Glazing unit with absorbent indicator

Abstract

A glazing unit 100, for a roof light, window, or door for a building, and a method of its manufacture are defined. The glazing unit 100 has first and second panes 102, 104 of glazing material, with a

Description

FIELD OF THE INVENTION Embodiments of the present invention relate to a glazing unit comprising an absorbent indicator, and a method of making the glazing unit. In particular, but not exclusively, they relate to a roof light, window, or door comprising the glazing unit, the glazing unit comprising an absorbent indicator. BACKGROUND TO THE INVENTION Some glazing units comprise multiple panes with one or more cavities therebetween. Each cavity is sealed and comprises a controlled atmosphere to reduce thermal conductivity. If the outside atmosphere can enter the cavity, the thermal conductivity and humidity within the cavity will increase. This can lead to diminishing insulation performance and internal condensation forming on the panes. The formation of a permanent seal depends on the choice of sealing materials, the manufacturing processes for applying seals, and the processes for replacing the atmosphere in the cavity. Once the cavity is sealed, the glazing unit may be visually inspected after a period of time. If no evidence of a poor seal is seen, such as internal condensation, the glazing unit may be sent to a customer. If a glazing unit is handled or transported improperly, the seal may be compromised prior to or during installation. This can lead to uncertainty regarding responsibility for the compromised seal. Furthermore, a very slow leak may not be evident until a long time after manufacture and installation. BRIEF DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION According to various, but not necessarily all, embodiments of the invention there is provided a glazing unit for a roof light, window, or door for a building, the glazing unit comprising: first and second panes of glazing material; a spacer between the first and second panes; a cavity between the first and second panes, defined by the first and second panes and the spacer, the cavity comprising a controlled atmosphere; and an absorbent indicator located in the cavity, configured to change appearance in response to a change of humidity and / or pH in the cavity, wherein the absorbent indicator comprises a first leg and a second leg transverse to the first leg, wherein the second leg extends along an inner face of the spacer, and wherein the first leg is clamped against the first or second pane by a side face of the spacer. An advantage is that means for diagnosing a compromised seal is provided. This will allow sealing issues to be identified prior to installation. Further advantages are that the absorbent indicator is passive and can be non-destructively fitted. A still further advantage is that an absorbent indicator can be a low cost and reliable component. Optionally, the absorbent indicator comprises a humidity indicator. Another term for a humidity indicator is a moisture indicator. Optionally, the humidity indicator is configured to change appearance in response to relative humidity in the cavity reaching a threshold. Optionally, the threshold is selected from the range 20% to 100%. Optionally, the absorbent indicator comprises a pH indicator. An advantage of a pH indicator is that it can act as an early warning signal for potential unit failure allowing for corrective measures to be taken early before any escalation. Optionally, the change of appearance comprises a colour change. Optionally, the absorbent indicator is located at a peripheral region of the cavity. Optionally, the absorbent indicator is secured to the spacer and / or clamped between the spacer and one of the panes. An advantage is that the absorbent indicator is in a discreet location. Optionally, the absorbent indicator is in the form of a test strip. Optionally, at least part of the absorbent indicator extends on an interior surface of the spacer, facing the cavity. An advantage is that the absorbent indicator only negligibly protrudes into the cavity. Optionally, the glazing unit comprises a substantially opaque strip (e.g., film layer) secured to a main surface of one of the panes, extending parallel to a perimeter edge of said pane. Optionally, the main surface to which the substantially opaque strip is secured is a cavity-facing surface of said pane. Optionally, the substantially opaque strip defines the peripheral region of the cavity where the absorbent indicator is located. Optionally, the absorbent indicator is concealed by the substantially opaque strip when the glazing unit is viewed from a direction perpendicular to a plane of the pane to which the opaque strip is secured. Optionally, the absorbent indicator is visible when the glazing