5 minute read
Getting glass rooflights right
Nick Henton of Lareine Engineering looks at the factors to consider for a successful and fully compliant rooflight specification
We all prefer light, airy daylit interiors and the benefits of exposure to daylight are well documented – as well as the potential for energy savings through reduced demand for artificial light.
These factors have all contributed to the huge rise in popularity of rooflights and glazed roof structures across all sectors in recent years. Plus of course, the fact that rooflights deliver up to three times as much daylight as the same area of vertical glazing. However, with the extremely wide choice of rooflight types currently available, specification raises a number of points for consideration.
Practical considerations
How much and what type of light is appropriate? What is the roof construction of the building? Is ventilation required?
Required levels of illuminance: values as listed in Cibse Guide A
To answer the first of these questions, the ideal light level for an internal space is largely dictated by its proposed use. For example, in stores and warehouses where visual tasks may not need a high perception of detail, 200 Lux is considered appropriate. Whereas for workshops or design studios where accurate detail perception is important, 2000 Lux is recommended. This has a bearing on the size, number and placement of rooflights.
For most applications, diffused light is the preferred choice, avoiding glare and the potential for unwanted heat build-up. This can be simply achieved, using diffused or solar control variants of the recommended glazing types.
A wide variety of rooflight designs are available to cater for both flat and pitched roof constructions. Flat glass or lantern style rooflights are the obvious choice for flat roofs and ‘in-plane’ roof windows are popular for pitched roof applications including loft conversions.
All types can be specified with opening sections for ventilation if required: from a simple hinged roof window, to a fully automated SHEV (Smoke & Heat Exhaust Ventilation) system for fire safety in larger commercial and industrial buildings. SHEV systems are a subject in their own right and Lareine Engineering operates a division specialising in these solutions.
Glass specification
Whilst alternative glazing materials are available for rooflights, for most residential and commercial applications, glass remains the preferred glazing choice. It offers excellent light transmission, outstanding aesthetics and longterm durability, with a huge range of options for matching specification to application – including solar control tints, etched surfaces for light diffusion, self-cleaning properties and others.
For any rooflighting project, glass specification should be among the first points to consider.
In the majority of building envelope applications, glass is fitted as a Double Glazed Unit (DGU), or increasingly, as a Triple Glazed Unit (TGU), in order to provide improved resistance to the passage of heat, thus improving energy efficiency and comfort to building users. These sealed units of differing pane numbers are collectively known as Insulated Glass Units (IGU).
A typical insulated Glass Unit, with laminated inner pane for safety of building occupants
Safety first
In respect of safety for people needing to access roof areas, rooflights are classified for non-fragilty according to the ‘Red Book’ ACR[M]0012 issued by the ACR (Advisory Committee for Roof Safety) and based on testing by the CWCT (Centre for Window & Cladding Technology).
The highest rating, Class 0, applies to rooflights which are designed for unrestricted access by building occupants. These are known as ‘walk-on’ rooflights and are becoming increasingly popular for use on roof terraces, allowing daylight into the rooms below.
Class 1 applies to rooflights which might be walked on for occasional cleaning/maintenance activities and which will therefore need to support both the weight of people on the glass and their equipment.
Class 2 covers rooflights for applications where people are not intended to walk on the glass, but which are required to be non-fragile to protect maintenance staff or others who may need to access roof areas.
Class 3 rooflights are to be considered as fragile. Adjacent roof areas should be equipped with barriers to prevent access.
For safety of building occupants, Lareine Engineering strongly recommends that a hermetically sealed DGU mounted within a rooflight frame should comprise a toughened glass outer pane, argon filled cavity and a laminated glass inner pane – as supported by NARM, the National Association of Rooflight Manufacturers, of whom we are a member company.3
Toughened glass is sufficiently durable and appropriate for outer panes, but for inner panes, the shatterproof characteristics of laminated glass prevent the possibility of glass debris falling onto building occupants in the event of breakage. In rare instances, toughened glass can shatter spontaneously, due to manufacturing imperfections, so it should always be avoided for inner panes.
Thermal performance
Approved Document L of the Building Regulations stipulates appropriate levels of thermal performance for rooflights. The required U-value for rooflights is dependent on their application and whether for new build or refurbishment. These values should be noted at the specification stage. It’s also important to note that the quoted U-value for a rooflight is for the whole assembly, not just the glazing. Some rooflight suppliers quote ‘centre pane’ U-values which apply only to the glazing, not the complete assembly – and are therefore misleading.
Thermal performance of rooflights has generally improved dramatically over recent years and it’s now widely understood that appropriately specified, compliant rooflights make a positive contribution to building energy efficiency.
Security
The Building Regulations state that rooflights deemed as easily accessible need to be proven to resist criminal attack or incorporate features proven to resist crime. To demonstrate this they need to have been tested to one of the security standards listed in Approved Document Q of The Building Regulations and have achieved a minimum performance standard.
Simplifying a complex subject: free Webinar
If you’ve read this far, you’ll know that rooflight specification is a fairly complex subject, requiring some knowledge of all the factors involved. Choosing a reputable supplier is always a recommended solution. However, if you’d like to gain greater understanding yourself, Lareine Engineering is running a RIBA CPD accredited webinar on the topic, at 12 midday, on Tuesday 21st September.
