Whole life costing: Solar shading

Peter Mayer

Solar Shading is an option in Part L for limiting summer
heat gains.
Peter Mayer of Building LifePlans looks at the options
and life-cycle

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.

 

Materials


• 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.

 

Further guidance


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.

 

Specification options

 

External solar shading louvres Capital cost
£/m*
Net present value for 60 years
£/m
Expected period to first maintenance (years)
Aluminium aerofoil sections, fixed, 1.2m projection      
Mill finished 220 400 > 40 +
Polyester powder coated to BS 6496, minimum 40 microns thick 270 600 > 15–25 +
Polyvinylidene–fluoride (PVDF or PVF2) minimum 25 microns thick 320 690 20 +
Anodised BS 3987, minimum 25 microns thick 250 480 30 +
Aluminium slat sections, fixed, 1.2m projection      
Polyester powder coated to BS 6496, minimum 40 microns thick 180 450 15–25 +
Aluminium louvre system adjustable with polyester powder coated to BS 6496      
Manually adjustable with crank 370 990 15–25 +
Motorised adjustment 595 1,650 15–25 +


• NB. Rates are for a linear metre based on the projection of the louvres out from the external wall.

 

 

Table notes


• A discount rate of 3.5 % is used to calculate net present values.

• Extruded Aluminium to EN 755. Period to first maintenance dependant on colour of finish and exposure to pollutants and solar radiation. In practice it may be more economical to replace solar shading rather than remove and recoat or carry out insitu redecoration. An alternative strategy may incorporate solar shading replacement or redecoration with a major façade refurbishment.

• Life cycle costs include servicing, inspection, cleaning, allowance for repairs and replacement at period to first maintenance (upper values have been used in the LCC table).

• A cost analysis based on project specific information is essential for a realistic best value appraisal.

First published in Building 2007

 

 

Further information


BLP provides latent defect warranties for buildings www.blpinsurance.com

Further information contact peter.mayer@blpinsurance.com or telephone: 020 7204 2450.

 

 

 

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