unit is viewed diagonally from a location towards a centre of one of the panes of the glazing unit. An advantage is that the absorbent indicator is in a discreet location, and hidden from view when viewed from a typical angle. Optionally, the glazing unit comprises a parallel pair of the substantially opaque strips, each proximal to a common edge of the glazing unit, each adhered to a main surface of a respective one of the first and second panes, and wherein the peripheral region is defined by the parallel pair of substantially opaque strips. An advantage is that the absorbent indicator is concealed from normal view from both sides. Optionally, the absorbent indicator is secured to the spacer. Optionally, the absorbent indicator is adhered to a face of the spacer. Optionally, the absorbent indicator is secured to an inner face of the spacer, the inner face of the spacer facing the cavity. Optionally, the absorbent indicator is adhered to the inner face of the spacer. Optionally, a portion of the absorbent indicator is secured to a side face of the spacer, the side face facing one of the panes. Optionally, the portion of the absorbent indicator is adhered to the side face of the spacer. Optionally, the side face of the spacer comprises self-adhesive. Optionally, the absorbent indicator is secured to the spacer by at least one mechanical fixing. Optionally, the absorbent indicator is secured to an inner face of the spacer by the at least one mechanical fixing, the inner face facing the cavity. Optionally, the mechanical fixing comprises a rivet. An advantage of a mechanical fixing is that no volatile organic compounds (VOCs) are emitted, whereas adhesives might emit VOCs. VOCs can cause impurities in the cavity which can lead to premature failure. The absorbent indicator comprises a first leg and a second leg transverse to the first leg. Optionally, the first and second legs are generally perpendicular to each other. Optionally, the first and second legs define an L-shape. Optionally, the first and second legs are connected to each other by a fold. The second leg extends along an inner face of the spacer. The first leg is clamped against the first or second pane by a side face of the spacer. Optionally, the side face of the spacer comprises an adhesive, e.g., self-adhesive, to adhere to the first leg of the absorbent indicator. An advantage is that assembly time is reduced, because it is quick to press the folded absorbent indicator onto a side face of the spacer before pressing the side face of the spacer onto the pane. The self-adhesive on the side face helps to keep the spacer and absorbent indicator in their correct positions until the next pane can be placed onto the other side face of the spacer. Optionally, the spacer comprises first and second side faces parallel to each other, wherein the first side face is secured to the first pane by adhesive, wherein the second side face is secured to the second pane by adhesive. Optionally, the adhesives are self-adhesives. Optionally, one of the first and second side faces of the spacer is further adhered to the first leg of the absorbent indicator. Optionally, the spacer is a primary sealant, and the glazing unit further comprises a secondary sealant. Optionally, the secondary sealant is an edge sealant. Optionally, the glazing unit comprises edge sealant bonded to an outer face of the spacer and to main surfaces of the first and second panes. Optionally, the outer face of the spacer and the main surfaces of the first and second panes define a U-shaped channel along which the edge sealant extends. An advantage is that the seal is improved because two levels of sealing are provided. Optionally, the first leg of the absorbent indicator comprises a protruding portion which protrudes beyond the side face of the spacer into a space where the edge sealant extends. Optionally, the edge sealant is further bonded to the protruding portion of the first leg of the absorbent indicator. The protruding portion may be a protruding tab of the first leg. An advantage is that the absorbent indicator is very secure because its first leg is secured by both the primary seal and the secondary seal. In other words, the first leg is secured by both the spacer and the edge sealant. Optionally, the protruding portion of the first leg of the absorbent indicator is buried (submerged) by the edge sealant, relative to an exterior face of the edge sealant, wherein the exterior face of the edge sealant is proximal to perimeter edges of the first and second panes. An advantage is that none of the absorbent indicator is exposed to an external atmosphere. Optionally, the edge sealant comprises one of: an elastomer, Polyurethane, Polysulfide and Silicone. Optionally, the edge sealant comprises an elastomer. Optionally, the elastomer comprises