Encapsulating stained glass in sealed double-glazed units is a technique we carry out occasionally, but in many cases it is not the best solution. Here are a few of the issues we have come across, along with a couple of alternative solutions.
Issues to consider with Encapsulation
Stained glass windows last a long time - the majority of domestic stained glass panels in the UK are of Victorian or Edwardian age (e.g around 80-120 years old). In contrast, even high quality double glazed units only last 15-20 years or so, and standard units much less than this. The polymer edge bond around a sealed unit is slightly vapour permeable, so to prevent the unit from misting the edge bond incorporates a desiccant in a perforated aluminium strip. Over time this desiccant becomes saturated, and moisture is then able to condense on the inside of the glass.
Encapsulating a panel in a double glazed unit increases the likelihood that the stained glass panel will receive constant contact with moisture once the panel "blows" or starts misting. The risk of this can be minimised by using "dry-mounting" techniques for the panel (avoiding the use of putty adjacent to the polymer edge bond cuts down on corrosion) but eventually the glazing unit will need to be replaced.
The energy savings attributed to double glazing by glass companies anxious to sell you complete double glazed units and frames can take as long as 30 years to recoup - bear in mind that, over that time, you will probably have to replace the units themselves at least once too. Given that, some experts recommend draught proofing as a more cost-effective way of saving on heating bills.
Effect on existing timber frames
In traditional timber framed windows, the size and weight of standard double glazed units can be an issue. Often the rebate on original timber frames is not deep enough to cope with a double glazed unit, and the sash weights on traditional sash and case windows cannot usually handle new units and need to be re-weighted, even with slimline units (which themselves cannot accommodate stained glass panels).
Another consequence of installing double glazed units in existing timber frames is that condensation is promoted by the removal of draughts, and, as a consequence, previously sound 100 year old timber frames may become subject to damp and subsequently rot.
Stained Glass Conservation
One of the biggest arguments against encapsulation, from the point of view of conserving stained glass, is that is effectively an irreversible process. The glass around the border of the panel needs to be cut down by almost 1": in total in to accommodate the polymer strip, spacer bars and desiccant around the border of the sealed unit. This may result, in the absence of a sacrificial border, in removal of a portion of the design. As noted above, the propensity of sealed units to trap moisture adjacent to the panel may also damage paintwork and speed up formation of corrosive salt deposits on lead.
Releading and cementing
Usually the reason clients ask about encapsulation is that the existing leaded panel rattles and is draughty and possibly leaking. Stained glass panels gain most of their strength from a cement forced between the glass and lead during construction. Over time, this cement dries out and after 100 years or so it is often dry enough to fall out, weakening the panel. Once a panel has been releaded and properly cemented, it is completely waterproof, with enough flex to resist almost all weathers. As a consequence, we normally try and persuade customers to reinstall their panels as single glazed units, with or without secondary glazing.
This is a broad brush term to cover the addition of a second layer of glass (or polycarbonate) between the panel and the elements. There are a multitude of secondary glazing options available. Some of these include:
Isothermal glazing (used most commonly on windows of historic significance in churches).
Polycarbonate (used most often as a protective layer against damage, again primarily in churches).
Toughened glass secondary glazing - most common domestically.
The last of these, installing a sheet of toughened glass on the outside of a stained glass panel, is probably the simplest way to to increase insulation without causing condensation issues, and can often be carried out without affecting the existing stained glass. Aesthetically it is far less attractive than stained glass when viewed from the outside of the building in daytime, as the texture of the panel is lost, but it is generally cheaper than encapsulation. One other noteable drawback is that the void between the panel and the secondary glazing is very attractive to spiders, which are impossible to remove without removing the external glazing sheet.
In some cases we do deploy encapsulation as a way of retaining stained glass in a window. When a customer is having new timber frames made, or if the window is situated in a particularly windy location, encapsulation may be a good choice, especially if the design will not be affected when the panel is resized. It is important to warn the client about the issues above, in particular the fact that the design may have to be altered.
Another setting we use encapsulated panels in is to fit a small panel in a modern front door. Encapsulation adds security to the entrance, but, more importantly, the door will already have been designed for the size and weight of a modern double glazed unit.