Conflicting Priorities: Protective-Glazing Systems in the Light of Current Energy-Conservation Strategies
In this month’s issue Sophie Wolf and Stefan Trümpler from the Vitrocentre Romont report on a recent research project on the thermal efficiency of external protective-glazing systems in Switzerland. First results were presented at last year’s Forum of the International Scientific Committee for the Conservation of Stained Glass (Corpus Vitrearum-ICOMOS) in Amsterdam and are published in the preprints of this conference. 
Protective glazing is among the most effective measures for the protection of stained glass. The effects of internally and externally ventilated protective-glazing systems have been systematically studied, and the efficiency of protective glazing in general, and isothermal glazing in particular, has been well established. As a preventive conservation measure protective glazing not only shields stained glass from the effects of the environment, but also helps to keep the number of interventions (restoration, conservation, maintenance) on the stained glass to a minimum. The strategy as it is applied today is fully in line with the conservation guidelines that were first established in 1989 by the International Committee of the Corpus Vitrearum for the Conservation of Stained Glass and the Stained Glass Committee of ICOMOS.
Protective glazing however is increasingly expected to fulfil energy-saving requirements (especially in view of the ongoing energy debate) – at least in countries where there is both an interest in retrofitting churches and funding available to preserve post-medieval stained glass. Due of rising energy costs and because of ecological concerns, church authorities in Switzerland are trying to lower the energy consumption of their buildings. Windows are often considered a weak point in terms of heat conservation, and there is a widespread belief that adding protective glazing with double-glazing units is an efficient way to save energy and heating costs. This explains why the amount of protective glazing with double-glazing units in Swiss churches, the majority of which have glazing from the Baroque period or stained glass from the 19th or early 20th centuries, has increased considerably in the past two decades (Figs 1–2).
Changes in building use, greater demands for comfort, and guidelines concerning measures aimed at conservation and the preservation of cultural heritage are developments that represent major challenges for church authorities. In the process of planning for the renovation and thermal retrofitting of churches, important questions are often not addressed adequately.
• Is the installation of protective glazing fitted with double glazing worthwhile in terms of energy conservation (and cost savings)? In other words, what is the cost-benefit ratio?
• Do protective-glazing systems with double-glazing units fulfil requirements from a conservation point of view? What are the risks associated with fitting double-glazing systems?
• How do these systems compare to simple protective-glazing systems that were typical in the first half of the 20th century?
These questions were the starting point for an interdisciplinary research project initiated by the Vitrocentre Romont. One of the main aims of the project was to question the ‘automatic’ introduction of systems with double glazing from a conservation point of view, and to challenge the assumption that the installation of such systems represents an efficient energy-conservation measure.
The study has provided new data on the thermal properties of protective-glazing systems used in Switzerland to protect post-medieval stained glass and has also led to a better understanding of their efficiency of in terms of conservation. Laboratory measurements and calculations have shown, for example, that the fitting of single-pane protective glazing reduces the thermal-transmission coefficient of a stained-glass window by a factor of 2, and double-pane glazing by a factor of 3.5. Yet despite the marked reduction of the thermal-transmission coefficient caused by adding protective glazing (whether single or double), further calculations demonstrated that the overall heat loss through the windows is negligible compared with that via the generally uninsulated church walls. In a case study and by way of simulation we were also able to prove that the installation of protective glazing with double-glazing units is much less energy efficient than other energy-saving measures: the comparison of various energy-saving options illustrated that by reducing the average heating temperature by 1–2°C, approximately 40% of heat energy could be saved; the relatively costly fitting of protective glazing, in contrast, would only yield an energy saving of 8%. Before taking any decisions on which measures to adopt, the effects of these options on the interior climate of the church and the consequences and risks (e.g., damage to materials and structures) would have to be assessed of course. The case we evaluated, however, clearly demonstrated that retrofitting windows with protective glazing does not yield the expected energy gain and is very expensive compared with other options.
As far as conservation efficiency is concerned, the results of dynamic climate measurements on selected protective-glazing systems as well as measurements in situ corroborated our observation that the protective-glazing systems that were installed in the first half of the 20th century (Figs 3–4) are effective in the preservation of post-medieval stained glass, and that they have many advantages over systems that came into use in Switzerland in the early 1980s (Figs 5–6). A major advantage associated with these early systems is that they are structurally independent from the stained-glass window, which therefore can remain in its original setting when the protective glazing is installed. In contrast, structural changes on the historic windows are necessary when fitting the newer ‘bonded’ protective-glazing systems. The stained glass usually has to be trimmed to fit the heavier new frame, and historic armouring has to be removed. From a structural perspective, the installation of the bonded systems, which nowadays are fitted with double glazing, can be critical. Initial observations of damage caused by condensation on walls adjacent to the windows are proof of this (Figs 7–8). The efficiency and sustainability of double-glazing systems therefore have to be questioned from a conservation point of view.
Finally, the study has provided arguments for a reappraisal of early protective glazing in Switzerland. The positive properties and historical importance of the simple and usually also aesthetically appealing ‘storm window’ systems are often underestimated (Fig. 9). They now constitute historically significant elements of buildings of cultural importance. Seen from a holistic perspective, these early systems are virtually as efficient as systems preferred today and are more sustainable in terms of materials and durability.
Together with the Association professionnelle suisse du vitrail (APSV), the Vitrocentre Romont is currently preparing a brochure on this topic that is intended to assist church authorities, architects and cultural-heritage managers in finding appropriate and sustainable solutions for protecting their stained glass and saving energy. The aim is to put forward practical recommendations for energy-saving measures based on an evaluation of various options. The qualities and risks associated with protective-glazing systems are weighed up in comparison with alternative proposals.
1. S. Wolf, S. Trümpler, K. Ghazi Wakili, B. Binder and E. Baumann, ‘Protective glazing: The Conflict between Energy-saving and Conservation requirements’, in H. Roemich and K. van Lookeren Campagne (eds), Recent Advances in Glass, Stained-glass, and Ceramics Conservation, Zwolle, 2013, pp. 99–108.
2. For a short review on this subject and references, see ibid., pp. 100–101.
3. The ‘Guidelines for the Conservation and Restoration of Stained Glass’ (second edition, Nuremberg 2004) is available in English on the British CVMA website. French and German versions of the text may be found on the website of the international Corpus Vitrearum (http://cvi.cvma-freiburg.de/documents/CVDirectivesConservation.pdf and http://cvi.cvma-freiburg.de/documents/CVRichtlinienKonservierung.pdf).
4. The project was carried out in close collaboration with Karim Ghazi Wakili and Bruno Binder from the Swiss Federal Laboratories for Materials Science and Technology (Empa) and Ernst Baumann from Baumann Akustik and Bauphysik AG (Dietfurt, Switzerland). It was funded by the Stiftung zur Förderung der Denkmalpflege in Zurich.
5. Also known as the U-factor and given as W/m2K. The rule is that the higher the U-factor, the higher the heat loss through the tested element. The U-factor measured for the unprotected stained-glass window was 5.8 W/m2K, while that of the stained-glass window with double protective glazing was 1.6 W/m2K. This latter value is higher than the U-factor assumed for modern double glazing. This is explained by the fact that – because of aesthetic considerations – the protective glazing was composed of three separate fields held in a metal frame, which acts as thermal bridge.