a rubber. Optionally, the edge sealant comprises a butyl rubber compound. Optionally, the edge sealant is a bead of setting material. Optionally, the edge sealant is a hot melt, such as a hot melt of the butyl rubber compound. Optionally, the glazing unit is a double glazing unit. Alternatively, the glazing unit is a triple glazing unit further comprising: a third pane of glazing material; a second spacer between the second and third panes; and a second cavity between the second and third panes, defined by the second and third panes and the second spacer, the second cavity comprising a controlled atmosphere. Optionally, the controlled atmosphere in the cavity is a shared atmosphere with the controlled atmosphere in the second cavity. Optionally, the glazing unit comprises a conduit between the first cavity and the second cavity, wherein the absorbent indicator is in fluid communication with the first and second cavities. Optionally, the glazing unit does not comprise an absorbent indicator in the second cavity. Alternatively, the first and second cavities are sealed from each other, and wherein the glazing unit comprises a second absorbent indicator in the second cavity, configured to change appearance in response to a change of humidity and / or pH in the second cavity. Optionally, if only one of the cavities comprises an absorbent indicator, the cavity comprising the absorbent indicator may be on an indoor side or an outdoor side of the glazing unit. Optionally, the controlled atmosphere comprises an argon-filled atmosphere. Optionally, the spacer is a spacer bar. Optionally, the spacer comprises an elastomer. Optionally, the elastomer comprises a rubber. Optionally, the spacer comprises a butyl rubber compound. Optionally, the spacer comprises polyisobutylene. Optionally, the spacer further comprises a desiccant to adsorb humidity in the cavity. Optionally, the desiccant comprises a 3A desiccant. The spacer may be impregnated by the desiccant. Optionally, the spacer further comprises a backing layer. Optionally, a surface of the backing layer is more hydrophobic than a surface of the desiccant. This 7 provides further protection against condensation. Optionally, a surface of the backing layer has a higher surface energy than a surface of the desiccant. This allows the edge sealant to adhere to the spacer by adhering to the backing layer. Optionally, the spacer comprises an inner face facing the cavity and an outer face facing a proximal edge of the glazing unit. Optionally, the inner face comprises the rubber. Optionally, the outer face comprises the backing layer. Optionally, the spacer is formed from a sealing tape. Optionally, the edge sealant described earlier is bonded to the backing layer of the outer face of the spacer. According to various, but not necessarily all, embodiments of the invention there is provided a roof light, window, or door for a building, the roof light, window, or door comprising the glazing unit. According to various, but not necessarily all, embodiments of the invention there is provided a method of manufacturing the glazing unit, the method comprising: providing the first and second panes, and the spacer; positioning the absorbent indicator against the spacer; securing a first side face of the spacer and a main surface of the first pane to each other, and a second side face of the spacer and a main surface of the second pane to each other, and wherein the absorbent indicator is located within the cavity. Optionally, the first and second side faces of the spacer each comprise self-adhesive to self-adhere the spacer to the first and second panes. Optionally, the method further comprises applying a bead of setting material to a perimeter region of the glazing unit between the spacer and an outer edge of the first and second panes. Optionally, the method further comprises replacing an atmosphere within the cavity with a controlled atmosphere. Optionally, positioning the absorbent indicator against the spacer comprises securing the absorbent indicator to the spacer and / or clamping the absorbent indicator between the spacer and one of the panes. Optionally, positioning the absorbent indicator against the spacer comprises securing the absorbent indicator to the spacer before securing the first and second side faces of the spacer to the first pane. Optionally, the absorbent indicator comprises a first leg and a second leg transverse to the first leg, wherein positioning the absorbent indicator against the spacer comprises positioning the first leg against the first side face of the spacer prior to securing the first side face of the spacer to the first pane. Optionally, positioning the first leg against the first side face of the spacer comprises adhering the first leg to self-adhesive on the first side face. