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The brand new U

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understanding part L - A guide to the manufacturing technicalities that need to be understood and implemented early on in the design process

The changes to Approved Document Part L (England & Wales) and Part J of the Technical Standards (Scotland), which come into force on 4 March, have been the subject of much debate.

Arguably, the final versions are still far from straightforward.

The most obvious change is that the new regulations demand significant improvements on previous levels of thermal performance for the building fabric, based on the elemental method.

In some cases, the called-for improvements are a massive 80 per cent better than previous demands.

Harmonisation standards Alongside the improvement in the thermal performance standards is a change to the method for determining the thermal insulation properties of building materials and products.

Behind this change is the development of test methods, ways of assessing performance and calculation procedures, which are all set out in the new European 'harmonisation' standards.These standards are referred to in the new Approved Document Part L (ADL) and are available from the British Standards Institution (BSI).

The new harmonised product standards were published in 2001 for factory-made insulation products; in many cases they replace existing British Standards. These testing standards will include new fire classifications of building products and changes in the procedures for establishing declared thermal conductivity values such that:

l90 per cent of the production output has a thermal conductivity not exceeding the declared value;

lFor foamed plastic materials, blown other than by air, the declared value must represent the average value over 25 years.

Under this ruling, manufacturers can choose to make their new declarations from 1 March 2002, with a mandatory declaration date by 1 March 2003. Further advice should be sought from individual manufacturers.

The combined method Under the incoming regime, designers must use a new method for determining elemental U-values. Known as the combined method of calculation, compared with the currently used 'proportional area method' it gives a more accurate assessment of the thermal bridges that occur in all parts of the building envelope. Together with the changes in the assessment of the thermal properties of building products, this will yield higher U-values than those currently experienced under the 'proportional area method'.

U-values should be calculated using the methods given in:

-BS EN ISO 6946 - for roof and walls, exposed floors and floors over unheated spaces. This standard accommodates the effect of thermal bridges such as mortar joints, metal wall ties and fixings, and air gaps (appendix A & B of Part L1 document);

-BS EN ISO 13370 - for ground floors (appendix C of Part L1 document);

-For procedures applying to elements such as metal cladding applications containing metal connecting paths, the reader should refer to BRE IP 5/98 for metal cladding and to BS EN ISO 10211-1 and 10211-2 for other cases.

Interpretation of these documents is not straightforward and will require a good understanding by the end user.

The designer's role The new ADL's more stringent thermal standards will undoubtedly place greater demands on designers.

Though many product manufacturers are happy to provide specification guidance for compliance, ultimately the responsibility for the overall building performance lies with the architect. Think about these points:

-Consider the acoustic and fire safety requirements in certain elements of the structure at the same time as specifying insulation which satisfies the thermal demands.

-Ensure that your thermal solutions do not contravene other approved documents.

-The ADL stresses the need to deliver the higher standards on the building site. Site checks will be essential.

-Part A structural and Part E acoustics are also under review, and are currently scheduled for enforcement by the end of 2002.

-Always compare like with like when it comes to manufacturers' data.Make sure the information you have corresponds to the ADL guidance and avoid anything that could result in technical risks occurring at a later date.

Ground-floor insulation As table A (page 60) shows, the new ADL (both L1 & L2) requires an 80 per cent improvement in the thermal resistance of ground floors over current values. Though this may not have too much impact on a ground-bearing concrete floor where insulation can be placed below the concrete, it could have a significant impact on suspended floors.

With pre-cast concrete and beam and block suspended floors, the insulation layer tends to be located above the deck, finished with a layer of 65mm screed or an interlocking chipboard floating floor. In the past, designers have often presented manufacturers with their completed design drawings before determining the required insulation thicknesses.

In such cases, unless they are predetermined, designers could run into problems with the overall floor-tofloor heights. Difficulties with ramped access and flush threshold details, in compliance with Part M access & facilities, could also arise.

With the new Part L in place, designers are urged to establish the design parameters and discuss them with the manufacturer at an early stage.

Air leakage standards The new ADL introduces a section covering the construction stage when the building fabric is being assembled and inspected.

First, there needs to be a check on the continuity of the insulation. The responsibility for achieving compliance rests with the person carrying out the work, who should provide a declaration that the provisions meet the requirements and state that:

-Appropriate design details and building techniques have been used;

or -Infrared thermography inspections have shown that 'the insulation is reasonably continuous over the whole visible envelope'.

Second, a site check for airtightness of the completed building has to be carried out such that:

-For all new buildings with a gross floor area greater than 1,000m 2, airleakage tests must show that the air permeability does not exceed 10m 2/hour at an implied pressure of 50Pa. If the completed building does not pass the air-leakage test, remedial works will have to be undertaken to seal the draughts. However, there will be lower acceptance criteria until the end of September 2003.

-For buildings of less than 1,000m 2 infloor area, the person carrying out the work should supply a declaration stating the 'appropriate design details and building techniques have been used to achieve reasonable conformity'.

The new regulations refer to the alternative of engaging a 'suitably qualified person' to issue declarations that there is continuity of insulation and that the airtightness is compliant.

Building control would check in advance that the person giving the declaration was suitably qualified.

Surveyors and architects offering this service would need to maintain the appropriate professional indemnity insurance.

Software for saps Though much software is available, a rigorous European software protocol that will ensure misinterpretations and misrepresentations are avoided, is not yet finalised. Until then, software suppliers may interpret the resistivity of mortar, for example, differently.

The final version is dependent on the revisions to Part L (England & Wales) and Part J (Scotland) and the introduction of SAP 2001. The software schedule is further complicated because enforcement dates for Part L (1 April 2002) and Part J (Scotland) (4 March 2002) are different. Part F for Northern Ireland is unlikely to change for at least another two years.

The new software must embrace designers' needs for absolute flexibility, make it easy for them to check compliance with Part L and Part J, and investigate the differences between SAP98 and SAP2001. The regulations also require U-values to be calculated using the combined method. Though many of the accredited software manufacturers are doing their best to finalise programs, they are heavily reliant on external events.

The BRE is currently in the process of finalising Conventions for U-value Calculations, details of which will be available on the BRE website. This document is intended for those who carry out or commission U-value calculations. It provides a guide to the relevant calculation procedures as given in the new standards and other documents. It indicates which calculation methods are appropriate for different construction types with additional information and data relevant to typical UK constructions.

Value added?

Roles have changed. Manufacturers are no longer looked upon as simply a supplier of insulation products.As the maze of new regulations and their implications become ever more complex, designers have increasingly come to rely on the technical support of the manufacturer. This means that selecting appropriate technical advice is an all-important process.

Although the Approved Documents L1 and L2 offer alternative methods of design compliance, the elemental method is still the most straightforward - it simply involves checking that the U-values for each of the elements are no worse than the stated values tabled within the documents. But this could be restrictive for designers of many contemporary buildings. Technical advice should be sought for the various specification solutions for compliance, ideally, manufacturers working with designers at an embryonic stage.Getting it right at the beginning is better than costly remedial or bolt-on solutions at the end.

Huw Evans is technical consultant of Rockwool www. rockwool. co. uk Rockwool's own guidance on compliance with Part L is out in early March.

For further information, please contact Anna Cherry on tel 01608 810692, anna@cosmic-cherry. com

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