21 May 2008
WORKING TOGETHER TO ACHIEVE MAXIMUM BUILDING PERFORMANCE
Designers, specifiers, systems manufacturers, materials manufacturers and contractors must all work together to achieve ‘airtight’ building performance.
Progressive changes to the Building Regulations requirements for limiting energy loss have placed greater emphasis on the as-built performance of the building envelope. This development started in 1995, continuing more recently in 2002 and 2006, and initiated a process to measure the actual performance achieved.
As part of the process, the designer and specifier are increasingly being made responsible for a building’s energy performance. To meet this requirement, they will need to delegate the responsibility to a systems manufacturer with tried and tested products, and work with the manufacturer to achieve the desired outcome.
In 1995, the traditional calculation of insulation U-value for metal cladding was altered by the necessity to include the effect of any repeating thermal bridges. As a result, the thickness of insulation in metal cladding systems was increased to allow for heat transfer through metal spacers and fixing screws.
In 2002, the testing of completed buildings for air permeability was introduced, making the junctions of components and the joints between parts of the building critical to overall performance.
In 2006, the National Calculation Methodology (NCM) was introduced making it necessary to check the CO2 emissions from installed heating, lighting and hot water systems against a limit for that type of building.
Energy ownership
In 2008, energy rating of existing non-domestic buildings will increasingly be an issue. Initially, public buildings will need a CO2 emissions certificate displayed in the entrance, rating the energy efficiency. This certificate will be based on the metered energy supply into the building measured over at least 12 months.
The long-term performance and durability of insulation and air leakage seals will therefore be measured on a large range of real buildings for the first time. It must be expected that owners, with buildings which may have complied with the NCM limits on completion, will complain if the measured operational ratings turn out to be poor.
Complaints against the design and construction team have already started where buildings owners who paid for good BREEAM design ratings have found lower measured BREEAM operational ratings. This is not just theoretical, as a heating system for a building with low design air permeability may not have sufficient installed heating capacity if the building is leaky; this would also be the case if parts of the insulation were left out.
Designing complete systems
Since architects, designers and specifiers cannot be experts in all building materials and components, they need to consider complete systems of envelope construction. The system should include the weathering surface, insulation and support spacers, internal surface, all seals and fixings. The package needs to last the lifetime of the building which for most metal clad roof and walls is at least 25 years with no replaceable items.
Metal sheeting will not leak air or water, it is the joints and any fixings through the sheets which cause problems. To achieve the almost maintenance-free life required with metal cladding there are four criteria to be considered:
- Durability and performance of the fixing and seals
- Quality and consistency of the product
- Suitability and ease of use in the UK’s often adverse weather conditions
- Workmanship which cannot be specified but will depend on the above three criteria, because nobody can do a good job with bad materials
To achieve the operational performance of air and rain seals to joints and fixings for an extended period of years, the product manufacturer needs to have invested time and money in research and product development.
Mind the gap
To give one example, a seal between two identical profiled metal sheets needs to fill a variable gap which is reduced by half down the sides of the rib at an end-lap as illustrated due to basic geometric shape. To achieve this thinning under moderate pressure it needs to flow but also to work in both hot and cold weather.
Fig. 1 Typical variation in gap between metal sheeting over the profile.
Buildings are constructed in adverse weather when seals are expected to adhere to damp surfaces. Even experienced roofers and cladders, often being paid on piece work rates, will quickly lose interest in doing a good job if seals do not stick, or screw drill points break. Materials which may be the cheapest can quickly reduce productivity and increase wastage due to poor performance in use. The additional costs of returning to rectify what is often incorrectly labelled ‘poor workmanship’ may then exceed not only the small saving for the cheap products but also any profit on the whole job.
Leaks from rain have always resulted in complaints to contractors. From 2008 onwards I foresee that call-backs to rectify problems with defective envelope assembly will increase as the Building Regulations draw owners’ attention to the measured energy use over the early years of a building’s life. Using suitable materials in accordance with manufacturer’s well researched guidance will minimise the number and costs of any such complaints.
Working towards a common goal
To achieve the optimum building energy performance, all parties involved in the design and construction progress must have a common goal, which is not compromised by the desire to save cash at the expense of performance.
Working together is the future as was demonstrated at SFS intec’s International Customer Forum in April 2007, at Valence France, which drew together over 250 construction industry leaders from 20 countries worldwide to discuss the value of partnership as the way to success.
The Forum demonstrated the way to the future by examining current projects coming to a successful conclusion through partnership, and asked the delegates: ‘Why can’t we work more like this?’