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of various examples of embodiments of the present invention reference will now be made by way of example only to the accompanying drawings in which: FIG. 1 illustrates a cross-section perspective view of a glazing unit with an absorbent indicator; FIG. 2 illustrates a side perspective view of the glazing unit; FIG. 3 illustrates an example triple glazing unit; FIG. 4 illustrates an alternative method of securing an absorbent indicator; and FIG. 5 illustrates a further alternative method of securing the absorbent indicator. DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION The Figures illustrate a glazing unit 100 for a roof light, window, or door for a building, the glazing unit 100 comprising: first and second panes 102, 104 of glazing material; a spacer bar 106 (spacer) acting as a primary seal between the first and second panes 102, 104; an optional edge sealant 108 acting as a secondary seal between the first and second panes 102, 104; a cavity 112 between the first and second panes 102, 104, defined by the first and second panes 102, 104 and the spacer bar 106, the cavity 112 comprising a controlled atmosphere; and an absorbent indicator 136 located in the cavity 112, configured to change appearance in response to a change of humidity and / or pH in the cavity 112. In examples, the absorbent indicator 136 comprises a test strip configured as a humidity indicator. The absorbent indicator 136 works by changing colour in response to the presence of moisture. The absorbent indicator 136 is coated or impregnated with a water-reactive chemical that reacts with water molecules. A first example chemical is cobalt chloride, which changes colour based on its hydration state. For example, cobalt chloride is blue when dry and turns pink when it absorbs moisture. In other examples, a different chemical can be used having an equivalent colour change effect. The water-reactive chemical is embedded in a substrate of the absorbent indicator 136, such as paper or a polymer film, that allows moisture to reach the 10 chemical easily. This substrate may be a wicking material, to wick moisture from the cavity 112. The specific colour change can be designed to indicate different levels of humidity. For example, it might turn a different colour at 20%, 50%, and 80% relative humidity, providing a visual representation of the moisture level. The chemical is configured to change appearance in response to relative humidity in the cavity 112 reaching a threshold such as a threshold from the range 20% to 100%. Additionally, or alternatively, the absorbent indicator 136 comprises a pH indicator. The pH indicator comprises a pH-reactive chemical that changes colour in the presence of acidity or alkalinity. As shown in the Figures, the absorbent indicator 136 is located at a peripheral region 122 of the cavity 112, against the spacer bar 106. In summary, FIGS. 1-3 show the absorbent indicator 136 clamped between the spacer bar 106 and the first pane 102, and protruding into the cavity 112. In FIGS. 4-5, the absorbent indicator 136 is secured to an inner face 128 of the spacer bar 106, facing the cavity 112. FIGS. 1-4 utilise adhesive to secure the absorbent indicator 136 to the spacer bar 106 whereas FIG. 5 utilises a mechanical fixing 500 to secure the absorbent indicator 136 to the spacer bar 106. Although useful, adhesives may be omitted in the examples of FIGS. 1-3 and FIG. 5 because they rely on mechanical clamping force without necessarily requiring the use of adhesives. FIGS. 1-2 show a double glazing unit 100 comprising two panes 102, 104, one of which is a building-exterior pane (wet side pane), and the other of which is a building-interior pane (dry side pane). The panes 102, 104 are separated by a spacer bar 106 which extends continuously around the entire perimeter of the glazing unit 100 to define a primary seal. A bead of edge sealant 108 extends 11 outside the spacer bar 106 to define a secondary seal. Therefore, the cavity 112 between the panes 102, 104 is sealed from the external environment. The panes 102, 104 may be formed from glass, such as tempered glass. The panes 102, 104 may be cut to a quadrilateral or triangular shape, for example. In an example implementation, the spacer bar 106 comprises a polyisobutylene rubber compound, additionally impregnated with a 3A desiccant to adsorb any residual humidity in the cavity 112. The spacer bar 106 has an extruded form. The illustrated spacer bar 106 has a rectangular cross-section comprising an inner face 128 and an opposite outer face 130, connected by narrower side faces 132, 134. The spacer bar 106 has a thickness defined by the width of each side face. The spacer bar 106 has a height defined by the width of the inner and outer faces 128, 130. A first side face 132 of the