itself has no value, this applies only to the construction element into which it is integrated does.
3.5.4 Uw value
Insulating glass is normally used in windows. The Uw value describes the heat conductivity of the construction element window. Based on the Ug value, it can be determined using three different methods:
– reading in the EN ISO 10077-1, Tab. F1
– measuring acc. To EN ISO 12567-1
– calculating acc. to EN ISO 10077-1 as per the following formula
|Wärmedurchgang des Fensters||Thermal transmittance from the window|
|Wärmedurchgang des Rahmens (Bemessungswert!)||Thermal transmittance from the frame (assessment value!)|
|Wärmedurchgang des Verglasung (Nennwert!)||Thermal transmittance from the glazing (rated value!)|
|Umfang der Verglasung||Periphery for the glazing|
|linearer Wärmedurchgang der Glaskante||Linear thermal transmittance from the glass edge|
The heat loss in the edge zone is more important than in the middle of the glazing, which is why thermally improved spacers are becoming increasingly important. Like Ug and Uf, the Uw values are nominal values, which only become rated values after having added the national supplements.
3.6 Dew point and condensation
There is always humidity in the air. Warmer air can hold more water than cooler air. Once the air cools down, the relative humidity increases, yetwhereas the water vapour volume remains the same. The dew point temperature is the temperature when the relative air moisture reaches 100 % and water vapour condenses.
This can occur at different places on the insulating glass:
3.6.1 In the interspace between the panes
This rarely occurs with today’s insulating glass, since they are hermetically sealed and filled with dried gases.
3.6.2 On the interior surface of the pane
Appears on poorly insulated windows or those with single glazing. Warm air cools suddenly near windows and transfers humidity to the cold inside pane – the temperature in winter is often below the dew point of the ambient air. The inside pane for modern insulating glass stays warm longer so that condensation very seldom occurs.
If the relative air humidity is very high, for example due to cooking, for example, washing or proximity to ain swimming pools, panes may condensate get steamed up more often. One way to correct this is to exchange the air by means of short and direct ventilation.
3.6.3 On the outer pane surface of the insulating glass
This effect appears with the advent of modern insulated glass, and is particularly noticeable during the early morning hours, when the moisture content in the outside air has sharply increased during the night.
The excellent insulating quality of these glass surfaces prohibit heat transfer to the outside, so that the outer pane remains extremely cold. When the sun’s rays start to heat the outside air faster then the temperature of the pane, it may lead to condensation, depending on the orientation of the building and the environment. This is not a defect, yet it is the resultbut the proof of an excellent thermal insulation of the insulating glass.
Guardian offers special coatings that allow a clear view through the glazing even during the morning hours (see > chapter 4.4).
Dew point diagram
|rel. Luftefeuchtigkeit (%)||Relative humidity (%)|
The outside temperature, at which the glazing on the inner side condensategets steamed up (= forming of condensation water = dew point), can be determined by the dew point diagram. The diagrams show a room temperature of 20 °C and a room humidity of 50 %. With a Ug value of 5,8 W/m² the dew point is already reached at an outdoor temperature of 9 °C, with Ug = 3,0 W/m²K at -8 °C, with Ug = 1,4 W/m²K at -40 °C and with Ug = 1,1 W/m²K at -48,2 °C.
3.7 g value
The total energy transmittance degree (g value) defines the permeability of insulating glass versus solar radiation. Solar protection glass minimize the g value due to appropriate choice of glass and coatings. The g value of transparent heat insulating glass is preferably high in order to optimize the energy balance of the component glass by passive solar gains.
3.8 b factor (shading coefficient)
The non-dimensional value serves as calculation for the cooling load of a building and is also called shading coefficient. It describes the proportion of the g value of a respective glazing versus a 3 mm float glass with a g value of 87 %.
Acc. to EN 410 (2011): b = gEN 410 / 0.87
3.9 Solar energy gains
Thermal insulation glazing allows allow a large proportion of solar radiation into the interior of the building. Furniture and fixtures, walls and floors absorb the short-wave solar radiation and convert it into long-wave heat radiation. This heat radiation cannot leave the room due to the thermal insulation quality of the glazing, and the heat warms up the air in the room. These real solar gains support traditional heating. Depending on the orientation of the windows, these gains are different, less when the windows face east and west, and more when the glazing faces southward. This energy is free of charge and helps to save on heating costs during the cold season. In the summer months, however, it may cause the building to heat up to an uncomfortable degree. This is called the “greenhothouse effect”. Therefore, the demands on summer heat protection must be taken into consideration (see > chapter 5.5).
3.10 Selectivity classification figure
Solar control glass works to minimize solar heat gain while maximizing the amount if light transferral into the building. The “S” classification number represents the proportion of the total energy (g value) and light transmittance (v) for a glazing. The higher this value, the better and more efficient the ratio is.
S = light transmittance Tv/g-value
The latest generation of Guardian’s solar control glass already exceed a ratio of 2:1, which has long been considered the maximum value.
3.11 Colour rendering index
Colour rendering is not only relevant for the physiological feeling of the observer but also for the aesthetical and psychological aspects. Sunlight that falls through an object or is reflected by it will be changed depending on the nature of the object (see > chapter 2.1).
The colour rendering index (Ra value) describes how much an object’s colour changes when it is observed through glazing. It defines the spectral quality of glass in transmission, and the value can range from 0 to 100. The higher the colour rendering index is, the more natural the reflected colours appear. A Ra value of 100 means that the colour of the object observed through the glazing is identical to the original colour.
A colour rendering index of >90 is rated as very good and >80 as good. Architectural glass based on clear float glass generally have an Ra value >90, and mass-coloured glass usually have an Ra value between 60 and 90.
The colour rendering index is determined according to EN 410.
3.12 Interference phenomena
When several parallel float glass panes exist, very specific lighting conditions can cause optical phenomena to appear on the surface of the glass. These can be rainbow-like spots, stripes or rings that change their position when you press on the glazing, also referred to as Newton rings.
These so-called interferences are of a physical nature and are caused by light refraction and spectral overlap. They rarely occur when looking through the glazing, but in reflection from outside. These interferences are no reason for complaint but rather are a proof of quality regarding the absolute plane parallelism of the installed float glass.
3.13 Insulating glass effect
Part of each insulating glass is at least one hermetically enclosed space, i.e. the interspace. Since this space is filled with air or gas, the panes react like membranes that bulge in and out in reaction to varying air pressure in the surrounding air.
Under extreme weather conditions, unavoidable distortions may show up despite the plane-parallel glazing. It can also occur due to extreme changes in air pressure, and influencing factors include the size and geometry of the pane of glass, the width of the interspace, and the structure of the pane of glass itself. With triple insulation glazing, the medium pane remains nearly rigid, which is the reason why the impact on both outer panes is stronger than on double insulating glass. These deformations disappear without any effects once the air pressure normalizes and represent no defect, but rather are an indication of the edge seal density.