More than a shade of difference
- Date: 06/07/2007
Building Magazine: Solar shading is an option in Part L for limiting summer solar heat gains. Peter Mayer of Building LifePlans takes a look at the options and life cycle costs.
The judicious application of solar shading can reduce solar gain in summer. This saves air-conditioning costs and allows solar heat gain in winter, thereby saving heating costs. Solar shading can be achieved with internal blinds or tinted glazing. The focus here is on external shading.
External solar shading devices are most effective on south-facing windows. Adjustable shading is more responsive to variations in weather, and other elevations, and can achieve a reasonable balance between shading and other performance requirements, such as daylight, control of glare and view. There may be a cost trade-off between the benefits of solar shading and the additional cleaning and maintenance required by solar shading components.
- External louvres, including brises soleil, are synonymous with modern solar shading and offer the best control of solar gain. Louvres may be positioned horizontally, vertically or inclined. Adjustable louvres can be more effective than the fixed type, but have a higher capital cost and require more maintenance. Adjustment may be manual, or automatic. Louvres, aerofoils, slats, fins, leaves or blades are commonly constructed from extruded aluminium. Finishes include: anodising to BS 3987, polyester powder to BS 6496, PVF2-coated, or mill finished.
Alternative louvre materials include: glass with a range of coatings or finishes, ceramics and acrylics, durable timbers such as western red cedar, fabric or plastic membranes. Sophisticated louvres incorporate photovoltaic cells to power the motors that drive the blades for optimum orientation to the sun.
- Overhangs can be incorporated as part of the building design to provide shade to windows. Typical examples include extended roof eaves, balconies, upper floor external walkways and deep window reveals
- Light shelves are fitted to the upper part of a window throwing shade below and reflecting light into the building from its upper surface
- Canopies and awnings are common in retail and leisure premises, based on textile materials and metal structure
- Shutters are a historic solution to solar shading, as well as providing security for dwellings. Usually constructed from softwood. If they are not naturally durable, a preservative treatment for a desired service life of 30 years to BS 8174, and waterproof adhesives, should be specified
- Vegetation may be planted to provide shade from the sun, as well as giving a pleasant outlook. Deciduous trees which lose leaves in winter are best
- External roller blinds are an option more often seen in continental Europe.
Notwithstanding the paradox that better insulated buildings are more susceptible to solar gain, solar shading is one of several design options to limit the effects of solar heat gain. Solar shading can be considered in combination with, or as an alternative to, window size and orientation, ventilation and thermal capacity. Solar shading should be considered early in the design and may require the input of a specialist. Solar shading should not be installed at the expense of daylight; BS 8206 refers to this.
There are no British Standards specifically for solar shading, or related components. Use corrosion-resistant fixings and confirm suitability for exposure and loadings, especially wind forces and maintenance. Adjustable systems will require commissioning. Guidance on solar shading and reducing overheating is published by Energy Savings Trust (EST), BRE and CIBSE.