spacer bar 106 is pressed against the first pane 102, and a second opposite side face of the spacer bar 106 is pressed against the second pane 104. The side faces 132, 134 of the spacer bar 106 comprise self-adhesive 110, to help the spacer bar 106 remain fixed in position between the first and second panes 102, 104 while the glazing unit 100 is being assembled. The inner face 128 of the spacer bar 106 presents the desiccant-impregnated polyisobutylene compound, to adsorb any gases in the cavity 112. The outer face 130 of the spacer bar 106 comprises a polymeric foil defining a backing layer. The spacer bar 106 is positioned inset from the perimeter edges 118, 120 of the panes 102, 104, leaving a U-shaped channel 144 running along a perimeter of the glazing unit 100, between the panes 102, 104 and the outer face 130 of the spacer bar 106. A bead of the edge sealant 108 extends continuously around the U-shaped channel 144 to bond the panes 102, 104 and spacer bar 106 to each other, and form a continuous seal. The exterior face 146 of the 12 edge sealant 108 may extend between the perimeter edges 118, 120 of the first and second panes 102, 104. In an implementation, the edge sealant 108 comprises a hot melt butyl rubber compound. Different compounds may be used in other examples. Other examples include other elastomers, or Polyurethane, Polysulfide, or Silicone. Once the edge sealant 108 has been applied, the atmosphere in the cavity 112 is replaced with a controlled atmosphere comprising Argon gas, or in some examples Xenon or Krypton gas. FIG. 1 further illustrates a pair of substantially opaque strips 124, 126 (e.g., film layers), each adhered to a respective main surface 114, 116 of a respective pane and extending along the peripheral region 122 of the cavity 112. The aesthetic strip can comprise a polymeric film which hides the spacer and edge sealant 108 from normal viewing angles. The aesthetic strip can comprise a black colour, or any other desired colour. Each substantially opaque strip 124, 126 has side edges with a width therebetween, and extends from corner to corner of the pane. An outer side edge of the substantially opaque strip 124, 126 extends along or proximal to the perimeter edge 118, 120 of the pane. The inner side edge of the substantially opaque strip 124, 126 is separated from the outer side edge by the width. The width of the substantially opaque strip 124, 126 (e.g., black film layer) is optionally selected from the range 5cm - 20 cm. A set of four substantially opaque strips 124, 126 adhered to a pane 102, 104 may collectively define a rectangular film frame for a rectangular window. FIG. 1 further illustrates the absorbent indicator 136, extending on the inner surface of the spacer bar 106. The absorbent indicator 136 is at the boundary of the cavity 112, close to the perimeter edges 118, 120 of the panes 102, 104. The absorbent indicator 136 changes colour when an above-threshold humidity 13 and / or pH is reached. The absorbent indicator 136 is hidden from normal viewing angles by the pair of substantially opaque strips 124, 126. The absorbent indicator 136 is externally visible when viewed from a certain angle. The absorbent indicator 136 is visible from a point of view which is diagonal to the plane of the panes 102, 104 such that the absorbent indicator 136 is visible around the inner side edge of the substantially opaque strip 124, 126. The viewing angle may be mostly parallel to the plane of the panes 102, 104. FIGS. 1 -3 illustrate a first manner of attachment of the absorbent indicator 136. The absorbent indicator 136 is folded or pre-folded into an L-shape, comprising a first leg 138 and a second leg 140, connected by a fold at an approximate right angle. The second leg 140 of the absorbent indicator 136 is shorter than the width of the inner face 128 of the spacer bar 106. Before the spacer bar 106 is attached to either pane, a portion of the first leg 138 of the absorbent indicator 136, close to the fold, is stuck to the self-adhesive 110 on the first side face 132 of the spacer bar 106. The absorbent indicator 136 is now held by the spacer bar 106 without needing to be separately handheld. This step takes only a few seconds at most. FIGS. 1 and 2 show the position and orientation in which the absorbent indicator 136 is secured to the spacer bar 106. The fold of the absorbent indicator 136 extends parallel to the longitudinal axis of the spacer bar 106, and the first and second legs 138,140 extend perpendicular to the longitudinal axis of the spacer bar 106. The fold of the absorbent indicator 136 is positioned against an edge of the spacer bar 106, the edge connecting the first side face 132 and the inner face 128 of the spacer bar 106. The spacer bar 106 is then adhered to the first pane 102 via the self-adhesive 