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In the December 2007 issue:
The Daylight Factor
With increased recognition of the importance of natural light and the benefits it can bring, ‘Daylighting’ techniques are employed in more and more constructions. In the residential sector, conservatories and sunrooms are hugely popular, providing the comfort, space and light expected by modern homebuyers. People in the 21st Century now spend as much as 90% of their time indoors, sometimes enjoying exposure to sunlight for less than an hour a day. This situation worsens over the winter months, when people commonly leave for work and return in the dark, yet the benefits of sunlight and the profound ways it can affect a person’s wellbeing are scientifically established. Natural (or ‘full-spectrum’) light improves mood and combats depression; boosts energy and increases production levels; makes interior spaces appear larger; renders colours true; reduces eyestrain; conserves energy and brings the outdoors in. On top of its many other benefits, daylighting also offers passive solar gain, contributing towards a reduction in energy consumption and carbon emissions. So exactly how much daylight is needed in any particular room or area? How can we measure this? And how can we ensure the optimum amount is provided?
The conventional measure of daylight in building is the daylight factor, a comparison of two illuminance measures. Illuminance is the spread or density of light flux falling on a surface and is measured in lumens per m² (lux). The lumen is the measure of luminous flux, and is used to quantify the visible output of a light source. The daylight factor at a point indoors is equal to the indoor illuminance expressed as a simultaneous horizontal illuminance under an unobstructed overcast sky. Although illuminance from the fully overcast sky varies with a change in altitude, it is stable, remaining unchanged in azimuth (around points of the compass).
Fig 1
The overcast sky represents worst case conditions. If we can design for these then variations in conditions will be an improvement.
The daylit appearance of a room is often judged on a comparison with what can be seen outside, and the daylight factor is a physical correlate of this comparison. Current guidance from the CIBSE states that a room with an average daylight factor of 5% or more will appear well daylit and bright. A room with an average daylight factor of less than 2% will appear dull and under lit. For the conventional densely overcast sky with an illuminance of 5000 lux, these translate to indoor illuminances of 250 lux and 100 lux respectively, while for a conventional side lit room uniformity also plays a part in determining the acceptability of the daylighting. Two boundary conditions are suggested as limits to non-uniformity in side lit rooms: Firstly, the no-sky line should not extend more than half way to the window wall. This is a question of geometry and is affected by external obstructions as well as window geometry.
Fig 2
Secondly, the average daylight factors in the two halves of the room bisected parallel to the window wall should not differ by more than 3:1.
Fig 3
This case holds when (L/W + L/H) <= (2/(1-Rb)), where L=length, W=width, H=height and Rb=reflectance at the back of the room.
The daylight factor at a point indoors is formed of three components: the sky component:
Fig 4
the externally reflected component:
Fig 5
and the internally reflected component:
Fig 6
These describe respectively light that reaches the point directly from the sky, light that reaches the point reflected from external surfaces and light that reaches the point reflected from internal surfaces. The proportions of these components vary with the position of the point within the room. Generally, the nearer the window, the greater will be the effect of the sky component and the further away from the window the greater will be the effect of the internally reflected component. Areas beyond the no-sky line rely completely on the externally and internally reflected components. The average daylight factor for a room is given by:
DF=T x Ag x θ(1 - R)
where DF = daylight factor, T = transmittance of glass, Ag =area of glass, θ =angle of sky subtended at window, A =total area of internal surfaces, R = area weighed average reflectance of surfaces. This equation can easily be rewritten to calculate window size from a given daylight factor:
Fig 7
Ag = 2 DF x A(1 - R) Tθ
Installing rooflights is a highly effective method of brightening interiors, as they are up to three times more efficient at introducing natural light than windows of a similar area. However, selecting an appropriate product to complement the architecture of the building while still meeting strict modern performance standards can be a challenge. Manufacturing the widest range of skylights in the UK, the Rooflight Company creates bespoke designs to suit any structure, and easily exceeds the eco-friendly performance levels customers expect by continually developing cutting edge production methods and materials. All of its products incorporate the patented Thermoliner® thermally decoupled lining, which ensures no condensation problems occur. As ??? states: “Through pioneering scientific research and development, we can deliver elegant and efficient daylighting solutions for any interior environment.”
Rooflight Company Champions Excellence In Architectural Design
An authentic period appearance and top-notch sustainability credentials led to two Cheltenham homeowners specifying a bespoke design from specialist manufacturer the Rooflight Company. Dean J. Mason Building Contractors carried out the construction work on the new extension for owners Joe Harrison and Mike Coltman, installing a Special Rooflight® in the listed property.
