How to get roof window and rooflight specification right

Appropriate building fabric specification

One of the benefits of a ‘whole building’ approach to energy efficiency  is the ability to compensate for poorer performing areas or elements by ‘beefing up’ others.

There are limits to that, so a level of common sense is necessary. Building regulations stipulate worst-case U-values for new buildings, so a minimum level of performance still has to be achieved.

For work to existing buildings, area-weighted U-values combine the performance of building fabric elements. This approach allows for larger areas of glazing to be incorporated in an extension, with surrounding elements improved to compensate.

Specifying lower building fabric U-values is all very well, but there is a point at which they become economically and practically infeasible. The addition of more and more insulation generates diminishing returns, so really low U-values in the order of 0.10 W/m²K require a thickness of construction that may not have been planned for.

Know the limits on roof glazing performance

As it is with insulation, so too with windows and doors – including roof glazing products. Manufacturers are always looking to improve the performance of their products, and current technology can produce roof windows with whole-unit U-values of around 0.80 W/m2K.

Eventually, maybe all roof windows, and glazing generally, will offer U-values this low – or even lower – but for now specifiers, installers and clients need to be aware of what is appropriate to their project.

Just because they exist, though, doesn’t mean they are right for every project – or available! A window with that sort of performance is a highly engineered, triple or quadruple glazed unit – something that may be beyond the budget of many projects. It comes back to taking a balanced approach, that offers flexibility without going to extremes.

Keep product substitution to a minimum

Quoting a certain U-value for a roof window as a performance specification, and then expecting a contractor to source a window of the correct size that meets the specification, risks compromising the compliance calculations if they don’t know where to look for one.

Product substitution is a risk inherent in all construction projects. It could be for reasons of cost or availability, or because those supplying or purchasing the materials do not understand the implications of differences in performance.

Uninformed or unauthorised specification changes are a consistent cause of design intent not being achieved.

Understanding roof glazing U-values

Where glazing products are concerned, widespread understanding of U-value declarations is not what it could be. If that leads to mistaken performance claims and the installation of roof windows that do not match the original specification, then compliance simply may not be achieved.

When a rooflight meets the needs of a project, every effort should be made to stick with the choice. Unlike other building materials, it is difficult to simply swap to a like-for-like alternative that costs less or is more readily available. If in doubt, contact the rooflight/roof window manufacturer for advice.

Better still, specify the right quality products from a reliable manufacturer at the start and set a tone for the project where quality and the end result is prioritised over short term gains. Ideally, choose to work with a manufacturer who is a member of the National Association of Rooflight Manufacturers (NARM).

This topic is explored in our ‘Thermal Performance of Roof Glazing’ white paper, available to download for free here.