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Profiled Metal Sheeting Part L2 of the Building Regulations by David Roy

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Introduction - Note the regulations changed in October 2010 for more information see here
The revision the Building Regulations (England & Wales) Part L - Conservation of fuel and power which came into force in April 2002, introduced the following requirements:
  • Decrease in U-values (increase insulation thickness)
  • No significant thermal bridges or gaps in insulation
  • Buildings should be reasonably airtight
For metal roofing and cladding systems Part L2 of the regulations refer to the Metal Cladding and Roofing Manufacturers Association (MCRMA) Technical Note 14: Guidance for the design of metal cladding and roofing to comply with Approved Document L 2002 Edition: for guidance in achieving the above.
The U-value requirements for buildings other than dwellings using profiled metal sheeting are:
  • Roofs 0.25 W/m K
  • Walls 0.35 W/m K
In calculating the effective U-value of a roof or wall, the thermal bridge of any spacers and/or fixings must be taken into account. In order to calculate the effect of the thermal bridge 2D and 3D thermal modelling is normally used.
Picture 1 Picture 2
For bar and bracket spacer systems for a given thickness of insulation, liner and outer profiles, the U-value will vary depending on the purlin/rails (bar) spacing and spacer bracket centres.
The major potential for thermal bridges occurs at details. The effect of a thermal bridge is twofold:
  • Increase heat loss through the detail 
  • Risk of internal surface condensation occurring at the detail during the cold weather.
Details can been analysed using a 2D finite element thermal modelling program.
Picture 3 Picture 4
The results are normally expressed by two values, linear thermal transmission value Ψ (psi value) and the minimum temperature factor fmin.
The Ψ value is a measure of the heat loss through the detail, the higher the value the greater the loss.
The fmin factor indicates the risk of surface condensation, the lower the value the greater the risk.

The minimum values of the f-factor are

  • Storage buildings - 0.3
  • Offices, retail premises - 0.5
  • Sports hall, kitchens - 0.8
  • Swimming pools, laundries, breweries - 0.9
The ratio of the heat loss through the details against the heat loss through the main areas is known as the a-value (alpha), for non domestic buildings it is recommended that this value should not exceed 10%.
In order to determine that a specific building complies with the regulations, it is necessary to calculate the total heat loss through the building fabric, including details and compare this with the limits set out in the regulations and associated documents.
The requirement for building to be reasonably airtight can be satisfied as follows:
  • For buildings under 1000m -submit evidence that appropriate design details and building techniques, have been specified, and that the work has been carried out in ways that can be expected to achieve reasonable conformity with the specifications.
  • Alternatively for buildings of any size, carry out of air leakage tests. From 1st October 2003, reasonable provision is test results showing air permeability not greater than 10 m /h/m , with an applied pressure difference of 50 Pascals.
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