Theme : Roofing
Through effective roofing solutions, businesses can save on energy demand, improve thermal performance and increase ventilation, among other benefits, to enhance the experience of interior environments and potentially save on heating expenditure in the long term
Different countries are taking different approaches to reducing their energy demand. In North America, where a high proportion of energy is used for running air-conditioning units during the warm summer months, lighter coloured roofs with high solar reflection are now recommended in Building Codes. 'Cool roofing' products have become one of the fastest growing sectors of the US roofing market.
Dark materials absorb more heat from the sun. In summer, heat flows in the reverse direction from in winter, with the hottest surface temperatures on the outside of the building. Thermal insulation is still important in reducing the heat flow to keep internal rooms at comfortable temperatures.
The amount of heat flow is directly proportional to the temperature difference between the outside and the inside of the building.
If the external surface temperatures can be reduced, there is less heat flow and less need for cooling.
In general, lighter-coloured roofs have higher solar reflectance, and can be 21 °C cooler on a hot day than a typical dark-coloured roof, according to tests carried out at the Lawrence Berkeley National Laboratory in California.
The effectiveness of a cool roof depends on the climate in which it is situated. Buildings in warmer Mediterranean climates that require significant air conditioning will benefit the most from cool roofs.
Two primary properties determine a roofing product's 'coolness':
l how much of the sun's heat is reflected away from the roof instead of absorbed (solar reflectance); and l how effectively the absorbed heat is released from the roof (thermal emittance).
US experience Studies of buildings carried out in the US have shown that cool roofs can result in savings in cooling energy of up to 40 per cent.
Experience in adopting lighter-coloured roof surfaces in North America during the past decade is claimed to offer the following benefits:
l reduced air-conditioning energy costs;
l increased roof-system lifespan, due to reduced thermal movement in roofing components; and l reduced air temperatures in central urban areas, minimising the 'heat island effect', with consequent reduction in air pollution and smog formation.
It has been found that some roofing products start out with highly reflective white surfaces when first laid but become dirty over time, reducing thermal benefits. This has led to debates over the desirability of regularly washing down roofs, in terms of cost and safety. The Cool Roof Rating Council has been formed to give independent guidance to building owners and specifiers on the longterm performance of different products.
Infrared-reflecting pigments are being developed for use in coatings for roof tiles and metal cladding to increase the solar reflectance and reduce surface temperatures. With this development, dark colours used on everyday roofing materials can have a higher solar reflectance and reduced surface temperatures.
Air conditioning In the UK, the market for air conditioning is relatively small compared with the US, although growth is predicted as a result of climate change, improved system efficiency and more widespread marketing. Those working in offices know that if the air temperature increases above 30 °C, it becomes difficult to concentrate for long periods, we become less effective and there can be a commercial loss.
Since staff costs are high, there is a business need to cool the air in the summer.
One study in southern England has suggested that a 2 °C increase in summertime air temperatures would increase the number of cooling degree days from 100 to 200. This in turn could lead to an increase in the number of new houses constructed with air conditioning, perhaps from 10 per cent to 30 per cent in southern England. More air conditioning would give an incentive to look at changing roof colours.
It will become necessary to consider the predicted seasonal energy trade-offs of cool roofs because, in the winter, cool roofs may lead to modest increases in a building's heating load, especially where insulation levels are low. However, the amount of energy reflected off a roof during winter is less than that reflected in summer because winter days are shorter and the sun is lower in the sky. Therefore, for northern climates, the cost savings in summer are relatively greater than the increase of heating costs in winter.
At present in the UK, the economic argument for changing roof specifications to use lighter colours is not proven. However, if we continue to experience increases in summer temperatures and a growth in the number of buildings with air conditioning, we could expect a commercial interest in 'cool roofing'. After all, it is not uncommon to find that what happens in America ends up happening in the UK a few years later.
Build tight, ventilate right With time, roof specifications evolve. A hundred years ago, natural slates were nailed to timber battens spanning between rafters, usually with no underlay. The open form of construction allowed the passage of currents of air, which could be reduced by the use of close timber boarding.
Fifty years ago, with the high cost of timber and the prohibition of its use as roof boarding for a period during and after the war, the undersides of tiled and slated roofs were 'torched' using haired lime mortar to reduce air flow through the roofing. In practice, this was found to be undesirable due to the retention of moisture and the restriction of thermal movements. Bitumen sarkingfelt was introduced, reinforced by a layer of jute hessian embedded in the coating on one side, and was found to be sufficiently strong to span between rafters without a boarded support. With the free flow of air through the slate or tile roofing, under certain wind conditions water or fine powdery snow could be blown through on to the underlay. On cold, still days, dew formed on the underside of tiles and slates. The underlay was laid so that any water falling on to it would be discharged harmlessly, clear of the external walls of the building and preferably into the eaves gutter.
The introduction of sarking-felt had an unfortunate side effect in that, occasionally, there were pockets of moist air trapped within attic spaces that led to an increase in reports of timber decay in the rafters and ceiling joists. This was overcome by the provision of ventilation of the attic spaces, which became mandatory within the Building Regulations.
In recent years, the need to improve the thermal performance of the roof has led to increased thicknesses of insulation, and measures to reduce the escape of warm air from the heated rooms within the building.