110 along the first side face 132 of the spacer bar 106, into the arrangement shown in FIGS. 1 and 2. The second leg 140 of the absorbent indicator 136 is parallel to and positioned against the inner face 128 of the spacer bar 106, and extends from the fold across the cavity 112 towards the second pane 104. The second leg 140 may have a length from the fold to its tip which is less than the corresponding height of the spacer bar 106, to prevent it from reaching the second pane 104. The first leg 138 of the absorbent indicator 136 is parallel to and positioned against the first pane 102. Part of the first leg 138 is adhered to the first side face 132 of the spacer bar 106. Since the first leg 138 is longer than the thickness of the spacer bar 106, the first leg 138 will comprise a protruding portion 142 which extends beyond the first side face 132 of the spacer bar 106 towards the perimeter edge 118 of the first pane 102, and in the U-shaped channel 144 behind the spacer bar 106 where the edge sealant 108 is later applied. The first leg 138 is short enough that it does not reach the perimeter edge 118 of the first pane 102. A bead of edge sealant 108 is then applied to substantially fill the U-shaped channel 144. The edge sealant 108 is a settable structural sealant which, when set / cured, structurally secures the facing main surfaces 114, 116 of the panes 102, 104 as well as the outer face 130 of the spacer bar 106. The edge sealant 108 will further secure the protruding portion 142 of the first leg 138 of the absorbent indicator 136. This prevents the absorbent indicator 136 from slipping out of position after manufacture, where it could become loose in the cavity 112. The protruding portion 142 will completely submerge / bury the protruding portion 142 of the first leg 138 of the absorbent indicator 136, so that none of the first leg 138 is exposed to the outside atmosphere. In a variant of FIGS. 1 and 2, the absorbent indicator 136 could alternatively be located between the second side face 134 of the spacer bar 106 and the second pane 104. FIG. 3 illustrates a variant in which the glazing unit 100 is a triple glazing unit. The triple glazing unit 100 further comprises: a third pane of glazing material; a second spacer bar 306 between the second and third panes 104, 302; an optional second edge sealant 308 acting as a secondary seal between the second and third panes 104, 302; and a second cavity 312 between the second and third panes 104, 302, defined by the second and third panes 104, 302 and the second spacer bar 306, the second cavity 312 comprising a controlled atmosphere. In FIG. 3, each cavity 112, 312 is sealed from each other and therefore comprises a separate absorbent indicator 136, 336. The absorbent indicators 136, 336 may be located proximally to each other, for example side-by-side. In another example, the cavities 112, 312 comprise a shared atmosphere, and only one absorbent indicator 136 or 336 is provided in one of the cavities 112, 312, to the indoor side or outdoor side of the glazing unit 100. FIG. 3 shows that the absorbent indicators 136, 336 are each secured in the manner shown in FIGS. 1-2. In other examples, one of or each of the absorbent indicators 136, 336 may be secured in the manner shown in FIGS. 4-5 described below. FIG. 4 illustrates another method of securing the absorbent indicator 136 to the spacer bar 106. In FIG. 4, the absorbent indicator 136 substantially consists of a flat sheet. One face of the flat sheet is adhered by adhesive or self-adhesive to the inner face 128 of the spacer bar 106. This involves the use of extra adhesive. FIG. 5 illustrates another method of securing the absorbent indicator 136 to the spacer bar 106. In this example, a mechanical fixing 500 is used such as a rivet. The mechanical fixing 500 extends through the absorbent indicator 136 and penetrates the inner surface of the spacer bar 106 without extending all the way through its thickness. This technique obviates the need for adhesives which can emit VOCs while curing. The glazing unit 100 described herein is suitable for use with roof lights, windows, and doors, for buildings. A roof light is a type of window which forms all or part of a roof space of a building, for daylighting purposes. Other terms for roof light are ‘skylight’ and ‘roof window’. The glazing unit 100 could also be used in wall windows attached to upright walls of buildings. The glazing unit 100 could also be used in glazed doors such as external doors. External doors can include single leaf doors, double leaf doors, folding patio doors, or the like. Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be 17 understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and / or shown in the drawings whether or not particular emphasis has been placed thereon. 29 10 24

