Skylights and roof windows
- Skylights and roof windows can let natural light into your home. They are especially useful for rooms without windows.
- Careful placement and choice of product can ensure your home receives light but not heat. Quite small skylights can deliver a lot of light, so be conservative when sizing them.
- Skylights are usually factory-made products; roof windows are usually built to suit your building.
- The Window Energy Rating Scheme (WERS) includes skylights. Check the WERS database for the energy performance of products you are interested in.
- Skylights and roof windows can be included if you are buying or building a home, or can be retrofitted to an existing home.
Understanding skylights and roof windows
What are skylights and roof windows?
Skylights and roof windows are glazed openings on a pitched or flat roof designed to provide more light to the home. As well as allowing natural light into your home, they can also allow fresh air to enter if they are openable or vented. As with conventional windows, they can be a major source of unwanted heat gain in the summer and significant heat loss in the winter.
There are a variety of shapes of factory-manufactured skylight and roof windows for sloping or flat roofs. Skylights and roof windows may also take the form of glazed areas constructed on site (for example, for an atrium, attached conservatory or sunroom).
Photo: Velux Australia
Why are skylights and roof windows important?
A skylight or roof window can admit more than 3 times as much light as a vertical window of the same size. Skylights and roof windows can thus increase the amenity of internal spaces that might otherwise require artificial lighting or ventilation, such as windowless rooms. The use of skylights or roof windows can ensure that spaces are predominantly lit by natural light, with little or no artificial lighting required.
They allow additional flexibility in architectural design. Skylights and roof windows can allow natural light and fresh air where vertical windows are not an option, where there are privacy issues, or when you want to create a different architectural look.
Achieving good skylights and roof windows
Skylights and roof windows can be installed in both new buildings and retrofitted to existing ones. Retrofitting will require a qualified builder or contractor.
Roof windows or conventional skylights are suitable for locations with a high incidence of cloudy skies. Tubular skylights are suitable for locations that are typically sunny with clear skies.
Effective delivery of daylight depends on the following factors:
- the path and position of the sun in your location and season
- how often your location experiences overcast versus sunny weather
- levels of air pollution and haze
- roof aspect
- shading from trees or other features.
Skylights and roof windows can be prone to damage during extreme hailstorm events, but careful design and choice of material will reduce the risk.
Skylights and roof windows can be included during the design stage if you are building a home, or can be retrofitted to an existing home during renovation.
Skylights, tubular skylights and roof windows
Skylights can be made of glass, acrylic, or single glazed ‘opal’ (ie light-diffusing) moulded units. The top glazing of the skylights can also be in clear or tinted glass, acrylic or polycarbonate. Skylights typically have long white-coloured or flexible light wells and a diffuser panel fitted at ceiling level.
Photo: Skydome Skylight Systems
Tubular skylights (sometimes called tubular daylighting devices or TDDs), capture direct-beam sunlight, transmit it down a highly reflective light well and diffuse it at ceiling level around the room. They can reduce absolute heat loss and heat gain compared with conventional skylights, because of their small cross-sectional area.
Tubular skylights work best in climates with a high incidence of clear, sunny days. On cloudy days, the amount of daylight admitted is considerably less than for a conventional, large area skylight.
Photo: Skydome Skylight Systems
In tubular skylights, a reflecting tube is used to direct sunlight downward. Using a straight tube with a silvered lining can achieve the best results. Flexible tubes can also be effective, provided their internal reflectance is high and the material is kept taut. Diffusers fitted to tubular skylights can reduce glare and throw the light over a broad area.
Photo: Velux Australia
LED virtual skylights are an alternative in situations where a skylight is not easy or possible to install. They can simulate the effect of daylight by providing artificial light from LED lights, powered by solar energy.
Roof windows are simply windows set into the roof of a home. Roof windows are usually combined with light wells or shafts in homes that have flat ceilings. Ceiling-level diffusers are rarely used with roof windows.
Almost all roof windows use sealed double glazing to reduce heat losses while minimising condensation. Typically, they are openable. This feature is highly recommended during summer conditions, especially in 2-storey houses where heat would otherwise tend to concentrate in the upper level.
As with vertical windows, roof window frames come in a variety of different materials and have a major impact on thermal performance. Frames are typically timber with external weatherproof cladding, but may be aluminium, steel or uPVC. Uninsulated metal frames can cause condensation during times of cooler weather, and lose heat to the outside through conduction.
Roof windows are commonly used for attic rooms where there is a cathedral ceiling but little roof space. They are also used for other living areas with a conventional flat ceiling where a plasterboard-lined light well and no diffuser is used to bring the outside in.
Photo: Velux Australia
Placement and spacing
The principles of delivering daylight differ between conventional vertical windows, which light from the side, and skylights and roof windows, which light from above. Top lighting increases the potential for uniform light distribution. Under an unobstructed, overcast sky, the amount of light from directly above is about 3 times as much as from the horizon.
