Replacing Victorian cast iron skylights

Conservation Rooflight is designed to resemble an authentic Victorian skylight, but with a much higher standard of insulation and user-convenience.

While practising in London in the late 1980s, architect and founder of the Rooflight Company Peter King observed a large number of Victorian cast iron skylights slowly disintegrating, with no suitable substitute available.

Contemporary roof windows were inappropriate for many historically significant properties, but single-glazed cast iron replacements were unable to meet modern building standards.

He therefore set about creating the Conservation Rooflight, a product designed to resemble an authentic Victorian skylight, but with a much higher standard of insulation and user-convenience.

One obstacle in the product's development was providing protection against the common problem faced by all metal-framed rooflights and windows - condensation.

By inventing the Thermoliner, a unique, patented thermal break device, this was a difficulty he was able to overcome.

Peter explains: "It has long been recognised that metal rooflights, like metal windows, are susceptible to condensation".

"In the past, people with metal frame windows were used to the resulting moisture that inevitably dripped down from them".

"They accepted this and lived with it because, at the time, there was no real way of preventing it".

"A house's occupants generate water vapour through everyday activities - whether it be bathing, washing, cooking or even just breathing, and it is this - inevitable - moisture vapour that condenses as water droplets on relatively cold surfaces, such as metal windows or rooflights".

The air we exhale is itself saturated with moisture.

In fact, in the average four-person household, the occupants themselves can generate from 8 to 24 pints of water every day.

When moisture can't escape to the atmosphere outside, it condenses on any available cold surface".

""Metal window manufacturers were aware of this," Peter continues, "but their answer to the problem was to put the blame on the moisture-laden internal air, and recommend that the internal window sills be tiled for protection and, with no other solution to hand, this was deemed to suffice".

"With a steel framed rooflight, the potential problem is even worse, as any condensate dripping down will be landing directly on the flooring, instead of just lying on a sill and evaporating".

"This will eventually cause rotting or other deterioration, so it had to be addressed when we were developing the new product".

The situation is exacerbated by the fact that these days people will tolerate neither condensation nor increased ventilation (which removes the moisture-laden air) Whereas previously a great deal of water vapour could escape via air leakage through cracks and gaps in the building envelope, there is now a recognition that we need to build airtight structures in order to improve energy efficiency, and this is reflected in the Building Regulations.

When implemented later this year, the revised Part L will require the energy efficiency of whole buildings to be assessed, with the extension of air tightness testing to cover all building types, including dwellings.

The only effective answer to condensation on a metal window or door is to incorporate an efficient thermal break or thermal lining to isolate the cold part of it from the relatively warm, moist internal air.

The Thermoliner is a thermally decoupled lining made from PVCu developed to address the potential for condensation by isolating the cold bridge at the rooflight frame.

To the rooflight user the Thermoliner presents a warm to the touch internal edging to the rooflight frame and, more importantly, there is no incidence of dripping condensation internally.

As Peter King notes, "The Thermoliner is innovative in that it does not set out to prevent condensation - it allows condensation to occur in a controlled manner/, /out of sight of the house's occupants".

Contained within the rooflight frame and generally matt black in colour, it is unobtrusive and is rebated for 12.5mm plasterboard or timber linings to aid internal finishing.

When finished correctly, none of the rooflight perimeter metalwork is directly visible.

Integral drainage slots ensure that any condensation that may occur is channelled out of the building.

In rooms where more moisture vapour is likely to be generated, e.g bathrooms and kitchens, a Thermoliner for the rooflight glazing bar(s) can be fitted on request".

"Because the Thermoliner did not already exist, it was necessary to invent it," says Peter".

"Its development has been key to the success not only of the Conservation Rooflight, but to the success of the company as a whole".

Thermoliner is exclusive to the Rooflight Company, and is available in black, white, dark and light grey to match standard BS colours.

Back to top