The last word on Part L

This is it. It might be a year later than originally planned, but the new version of Part L of the Building Regulations has finally been published. As it stands, the document bears a large "interim draft" stamp, pending formal publication in August 2001, but the DETR has taken the unusual step of releasing it now to give suppliers and designers a chance to prepare for its implementation.

Approved Document L deals with the energy use of buildings. Officially entitled Conservation of Fuel and Power, it aims to cut carbon dioxide emissions from buildings – part of the government's drive to reduce CO2 to 80% of 1990 levels by 2010.

The document incorporates much of what was proposed in the consultation, which was published last June in a simplified format. In compiling the final version, the DETR considered almost 400 responses to the consultation. Respondents included the larger institutions and trade bodies, suppliers and consultants, local authorities and individuals. It is disappointing that more architects did not feel strongly enough to respond, given that the onus will be on designers to match specifications to the new requirements, but hats off to those that did take the time to fill in the 22-page questionnaire.

One reason the document has been published as a draft is that it may need amending to incorporate changes to the acoustic regulations, Part E, the consultation period for which has only just been completed. The two documents will then need to be aligned to ensure that there are no contradictions. There will then be a six-month grace period before the new rules become mandatory. Theoretically, plans deposited for building control approval before February 2002 may use current standards, but it is hoped that designers will adopt the new tighter rules in all designs from now on.

Although the draft has been published as one document, the final version will emerge as two editions: L1 to cover dwellings, and L2 to cover other building types, so housebuilders do not have to wade through pages of information relating solely to offices and factories.

What's new in Part L
As in the consultation document, the scope of the regulations has been widened to encompass alterations to buildings. However, rather than change the definition of "material alteration", as originally proposed, "controlled service or fitting" is being extended to embrace relevant alteration and extension work, but without covering minor or emergency repair work.

For dwellings, this is specifically limited to alterations that affect only glazed areas or heating systems. And in all buildings, no extension is deemed exempt because it is too small. Building control officers will have discretion in assessing individual cases such as unheated porches, and particular guidance is now given to "special cases" to clarify what changes might be considered reasonable in historic buildings and to acknowledge the demands of portable buildings.

For new buildings, the great U-value debate has resulted in a required insulation value for walls of 0.35 W/m2°C, rather than the more onerous 0.3 originally proposed. Some may feel this is a climbdown in the face of intense lobbying from the masonry business, but it is still a significant improvement over the previous value of 0.45. Also, the new method for calculating U-values will make the 0.35 figure more difficult to achieve. However, even with the new calculation method, brick-and-block manufacturers are confident of meeting the regulations using conventional cavity construction without a substantial increase in the building footprint.

The controversial plan to tighten insulation values in two phases has also been eliminated. Instead, the more demanding fabric U-values proposed originally will be introduced immediately, rather than 18 months later. It is worth pointing out, however, that the DETR has not abandoned its goal of tightening the rules further. Clearly, the desire remains to achieve 0.3 for walls in the not-too-distant future.

For windows, two values are quoted depending on frame type: 2.2 for metal frames and 2 for timber or PVCu (which generally have greater frame proportions and so transmit less heat).

For designers, there are still three ways to demonstrate that a building envelope has sufficient insulation – the target method, the elemental method or a carbon index – but the calculations to show compliance have been clarified. The target method, which allows a designer to offset one area of a building that is poorly insulated against another area where the insulation exceeds the regulations, only involves one calculation. The elemental method, where each element of a building's fabric must meet a minimum insulation value, has also been simplified; and the notion of using a carbon index, which would measure how much carbon is released when heating a building, received broad support from respondents.

The U-values are the same for domestic and non-domestic buildings. However, the requirement for shop display windows to be included in a building's overall heat loss has been removed because the DETR has acknowledged the cost of meeting the tougher standard for such large expanses of glass is too high, at least for now.

The DETR has also introduced further constraints to prevent abuse of the trade-off between construction elements, such as adding more insulation in the roof to allow bigger windows to be installed; an absolute poorest U-value for a particular element has been introduced, at the insistence of many respondents to the consultation document.

Air-tightness and overheating
The industry seems to have welcomed the new emphasis placed on the air-tightness of buildings to prevent heat loss through unwanted ventilation. This means that for many commercial buildings, contractors will have to arrange for specialists to test buildings to ensure they are air-tight. Further guidance to show designers and contractors how to meet the air-tightness standards will be provided in the form of "robust standard details" and a calculation procedure generated by BRE is promised to coincide with the document's formal publication in August.

The document acknowledges that the requirement to demonstrate compliance with 10 m3/hour of air leakage for every square metre of the building envelope area will place an additional burden on testing and commissioning periods for contractors and would also necessitate the development of new equipment to service the demand. However, the Detr did not deem this an insupportable burden, and the requirement remains unchanged, although it is acknowledged that buildings with a floor area of less than 1000 m2 do not necessarily need the practical test, provided they are detailed correctly.

The new document introduces guidance to avoid solar overheating in commercial buildings, along with a calculation method to demonstrate compliance. This has been included to stop well-insulated buildings getting so hot in summer that they require air-conditioning.

In the section on assessment of heating system efficiency, a number of refinements to the proposals have been adopted, including making allowances for community heating schemes and localised heating in industrial buildings that may have poor insulation but have a low level of heating.

The figures for carbon emitted by burning different fuel sources have also been adjusted. Methods of calculation have been clarified here, too, with the carbon performance index changed to a carbon performance rating, expressed in kilograms of carbon emitted per square metre of treated area a year. This means a check can be made relatively easily between design intent and actual operation.

Cross-references to benchmark data are also included for ease of use.

Record breakers
The sections on testing, commissioning and provision of information on building services remain largely unchanged from the consultation draft, although useful guidance is included under the metering section to aid designers and operators in selecting appropriate metering strategies for services.

The idea of building log-books to record energy use has been cautiously welcomed by the industry, although it remains to be seen how successful contractors will be at separating out log-books, operating and maintenance manuals and health and safety files – no doubt extra paper will be generated before reasonable systems are put in place.

The DETR does not consider the increase in resources that will be required to deal with the extra workload that these changes generate for building control. It must be hoped that good intent is not compromised by the inability of the authorities to implement it.