Under International agreement the UK has committed to reduce CO2 emissions by 12.5% below 1990 levels during the period 2008 - 2012. The UK government has taken this a stage further and pledged to reduce CO2 emissions a further 7.5% by 2010. As half of the UK's CO2 emissions are produced from energy used in buildings radical changes to traditional construction practice were necessary to meet these ambitious targets.

Following industry consultation, revisions to Approved Document L (Conservation of Fuel & Power) of the Building Regulations (England & Wales) were published in October 2001 and came into effect on April 1st 2002. Divided into two publications Approved Document L1 deals with requirements for dwellings whilst Approved Document L2 covers requirements for buildings other than dwellings. These documents replace the contents of the 1995 edition of Approved Document L of the Building Regulations (England & Wales). Revisions to Part J (Conservation of Fuel & Power) of the Technical Standard (Scotland) have been implemented in parallel and came into force on 4th March 2002.

MAIN CHANGES

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The overriding theme of these new Regulations is to manage the amount of energy required to heat and cool a building by looking at how the building is constructed and operated as a 'whole', rather than as a set of individual elements. Not only does the building fabric and its services (e.g. heating, lighting, air-conditioning etc) have to be appropriately designed and constructed, they also have to be operated efficiently. To demonstrate compliance declarations/test certificates must be submitted at both the design and construction stage*, which will involve specifiers, engineers and contractors working more closely together. 'Getting it right first time' is essential to avoid costly remedial work and potential disputes at a later date. However well designed a system, unless it is installed properly it will fail and require expensive re-testing which could delay the completion of the building.

Work on existing buildings has been included for the first time and is designed to close the gap in operating efficiencies between the high number of old buildings and the relatively small number of new developments.

• The main changes affecting the design and construction of buildings using insulated composite panels or metal cladding systems can be summarised as follows:
• Significant increase in insulating performance standards (U-values)
  
• A change in the U-value calculation method to introduced a more rigorous method of dealing with repeated thermal bridges
  
• Increased standards of design detail and site workmanship to reduce incidence of gaps in insulation and effects of cold bridging
  
• A rise in standards of fabric air tightness to minimise unwanted air change
  
• Thermographic imaging and envelope pressure testing will be required to demonstrate compliance *

* Not a requirement of Part J of the Technical Standards (Scotland)

NEW U-VALUE CALCULATION PROCEDURE

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The effects of repeated thermal bridges on heat loss were first taken into account in the 1995 revisions to Approved Document L though the 'Proportional Area Method'. This assumed that heat flow is locally higher through higher conductivity elements where they cross the insulation (e.g. mortar joints), but only considered heat flow as being one-dimensional. This method therefore tends to underestimate the U-Value, as any sideways flow of heat between the bridged and unbridged elements is not taken into account. In metal buildings insulation can be bridged many times by metal elements which have exceptionally high levels of thermal conductivity making the Proportional Area Method very inaccurate.

To make the calculation of U-Values more accurate the latest revisions to the Building Regulations have introduced a more rigorous method of dealing with repeated thermal bridges. U-Values now have to be calculated to take into account the effects of two and three dimensional heat flow by using computer software to model the building structure.

Design

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To achieve energy efficiency in practice, the building fabric and its service systems (e.g. heating, ventilating, air-conditioning, lighting etc) should be appropriately designed.

Approved Document L2 offers three methods for demonstrating that reasonable provision has been made for the conservation of fuel and power. Each method offers increased flexibility in return for greater complexity in terms of the extent of calculation required.

The Elemental Method is applicable to any building type and requires a minimum level of performance for each building element. Some trade-off between insulation standards and heating system performance is allowed.

The Whole-Building Method, which replaces the Calculation Method, is only applicable to schools, offices and hospitals and considers the performance of the whole building against a set of benchmarks.

The Carbon Emissions Calculation Method, which replaces the Energy Use Method, is applicable to any building type and again considers the performance of the whole building. Carbon emissions should not exceed those from a reference building constructed to the elemental standard.

For the purpose of this guide, compliance will be based on using the Elemental Method and assumes no trade-offs. For guidance on compliance using either the Whole-Building Method or the Carbon Emissions Calculation Method please refer to Approved Document L2 of the Building Regulations.

