Both L1 and L2 begin with a reminder that designing to minimise energy consumption may carry the risk of technical problems in other areas. High levels of thermal insulation, and careful attention to draught proofing can cause problems due to:
• Interstitial condensation
• Surface condensation in roof spaces
• Inadequate ventilation for occupants
• Inadequate ventilation and air supply to combustion systems and flues. Other potential problems are:
• Rain penetration causing, among other things, damage to thermal insulation. This is often associated with failure of flat roof coverings due to the high levels of thermal stress induced by high levels of thermal insulation within the roof construction.
• Sound transmission, due to the fact that materials which provide good thermal insulation are normally very poor at providing insulation against the transmission of sound.
Guidance on the avoidance of these related technical risks must be found elsewhere, and a number of sources of information are suggested:
• BRE Report No. 262: Thermal Insulation: avoiding risks, 2002 edition 
• Approved Document F, Ventilation
• Approved Document J, Combustion appliances and fuel storage systems
• Approved Document E, Resistance to the passage of sound
• Limiting thermal bridging and air leakage: Robust construction details for dwellings and similar buildings .
Both L1 and L2 also contain a series of definitions, as follows, that apply throughout the documents.
Thermal conductivity (the X-value)
This is the rate at which heat will pass through unit area of a material when there is unit temperature gradient across the material. It is usually expressed in watts per square metre for a temperature gradient of one degree Kelvin per metre, and is given the symbol X (Greek lambda). The units of X are thus W / m, which simplifies to mK.
Thermal transmittance (the U-value)
This is the rate at which heat will pass through unit area of a material (or a construction made up of several materials) when there is unit temperature difference between the environments on the opposite sides of the material. It is usually expressed in watts per square metre for a temperature difference of one degree Kelvin, and is given the symbol U. The units of U are thus mrK.
If measured test results for X and/or U-values are available, these should be used. Manufacturers are often able to provide such information for the X-values of their materials, but where these are not available, values may be obtained from data published in Appendix A of the Approved Documents or in any other authoritative publication (e.g. BS EN 12524  or CIBSE Guide, section A3 ). The measurement standards that should be followed are BS EN 12664 , BS EN 12667  and BS EN 12939 . Manufacturers sometimes also provide test results for the U-values of various construction elements that incorporate their materials, the relevant measurement standards being BS EN ISO 8990  or, for windows and doors, BS EN ISO 12567-1 . More usually, the U-values supplied by manufacturers are obtained by calculation from X-values. Where calculated values are used, care must be taken to ensure that proper allowance has been made for thermal bridging effects. These are most likely to arise when the construction includes joists, structural or other types of framing, or any material or component that breaks through an insulation layer. The bridging effect can only be ignored when the bridged material and the bridging material have sufficiently similar thermal properties (i.e. if the difference in their thermal resistances is less than 0.1 mK). This will normally apply to mortar joints in brickwork, but not necessarily to mortar joints in lightweight blockwork. Calculation procedures for U-values are specified in:
• BE EN ISO 13789  and BRE/CRC  for calculation methods and conventions
• BS EN ISO 10077-1  or prEN ISO 10077-2  for windows and doors
• BS EN ISO 13370  or BCA/NHBC Approved Document  for basements
• BS EN ISO 10211-1  and BS EN ISO 10211-2  for thermal bridges.
Appendix B of AD L1 and L2 provides a simplified method, based on BS EN ISO 6946 , which is suitable for the calculation of the U-values of most wall and roof constructions. Chapter 5 gives example calculations.
This means an element exposed to the outside air, and includes:
• A suspended floor over a ventilated or unventilated void
• An element exposed to the outside air indirectly via an unheated space
• An element in a floor or basement that is in contact with the ground.
It should be noted that an element exposed to the outside air indirectly via an unheated space was previously known as a 'semi-exposed' element. The calculation of the U-value of such an element must now use the method given in SAP 2001 . It should also be noted that a wall separating a dwelling from any other premises that are heated to the same temperature does not require thermal insulation.
This is defined as a window in the plane of a pitched roof. Within AD L1 and L2 such a window may be considered as a rooflight.
When evaluating areas, measurements should be taken on the internal faces of external elements. The areas of projecting bays must be included. Roof areas should be measured in the same plane as the roof insulation, and floor areas should include non-useable space (e.g. stairwells and builders' ducts).
This is a measure of the leakiness of the building fabric to unwanted internal to external air exchange. It is defined as the average volume of air, per unit area, which passes through the building envelope, when there is an internal to external pressure difference. It is normally expressed in cubic metres per hour, per square metre of building envelope area, at a pressure difference of 50 Pa. Within AD L1 and L2, the envelope area is taken to be the total area of walls, floor and roof separating the interior volume from the external environment. Air permeability does not include deliberate leakage paths between the inside and the outside, such as flues, ventilation ducts, air bricks, etc. These are sealed up during any measurement procedure, so that only cracks, gaps at joints and similar leakage paths are included. The lower the air permeability the better, as this shows that the building fabric is more airtight.
Standard assessment procedure (SAP)
This is the UK government's procedure for rating the energy cost performance of dwellings , and is defined in Chapter 9. Separately and independently of Part L1, Building Regulation 16 requires that whenever a new dwelling is created (either by building work or by a material change of use), the energy rating of that dwelling must be calculated by means of the standard assessment procedure. The result of the SAP calculation cannot be used by itself to demonstrate compliance with Part L1. However, the SAP calculation can be continued to find the carbon index of the dwelling. If the carbon index is above a specified minimum, compliance has been demonstrated (see section 2.3.4).
Building Regulation 18 has been extended. Previously it gave local authorities the power to test drains and private sewers for compliance with Part H. Now, Regulation 18 allows a local authority to make such tests as may be necessary:
• For compliance with Building Regulation 7, which specifies that all building work should be carried out with proper materials and in a workmanlike manner, and also
• For compliance with any of the applicable requirements of Schedule 1.
The inclusion of Schedule 1 within Regulation 7 confers very wide powers on a local authority to require testing. Much of that testing could be done beforehand on components, using approved test procedures, and the results provided in the form of appropriate certificates. However, there are several areas where testing can only be carried out and be effective when the building is complete, and two of these are of particular significance to Part L:
• Airtightness testing of buildings to test for compliance with the air permeability criterion
• Testing for continuity of insulation and the avoidance of thermal bridging.
In the case of airtightness testing, the Approved Documents point to only one type of test, the fan pressurisation test, and the criterion for compliance is written specifically in terms of the result which this test provides. Indeed, for large buildings above 1000 m2 floor area, Approved Document L2 appears to offer no alternative to the test as a means of demonstrating compliance.
For the testing of the continuity of insulation, there is a similar lack of flexibility. Unless an authoritative certificate can be provided stating that the design details and building techniques are appropriate, the Approved Documents require satisfactory results from a test of the whole of the visible external envelope using infrared thermography.
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