To determine the best way to size and space skylights and roof windows, a 3–5% floor-to-skylight area ratio is typically used, while 1-2% is sufficient for high-performing tubular skylights.
A rule of thumb for skylight spacing that provides even light distribution is that the distance between skylights should be approximately 1.5 times the height between floor and roofing. Skylight spacing is critical in large spaces.
Fire safety requirements under the National Construction Code (NCC) specify that if skylights are deemed combustible, the aggregate area of the skylights must not exceed 20% of the roof or part of the roof.
In addition, the NCC specifies minimum distances such skylights must be from property boundaries and adjacent buildings, from separating walls on adjoining buildings, and from any other roof lights.
In bushfire areas, skylights must comply with the Australian Standard AS 3959-2009 Construction of buildings in bushfire-prone areas. To comply, a product needs to be tested or demonstrate that it complies with the Bushfire Attack Level (BAL) requirement of your home. Check with the supplier that your preferred product meets the Australian Standard requirements.
The impact of skylights and roof windows on the thermal performance of a building is complex and depends on interacting factors: climate, orientation, materials, size and location of the skylights/roof window, shading, and their thermal properties.
Energy-efficient technologies used for windows may be applied to skylights and roof windows. Any design should meet the current regulatory requirement for building compliance.
Skylights and roof windows can be a source of unwanted heat gain in a home. This can be most acutely felt when exposed to direct sunlight when the sky is clear. In these cases, shading or other solar control measures should be considered.
Glazing can be designed to reduce, block or facilitate light transfer according to sun angles. For example, it is possible to avoid direct sunlight when the sun is at its highest points in summer while still admitting indirect light. Diffuse glass or acrylic glazing can be used to achieve even light distribution and solar control. Diffuse glazing has a back-scattering effect on incoming solar radiation. This promotes soft, glare-free lighting.
Skylights and roof windows can reduce their solar heat gain coefficient (SHGC) and increase their thermal insulation (reduce their U value) through the use of shafts, tubes, ceiling diffusers and supplementary blinds or integrated shades. This is crucial, because roof glazing receives almost twice as much heat as an unprotected west-facing window of the same area.
In Australia, the limits on skylight size helps to minimise unwanted solar heat gain. Quite small skylights can deliver a lot of light, so be conservative when sizing them.
Consideration should also be given to preventing undue heat loss or heat gain by conduction. The rate of heat transfer (from the inside to the outside of the building fabric) of sloped glazing is greater (typically by 40%) than that of vertical glazing.
Refer to the National Construction Code, Volume 2, for further information on maximum allowable aggregate areas of roof lights and the SHGC and U value requirements.
The shape and dimensions of the shaft affect both the light transmission and actual solar heat gain obtained from skylights. The longer the shaft or tube, the less light transmitted by the skylight system. Less solar heat is also admitted. A skylight with poorly performing top glazing may be improved thermally by using a long shaft, provided adequate overall light transmission is maintained. Best practice includes additional thermal insulation in a shafted skylight. This prevents unwanted heat loss or gain from the roof space or attic and is highly recommended.
Skylight products are required to meet minimum performance standards set out in the National Construction Code. The Window Energy Rating Scheme (WERS) maintains a comprehensive, independent database of skylight products called WERS for Skylights, rated according to protocols set by the Australian Fenestration Rating Council.
The database provides U value and SHGC for products, as well as a star rating system that ranks the performance of each product when heating and cooling your home, and the potential the system has for providing natural lighting.
Maintenance and long-term performance
Roof windows and skylights are made from a variety of materials including plastics (ABS, acrylic, polycarbonate, uPVC, and others), glass, aluminium (plain and powder-coated), steel, stainless steel and timber. Generally, these materials have a long life in Australian conditions if maintained properly.
Maintenance should include regular cleaning of the external roof and visible internal surfaces. Leaf debris should not be allowed to pile up on skylight materials since rainwater leaches decomposed chemicals out of the leaf litter and causes severe staining. Leaf debris can also cause corrosion and subsequent roof damage.
In harsh environments (for example, close to the sea, industrial pollution, or in heavily wooded areas), skylight exteriors should be cleaned every 6 months; in other locations they should be cleaned every 2 years.
Openable and ventilating skylights (for example, openable roof windows and combined skylight/roof ventilators) may require occasional lubrication of moving hardware.
References and additional reading
- Australian Fenestration Rating Council
- Lyons P (2004). Properties and rating systems for glazings, windows and skylights (including atria), Environment design guide, PRO 32, Australian Institute of Architects, Melbourne.
- Skylight Industry Association.
- Window Energy Rating Scheme, Certified products hub.
- Read Glazing to find out how glass and framing choices will affect your home’s thermal performance
- Read Shading for ideas on the best way to shade your skylight or roof window
- Explore Renovations and additions to find ways to improve your home
Original author: Dr Peter Lyons
Updated: Richard Hamber 2013, Scott Dwyer 2020