The Elemental Method of compliance is by far the simplest way to comply with new Building Regulations and is particularly suited to the construction of 'metal' buildings. If the thermal performance of the individual construction element is no worse than those listed in the table below then the requirement has been met.

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Notes to Table

1. Classed under 'Roofs up to a 10° slope' in Technical Standard J (Scotland)
2. Roof of pitch not exceeding 10°
   
3. 0.03 W/m²K Technical Standard J (Scotland)
   
4. Display windows, shop entrance doors and similar glazing are not required to meet the standard given in this table
   
5. This standard applies only to the performance of the unit excluding any upstand. Reasonable provision would be to insulate any upstand, or otherwise isolate it from the internal environment
   
6. For the purposes of Approved Document L2, a roof window may be considered as a rooflight

Standard U-Values for Non-domestic Buildings

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Thermal bridging usually occurs at junctions between building elements (e.g. the roof and walls), around openings and penetrations. These thermal bridges increase the total energy demand of a building, as heat flows are always higher where materials with a higher thermal conductivity 'bridge' the insulation layer.

Approved Document L2 states that 'the building fabric should be constructed so that there are no significant gaps or thermal bridges in the insulation layer(s) of the building fabric. Care should be taken at joints between individual elements, and at edges of elements such as those around window and door openings.'

It is also necessary to take into consideration the risk of surface condensation (the f-factor) on individual thermal bridges and the effect that the increased heat loss (the -value) through these will have on the overall heat loss of the building.

Compliance can be demonstrated by following the procedures detailed in BRE IP 17/01 and the MCRMA Technical Paper 14 which require two-dimensional models to calculate the f-factor and the -value for each detail.

For further information on surface condensation and heat loss through thermal bridges, please contact our Technical Department.

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Surface condensation can be a consequence of thermal bridging as increased heat flow lowers the internal surface temperature. The severity of the thermal bridge and therefore the risk of condensation are determined by the f -factor. This is calculated by modelling the building structure, and taking into account the local surface temperature of the particular component together with the internal and external air temperatures.

BRE IP17/01 gives guidance on limiting the risk of surface condensation in buildings by providing the following table of minimum critical temperature f-factors based on likely internal environments. The higher the f-factor the wider the usage of the building e.g. for a high humidity building such as a swimming pool a higher f-factor would be required.

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Heat loss through linear thermal bridges is expressed in terms of the linear thermal transmission or -value. This is the additional heat loss through the thermal bridge that is not accounted for in the U-value of the plane building elements or elements containing the thermal bridge. For example in a corner detail the internal facing of an insulated composite panel bridges the insulation core of the other panel. This scenario would be identical in a twin skin system where the liner sheet on one wall could bridge the insulation of another where they meet.

By using a two-dimensional thermal model the heat flow from the inside of the building to the outside can be determined and the linear thermal transmission (-value) calculated. This calculation takes into account the heat flow through the model, internal and external temperatures and the U-values of each construction element.

Once the contribution of individual thermal bridges to the heat loss of the building are calculated they are added to the heat loss figure for the plane area to give a total building fabric heat loss figure.

BRE IP 17/01 includes a table of maximum thermal transmission values (-values). If the -values is equal to or less than those listed in the table then the requirement is met and compliance has been achieved. However most examples of thermal bridges covered in BRE IP 17/01 are only applicable to domestic type construction so further calculation may be required to determine the a-values of the building fabric.

a = (sum of heat loss at junctions/thermal bridges)
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(sum of heat loss through plane areas)

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To satisfy the requirements of Approved Document L2 the a-value must be equal to or less than 0.10 (10%) for non-domestic buildings. If the total heat loss through the thermal bridges is more than 10% of the total heat loss through the plane areas, individual thermal bridges must be modified to reduce the heat loss.

Provided there is no condensation risk, relatively high -values are acceptable from individual details provided the heat loss from other building details are controlled to counteract it.