Claims

1. A glazing unit for a roof light, window, or door for a building, the glazing unit comprising:first and second panes of glazing material;a spacer between the first and second panes;a cavity between the first and second panes, defined by the first and second panes and the spacer, the cavity comprising a controlled atmosphere; andan absorbent indicator located in the cavity, configured to change appearance in response to a change of humidity and / or pH in the cavity, wherein the absorbent indicator comprises a first leg and a second leg transverse to the first leg, wherein the second leg extends along an inner face of the spacer, and wherein the first leg is clamped against the first or second pane by a side face of the spacer.

2. The glazing unit of claim 1, wherein the change of appearance comprises a colour change.

3. The glazing unit of any preceding claim, wherein at least part of the absorbent indicator extends on an interior surface of the spacer, facing the cavity.

4. The glazing unit of any preceding claim, comprising a substantially opaque strip secured to a main surface of one of the panes, extending parallel to a perimeter edge of said pane.

5. The glazing unit of claim 4, wherein the absorbent indicator is concealed by the substantially opaque strip when the glazing unit is viewed from a direction perpendicular to a plane of the pane to which the opaque strip is secured.

6. The glazing unit of any preceding claim, wherein the absorbent indicator is secured to the spacer and / or clamped between the spacer and one of the panes.

7. The glazing unit of claim 6, wherein the absorbent indicator is adhered to an inner face of the spacer, the inner face of the spacer facing the cavity.

8. The glazing unit of claim 6 or 7, wherein a portion of the absorbent indicator is secured to a side face of the spacer, the side face facing one of the panes.

9. The glazing unit of claim 6, 7, or 8, wherein the absorbent indicator is secured to the spacer by at least one mechanical fixing.

10. The glazing unit of any preceding claim, wherein the side face of the spacer comprises an adhesive to adhere to the first leg of the absorbent indicator.

11. The glazing unit of any preceding claim, wherein the first leg of the absorbent indicator comprises a protruding portion which protrudes beyond the side face of the spacer into a space where an edge sealant extends.

12. The glazing unit of claim 11, wherein the edge sealant is bonded to an outer face of the spacer and to main surfaces of the first and second panes, and wherein the edge sealant is further bonded to the protruding portion of the first leg of the absorbent indicator.

13. The glazing unit of claim 12, wherein the protruding portion of the first leg of the absorbent indicator is buried by the edge sealant, relative to an exterior face of the edge sealant, wherein the exterior face of the edge sealant is proximal to perimeter edges of the first and second panes.2014. The glazing unit of any preceding claim, wherein the spacer comprises an elastomer.

15. The glazing unit of any preceding claim, wherein the spacer is formed from a sealing tape, and wherein the edge sealant is a bead of setting material.

16. The glazing unit of any preceding claim, further comprising:a third pane of glazing material;a second spacer between the second and third panes; anda second cavity between the second and third panes, defined by the second and third panes and the second spacer, the second cavity comprising a controlled atmosphere.

17. The glazing unit of claim 17, comprising a conduit between the first cavity and the second cavity, wherein the absorbent indicator is in fluid communication with the first and second cavities, orwherein the first and second cavities are sealed from each other, and wherein the glazing unit comprises a second absorbent indicator in the second cavity, configured to change appearance in response to a change of humidity and / or pH in the second cavity.

18. A method of manufacturing the glazing unit of any one or more of the preceding claims, the method comprising:providing the first and second panes, and the spacer;positioning the absorbent indicator against the spacer;securing a first side face of the spacer and a main surface of the first pane to each other, and a second side face of the spacer and a main surface of the second pane to each other, and wherein the absorbent indicator is located within the cavity.

19. The method of claim 18, wherein the first and second side faces of the spacer each comprise self-adhesive to self-adhere the spacer to the first and second panes.

20. The method of claim 18 or 19, wherein the method further comprises applying a bead of settable material to a perimeter region of the glazing unit between the spacer and an outer edge of the first and second panes.

21. The method of claim 18,19, or 20, wherein the method further comprises replacing an atmosphere within the cavity with a controlled atmosphere.

22. The method of any one of claims 18 to 21, wherein the absorbent indicator against the spacer comprises securing the absorbent indicator to the spacer and / or clamping the absorbent indicator between the spacer and one of the panes.

23. The method of claim 22, comprising securing the absorbent indicator to the spacer before securing the first and second side faces of the spacer to the first pane.

24. The method of claim 23, wherein the absorbent indicator comprises a first leg and a second leg transverse to the first leg, wherein positioning the absorbent indicator against the spacer comprises positioning the first leg against self-adhesive on the first side face of the spacer prior to securing the first side face of the spacer to the first pane.

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