Two decades ago, 'breathing' membranes were introduced into timber-framed wall constructions behind the external cladding.
They reduced the entry of wind-driven rain from the outside, while allowing water vapour to escape from inside the building. In the UK, the use of membranes with low vapour resistance was subsequently extended for use as an underlay in pitched tile and slate roof constructions. They allowed any water vapour within the attic space below to migrate slowly through the vapour-permeable underlay, around the tiles and slates, and out of the building. Tests and monitoring of actual buildings suggested that it was no longer necessary to ventilate the attic spaces, simplifying the detailing of roofs around the perimeters.
Also, the reduction in air movements could lead to greater thermal efficiencies in fibrous thermal insulation. However, it is also a requirement that the vapour-control layer at ceiling level is continuous and not punctured by service penetrations, raised light fittings or unsealed loft access hatches. Water tanks must remain covered.
Above the underlay, ventilation is provided either by proprietary openings around the roof perimeters or by allowing for the movement of air through the laps in the tile or slate roofing. The latter alternative is not strictly compliant with the current version of the slating and tiling code of practice, BS 5534: 2003. With reduced provision for drying out by natural ventilation, it is important that the vapour-permeable underlays drain freely to the eaves and remain watertight for the lifetime of the roof covering.
Recent advertising campaigns by manufacturers of vapour-permeable membranes have promoted reduced ventilation within pitched roofs and, in particular, in the space above the underlay and below the tile or slate roofing. Manufacturers of the different types of roofing components have raised questions about the effectiveness of each other's vapour-permeable underlays and/or ventilation products. This led to the establishment of a Partners in Innovation Project several years ago to look at the performance of cold pitched roofs. The working party included representatives of roofing-component manufacturers, trade organisations, technical experts and the Building Research Establishment. Findings of the project are expected to be released later this summer and may lead to revisions of BS 5250, the 'Code of Practice for the Control of Condensation in Buildings'. In the meantime, to assist designers, the British Board of AgrÚment (BBA), has produced a 'question and answer' sheet on the use of breathable roof-tile underlays on cold roofs, and this is available on the BBA website at www. bba certs. co. uk When selecting an underlay, remember why they were originally introduced: to prevent water dripping into a building by providing a secondary drainage path down to the eaves. It is therefore essential that the underlay has a service life equal to that of the tile or slate roof covering.
The best advice at present is to provide an airtight ceiling construction, to minimise water-vapour entry into the attic space, and to provide effective ventilation to allow moisture out of the roof system to reduce the risk of long-term timber decay. In summary: 'Build tight, ventilate right.'
New energy-efficient roofing products Sunny outlook Kalzip AluPlusSolar is a new roofing system developed by a partnership between Corus Building Systems and the specialist firm SolarCentury, which specialises in the design and installation of photovoltaic (PV) systems to generate electricity from the sun. The product was launched at this year's Interbuild exhibition and it is understood that several projects currently at the design stage will be using Kalzip AluPlusSolar.
The product comprises a PV laminate that is factory-bonded directly to the outer surface of the aluminium Kalzip sheets, prior to delivery to site. The laminate consists of a triple-junction thin-film silicon cell deposited on to a stainless-steel foil and encapsulated in an ethane vinyl acetate copolymer protective envelope. A special wide flat-pan Kalzip profile has been developed for the system, which will be fixed and seamed in the normal manner.
Where the system has a distinct advantage is that the assembly, bonding and support to the PV laminates are carried out under factory-controlled conditions, reducing the tasks carried out by a roofing contractor on site to a minimum.
Extensive areas of metal roofing can provide ideal locations for photovoltaic cells to generate renewable electricity. The expectation of reduced manufacturing costs for PV laminates, combined with the long-term growth of the Kalzip system and the support of the Teamkal network of installers, should give this new roofing product a good chance of commercial success.
Photovoltaic variety An alternative form of PV cells, more readily suited to small domestic roofs, comprises individual rectangular units, typically 1,200 x 400mm, laid on timber battens or directly above tile or slate roofing. These have been promoted actively in the Netherlands for the past decade and, with the introduction of grant schemes, could become widespread in the UK. One new hybrid product recently launched is SolarSynergy, which combines solar PV and solar water-heating systems.
SolarCentury is also working with Corus manufacturing Uni-Solar shingles. It claims the C21 solar tile integrates seamlessly with common roof tiles - such as the Redland Richmond, Landmark and Saxon interlocking concrete tiles.
Light touch Catnic, manufacturer of steel lintels, has introduced a lightweight metal roofing system, which it claimes offers easy on-site handling, fast installation and rapid enclosure. Catnic offers four different profiles in a variety of colours that are suitable for all types of pitched roofs for both residential and commercial buildings.
Euroclad has introduced the Eurobar Extra system, which, it claims, reduces thermal bridging in twin-skinned metal roofing.
This includes a polypropylene section of a bracket with low thermal conductivity, to reduce the heat loss at the support positions. The company claims that the 'double leg' design for the metal bracket provides the strength required to support the outer skin without the need for anti-sway brackets.
Keith Roberts is a chartered civil and structural engineer based in Oxfordshire who specialises in roofing and cladding. He can be contacted at www. robertsconsulting. co. uk