Provision should be made to limit the rate of heat loss through any glazed elements of the building. To enable greater design flexibility trade-offs between construction elements are permitted and poorest acceptable U-Values are detailed in Approved Document L2 to facilitate this. However for the scope of this document compliance is based on limiting the total area of openings which are expressed as a percentage of the total roof or wall area. For information on trade-offs between construction elements please refer to page 16 of Approved Document L2.

Building Air Leakage Standards

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Approved Document L2 requires buildings to be designed so they are reasonably airtight and therefore avoid unnecessary space heating and cooling. Ventilation systems should also be able to perform effectively. When looking at building air leakage the requirements of Parts F & J should be taken into account to ensure adequate ventilation and air for combustion appliances.

One way of achieving the requirement would be to incorporate the sealing measures to achieve the performance standards that are detailed under 'CONSTRUCTION - Building Air Leakage Standards':

• Providing a reasonably continuous air barrier in contact with the insulation layer over the entire thermal envelope. Special care should be taken at junctions between elements and around door and window openings.
  
• The positioning of this air seal line is important, as it needs to be visible for site inspection and access in case remedial work needs to be carried out.
  
• Sealing gaps around service penetrations
  
• Draught-proofing external doors and windows

For further information on air tightness, please contact our Technical Department.

CONSTRUCTION

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To avoid excessive thermal bridging appropriate design details and fixing should be used so there are no significant gaps or thermal bridges in the insulation layer(s) of the building fabric. Responsibility for achieving compliance rests with the person carrying out the work. In the case of a newbuild project this would be the main contractor who has carried out the work directly or indirectly through a subcontractor.

Insulation continuity can be proven by the declaration of a 'suitably qualified person' (e.g. a surveyor or architect offering this service) whose suitability would have to be verified in advance by Building Control.

To confirm that provisions meet the requirements of Approved Document L2 it must be proved that appropriate design details and building techniques have been used and that the work has been carried out in a way that can be expected to achieve reasonable conformity. Alternatively, thermal imaging can be used to obtain measurements of surface temperatures to demonstrate that the 'insulation is reasonably' continuous over the whole visible envelope'.

In theory Infra red thermography is an excellent way of demonstrating compliance, but in practice requires a combination of unusual weather conditions, specific internal and external temperature conditions and is wholly reliant on correct interpretation. When you consider that the procedure is expensive and there are limited organisations in the UK that carry out this type of work, demonstrating satisfactory design and construction is a far more attractive option.

Further information on carrying out thermal imaging surveys is given in BRE publication 176 'A practical guide to infra-red thermography for building surveyors'

Airtightness

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Air barriers should be installed to minimise air infiltration through the building fabric as outlined under 'DESIGN - Building Air Leakage Standards'.

For buildings of less than 1,000m² gross floor area compliance can be demonstrated by the declaration of a 'suitably qualified person' (whose suitability would have to be verified in advance by Building Contro)l stating that 'appropriate design details and building techniques have been used to achieve reasonable conformity'

For buildings of any size the results of air leakage tests carried out in accordance with CIBSE TM23 (Testing buildings for air leakage) are a satisfactory method of demonstrating compliance:

• From 1st October 2003 air permeability not exceeding 10 m3/h/m² @ 50Pa
• Up to and including 30th September 2003, reasonable provision in the event that the initial test results are unsatisfactory would be the results of further tests carried out after appropriate remedial work.
  (i) an improvement of 75% of the difference between the initial test result and the target standard of 10 m3/h/m² @ 50Pa
  OR, if less demanding
  (ii) a performance no worse than 11.5 m3/h/m² @ 50Pa

If a building fails to meet the required level of air permeability remedial work will have to be undertaken to seal draughts. The easiest way of locating air leakage routes is by releasing smoke from generators or pencils and then simply viewing the outside of the building to see where the smoke is released. Once leaks have been identified additional sealing will have to take place and the testing procedure carried out again until satisfactory results are achieved.

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It is important to remember that the insulated composite panel or metal cladding system is only one area of the building fabric that can contribute towards air leakage. Doors, loading bays, windows, rooflights and penetrations can all contribute to excessive air leakage so need to be installed correctly. By incorporating correct sealing measures and ensuring that all side/end lap joints and perimeter joints are effectively sealed will not only minimise air leakage but also provide vapour control.