Conservation Rooflight Energy Efficiency: U-values, Glazing and Listed Buildings

Tuscan Foundry has supplied cast iron for listed and period buildings since 1893. We work on historic roofs every week, and the question we hear most about rooflights is how to make an old roof warmer without spoiling it. This page explains what drives the energy performance of a conservation rooflight, how to read the figures, and what to do with an original cast iron rooflight on a listed building.

Reading U-values properly

A U-value measures heat loss: the lower the number, the better. Two figures are often confused. Ug is the centre-of-glass value and always looks good. Uw is the whole rooflight, including the frame and any glazing bars, and is the figure that matters. Values also change with size and with the angle at which the rooflight is installed, so compare quotes only at the same reference size and pitch. Our chapter on U-values and g-values explains how to compare them fairly.

Air tightness matters as much as glazing

A rooflight that leaks air loses more heat than one with slightly poorer glass. Air permeability is classified under BS 6375-1, and the best performers reach the highest class. Ask for the class achieved, and make sure the installer seals the frame to the roof structure and the internal lining, because a gap around the unit undoes a good test result.

Glazing choices for heritage roofs

• Double-glazed sealed units with low-emissivity coatings and warm-edge spacers are the standard and suit most conservation rooflights.
• Triple glazing improves the figures but adds weight and thickness, which can push the frame above the slates and spoil the flush profile that a conservation rooflight is meant to have.
• Vacuum glazing gives very good performance in a slim unit and is worth asking about where the sightline is critical.
• Solar control coatings reduce the g-value and help on south-facing slopes, where overheating can be more of a problem than heat loss.

See the glazing chapter for the detail.

Frames, thermal bridges and condensation

Metal is a good conductor, so the frame is the coldest part of a rooflight. Thermal breaks, an insulated upstand and an insulated internal lining reduce heat loss and the risk of condensation forming on the frame. Some condensation in cold weather is normal in a room that produces moisture, such as a bathroom or kitchen; ventilation is the cure. Our condensation chapter helps tell condensation from a leak.

Materials and long life

Steel and cast iron are among the most recycled materials in construction, and both can be recycled again at the end of their life. In our experience the bigger environmental gain is longevity. A rooflight that lasts fifty years, can be repaired, and has a replaceable glazing unit uses far less material over its life than a cheaper unit replaced twice. The same principle runs through all our cast iron work: repair first, replace only what has failed, and keep the paint in good order. Linseed oil paint, which we use on our own castings, is a good example of a low-impact finish that is easy to maintain.

Original cast iron rooflights on listed buildings

An original cast iron rooflight is part of the special interest of a listed building and should normally be repaired rather than replaced. Repairs can include new glass, fresh putty, freeing seized stays and repainting. Where more warmth is needed, there are options that leave the historic frame in place, such as discreet secondary glazing on the inside. Replacing an original rooflight, or changing its glazing, is likely to need listed building consent, so talk to the conservation officer early. Our guide to rooflights, consent and planning sets out when permission is needed.

Building Regulations

A new or replacement rooflight in a dwelling generally has to meet the energy requirements of Part L. Approved Document L recognises that historic and traditional buildings need a balanced approach, aiming to improve energy efficiency as far as is reasonably practicable without harming the building’s character or causing problems with moisture. Your building control body and conservation officer can agree what that means for a particular building. The Building Regulations chapter covers Parts L, F, B and K.

Where Tuscan Foundry fits in

We do not make rooflights. We are cast iron specialists with long experience of listed buildings, and this information is shared to help owners and specifiers make good decisions. For a new conservation rooflight we refer enquiries to The Rooflight Co. and Clement, and our Conservation Rooflight Guide covers the subject in depth. For gutters, hoppers and downpipes we supply cast iron rainwater goods, and we are glad to advise on the roof as a whole. Get in touch with photographs of your building.

Frequently asked questions

What is a good U-value for a conservation rooflight?

Compare the whole-rooflight Uw figure, not the centre-pane Ug figure, at the same size and pitch. The current Part L limits for your project can be confirmed with your building control body.

Can I fit triple glazing in a conservation rooflight?

Sometimes, but the extra thickness can raise the frame above the roof covering and spoil the flush profile. Slim double glazing or vacuum glazing often gives a better balance on a historic roof.

Why does my rooflight get condensation?

The frame and glass are the coldest surfaces in the room, so moist air condenses on them first. Better ventilation, an insulated lining and a well-sealed installation usually solve it.

Can an original cast iron rooflight be made more energy efficient?

Yes, often without removing it. Repairs to glazing and putty, draught sealing and internal secondary glazing can all help. Changes to a listed building may need consent, so speak to the conservation officer first.

Does Tuscan Foundry supply rooflights?

No. We specialise in cast iron rainwater goods for period buildings and refer rooflight enquiries to The Rooflight Co. and Clement.