“When we bought the house 18 months ago we decided it would be a good idea to build a single-storey extension on the north-facing side”, explains Joe. “We were unable to include windows on two sides of the extension because they would overlook the neighbours, so we decided fairly quickly on a rooflight.” With this in mind, they browsed the products available on the market before selecting a dark grey Special Fixed Lantern Rooflight with a 30 degree pitch on the glazing. Featuring an internal kerb measuring 2.4 x 1.5m, it provides a great deal of daylight to the interior, reducing the need for artificial light and thus cutting energy consumption.
“The house is a Grade II* listed building, so the rooflight had to be sympathetic in order to pass the planning processes,” Joe continues, “and our architect recommended a lantern-style model. We looked at a couple of manufacturers - the other products being constructed from timber - and liked the simplicity and clean lines of the products in the Rooflight Company’s brochure. We showed the pictures from the brochure to the planning authorities and they approved the design, as it was deemed to fit sympathetically with the style and character of the existing structure, as well as the surrounding architecture.”
Along with ensuring that any work carried out on listed buildings is inkeeping with the original design, Cheltenham Borough Council is also strongly committed to the principle of sustainable development. The council has been working with other local organisations and groups to develop a strategy setting out what needs to be done to ensure that Cheltenham is able to minimise its contribution to climate change, therefore maintaining a good quality of life for everyone who lives and works in the town. The target is to reduce greenhouse emissions in Cheltenham by 20 percent from 1990 levels by 2010, and the long-term vision is to become a carbon neutral borough. This overarching principal informs the council’s decisions on many different issues, and often a balance has to be achieved between architectural authenticity and strict modern energy efficiency standards.
Joe and Mike then got in touch with Clive Barnett, Specials Manager at the Rooflight Company, who said: “Our bespoke design service helps customers to accentuate a buildings' character, be it a contemporary project or the refurbishment of a historic property. We have unrivalled experience in the careful reproduction of existing designs and the creation of new rooflights to suit any building. Providing an individually-tailored alternative to mass produced skylights, our thermally efficient models allow architects, designers and homeowners to achieve the design results they want.”
Sow's Ear To Silk Purse
In order to preserve the essential character and period look of a Grade II listed barn in Oxfordshire, RIBA Conservation Award winner Allan Woodfield of Spratley & Woodfield selected Conservation Rooflights® from specialist manufacturer the Rooflight Company to feature in its dramatic renovation. Involving the repair and conversion of the building at Princes Manor Farm, Harwell, the restoration has provided elegant new office accommodation in the heart of the countryside. However, prior to the refurbishment, which was carried out over a 10-month period from April 2006 to February 2007, the barn had been used for a far less stylish purpose - housing pigs and farm machinery.
Archaeological studies have shown that the earliest elements of the barn can be dated as far back as the 16th Century, while its reconstruction on the present site, with subsequent extensions and the addition of a second barn to the south, dates to the early 17th Century. Extensive specialist repair works to the original frame were carried out by principal contractor Donovan Construction (South West) Ltd, including the sensitive reconstruction of a number of bays in new green oak, before their realignment and new roofing works could commence. To the exterior, a natural palette of handmade clay plain roof tiles and untreated western red cedar provide natural weather resistance and create an authentic, traditional appearance. 14 CR14/2 Conservation Rooflights® were installed to enhance room ambience, creating a dramatic impact on daylight and space.
Winners of the 2004 RIBA South Conservation Award for their refurbishment of another listed Oxfordshire property, the Granary at Crowmarsh Battle Farm in Preston Crowmarsh, Spratley & Woodfield have extensive experience in designing sympathetic refurbishments of historic buildings. Allan Woodfield comments: “Preserving and enhancing traditional or historic buildings involves taking into account the visual characteristics of original features and matching them as closely as possible, while still conforming to contemporary standards and regulations. Planning and Conservation Officers will often refuse applications if they change a building’s character too much, and the alteration with most potential to change character involves fenestration. In barn conversions such as this, there is often a difficulty in providing natural light and ventilation without inserting numbers of new windows.”
However, installing rooflights is a highly effective method of brightening interiors, as they are up to three times more efficient at introducing natural light than windows of a similar area. This means that the rooflights can be smaller, or they can admit more light – whichever is more suitable for the project. However, it is the retention of character that is vital in winning support from planners, so wherever rooflights are used, these should be as near to the cast iron models farmers would originally have used as possible.
Favoured by English Heritage and the National Trust, the Conservation Rooflight® has been especially designed to look like a cast-iron rooflight, with a low profile and slim steel glazing bars, but has been fully adapted to use in habitable accommodation. Meeting or exceeding all current building standards, it is leak-proof, draught-proof, double-glazed, and features the patented Thermoliner thermal break device to obviate condensation. Over 20 different sizes are available to suit each particular application.
Allan concludes: “Research into the history of these barns has revealed that their conversion and adaptation has been ongoing for over four hundred years so, with the latest work, it is hoped that the future of the barns at Princes Manor Farm has been secured for even longer still.”
