Dome rooflights are a great way to get light into a building. Peter Mayer of Building LifePlans considers the specification options, durability and whole-life costs
Rooflights can save energy on lighting and provide evenly distributed light that enhances the internal environment.
Rooflights with kerbs, also known as out-of-plane rooflights, are used on flat or low-pitched roofs. They can also provide ventilation, either through the light opening or by incorporating ventilation grilles in the kerb or frame. Dome rooflights come in various shapes, including pyramidal, dome or trapezoid. Other types of out-of-plane rooflight include:
- Vault rooflights: Curved units available as modular units or complete system formats to create much larger areas of roof lighting.
- Pitched rooflights: Rooflights configured to create
Specification options: glazing
The glazing material may be single, double or triple skin. The more layers, the greater the thermal efficiency. The extra costs for multi-layer glazing may be compensated by reduced heating costs. Glazing may be clear or translucent and include anti-glare coatings or tints.
- Polycarbonate has excellent light transmission and impact resistance qualities. Its expected service life is 15-25 years.
- PVC is mainly used for domestic applications as it does not meet the non-fragility requirements for industrial applications. Its expected service life is 15-20 years.
- Glass options include safety glass such as Georgian wired, laminated or toughened glass. The service life of insulated glass units is 10-25 years; for single glazing, it is 60 years.
Specification options: kerbs
Kerbs may be part of the unit or constructed on site. Factory-made kerbs are manufactured from aluminium, galvanized steel, glass-reinforced plastics or PVCu. Metal kerbs may require additional insulation to minimise the risk of condensation.
The service life of plastic kerbs is 20-30 years. The service life of aluminium is related to its thickness, whereas the service life for steel is directly related to the galvanizing thickness. Initially, durability will relate to the decorative finish. Surface coating type, orientation and environment will determine the period before redecoration is needed, which can range from 10 to 30 years.
The durability of glazing material is mainly a function of its resistance to solar radiation and loading, especially impacts. Exposure of plastics to ultraviolet light leads to embrittlement, discolouration and reduced transparency.
Specifiers must identify what tests the plastics should meet to confirm resistance to the detrimental effects of ultraviolet light. "UV-protected" means little unless backed up by evidence. Some rooflights have a statement of expected durability from a third party.
BS EN 1873 for rooflights
The recent European standard for plastic rooflights BS EN 1873:2005 describes various tests and classes for the glazing material. Performance is judged by how the glazing material changes in response to artificial ageing under exposure to the energy from a xenon-arc lamp.
Two light transmission properties are measured: total luminous transmittance and yellowness index. Durability is a measure of the change of each property due to artificial ageing. There are nine durability classes for light transmission: from A (most resistant) to I (least resistant).
Mechanical properties that influence the impact and loading performance of the glazing are also classified by the degree of change after artificial ageing. Reduction in tensile strength and elasticity are classified in 10% bands from 0% to greater than 30%. Plastics are exposed to other stresses, including damp, temperature fluctuations and pollutants, which influence durability.
The maintenance regime is related to the complexity of the rooflight assembly, use and environmental factors.
Opening rooflights are more likely to need expensive maintenance. Typical requirements include cleaning, inspection of seals and ancillary components. Some parts, including seals and fixing caps, may need replacing during the life of the rooflight.
Notes for table below
- A discount rate of 3.5% is used to calculate net present values. Average service lives are used to calculate net present values.
- Costs are based on 600 × 600 mm rooflights and for glazing only unless otherwise noted.
- Installation, replacement, maintenance and cleaning are allowed for.
- A cost analysis based on project-specific information is essential for a realistic best value appraisal.
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