Historic Churches 2019

BCD SPECIAL REPORT ON HISTORIC CHURCHES 26 TH ANNUAL EDITION 21 understood. Air flow was clearly critical, but how wide apart did the glazing planes need to be, and how much of a barrier to flow were elements in the interspace such as ferramenta? How should the interspace be ventilated? Ventilation towards the interior ensured the stained glass was embedded in the warmer internal air of the building, but would ventilation to the exterior work as well? To answer these and other technical questions, in 2014 Historic England commissioned a research project centred on in situ and laboratory testing, and the assessment of a wide range of existing EPG installations. The project was led by Tobit Curteis of Tobit Curteis Associates and Leonie Seliger of The Cathedral Studios, Canterbury, with contributions from specialists in microbial growth and in modelling air flow. The report on this research has now been released, together with a guidance note that puts EPG in the wider context of environmental deterioration of stained glass. TECHNICAL CONSIDERATIONS FOR EPG The research confirmed that the key to successful EPG is the ventilation of the interspace between the two panes of glass at the top and the bottom of the window. If the building fabric is in good condition, with no serious moisture problems to increase the interior relative humidity, properly ventilated EPG prevents almost all instances of condensation on the stained glass. Even in poor conditions, condensation events are much reduced. The airflow is driven by the difference between the buoyancy of the air in the gap between the glazing planes (the interspace), and the buoyancy of the air in the space to which the gap is being vented. Buoyancy is primarily driven by air temperature, and to a lesser extent by relative humidity: hotter air and more humid air will both rise. Because glass is very sensitive to ambient temperature changes, the natural heating and cooling of the exposed exterior protective glazing will condition the air in the interspace. Solar gain and changes in air temperature produces remarkably strong vertical airflows, the direction of which will change according to the relative temperatures of the interior and exterior air. Flow is easily strong enough to overcome pinch-points such as ferramenta in the interspace. The minimum depth should generally be no less than 40mm, but as long as there is a minimum of 10mm gap at any pinch-points, the air flow can overcome any problems of turbulence. It is, of course, best to minimise obstructions as much as possible. Although the necessary depth can often be achieved with the stained glass left in its original glazing groove, in some cases it would need to be framed and moved slightly forward in the window splay. Ventilation was found to be most effective when the gaps at the top and bottom ran across the entire width of the glazing; but even smaller or less even vents can still produce acceptable flow, especially in tall windows. For maximum effect, the vent height should be at least one-third of the interspace depth: in other words, if the interspace depth is 60mm, the vents would need to be at least 20mm high. Wherever possible the ventilation should be towards the building interior (through the stained glass); but in instances where that cannot be achieved (whether because of the configuration of the window, or perhaps the fragility of the glass, or some other reason) we found that ventilating towards the exterior, through the protective glazing, will still give a marked reduction in condensation events. It is important to caution, however, that exterior-ventilated EPG will be at much greater risk of rainwater penetration. Condensation may be reduced or prevented on the stained glass, but it will still occur on the secondary glazing; and there will always be some risk of rainwater penetration as well. Therefore the interspace will need to incorporate some means of collecting any liquid water, and usually way of removing it from the interspace. The most effective method is to place a lead-lined collection channel at the base of the window, with drainage out to the exterior (through the secondary glazing). This channel should be filled with graded gravel, which will greatly reduce any evaporation of the collected water back into the interspace, as well as cutting air ingress into the interspace from the exterior. Schematic diagrams showing how EPG works against condensation: the direction of airflow will be dynamic, changing continually throughout the day and night in response to the exterior conditions. (Diagram: Historic England/Robyn Pender) INTERIOR INTERIOR INTERIOR INTERIOR Exterior ambient temperature = 1°C Surface temperature of stained glass = 5°C Interior ambient temperature = 15°C Ambient relative humidity = 75% Dew-point temperature = 10°C Condensation on stained glass Exterior ambient temperature = 1°C Surface temperature of stained glass = 11°C No condensation on stained glass Interior ambient temperature = 15°C Ambient relative humidity = 75% Dew-point temperature = 10°C Exterior ambient temperature = 25°C Interior ambient temperature = 15°C Exterior ambient temperature = 5°C Interior ambient temperature = 15°C Interspace air temperature = 10°C Interspace air temperature = 20°C Protective glazing