Part L Building regulations


Statutory Requirements for Energy Conservation
All development is regulated under the Building Regulations. Part L 'The Conservation of Fuel and Power' (or Part J of the Scottish Building Regulations) applies strict limitations on the heat loss permissible from new buildings, from extensions where the floor area proposed exceeds 10m², and in some cases from existing buildings affected by alterations. In an existing building it is usually only the components being altered which are affected by the Regulations. However where a building is being converted to flats or from non-residential to residential and for certain other changes of use, they can apply to the whole building. Residential conversions will be required to achieve an energy rating which satisfies a 'Standard Assessment Procedure'. Part L (or J) does not apply to buildings and extensions which are not habitable. A conservatory for example, may not need to comply if it is separated from the interior of the building by doors or if it is under 10m².

The insulation standards, which are measured in units of thermal transmittance or 'U-value', favour double rather than single glazing, but there is a considerable degree of flexibility, and Part L specifically states that double glazing 'could be inappropriate in conservation work'. Acceptable standards are given by the ratio of window opening to floor area for single as well as double glazing, and variation from standards are also acceptable 'if compensatory provisions are made'; increases in insulation to the roof space for example, could be used to off-set the heat lost through single glazing; solar gain through south-facing windows may also be taken into account.

It is interesting to note that aluminium and PVCu does not perform as well as timber in terms of their thermal efficiency. In addition both involve a much greater consumption of energy in their production than is used to produce timber - over 45 times as much in the case of aluminium (TRADA figures). Wood, as a natural renewable resource, is clearly preferable to any man-made alternative.


Making Sensitive Improvements
For older buildings and for listed buildings in particular, where it is essential to improve the thermal efficiency of the existing windows, the least obtrusive option is to introduce draught proofing measures. For casement windows the process is simple, with a wide range of products available, including durable rubber seals which are discretely rebated into the window frame. The draught proofing of double hung sash windows is more complex, requiring the replacement of the parting bead with a new component incorporating rubber blades to maintain the seal at the sides, and compression seals to the meeting rail, window head and sill. These seals may be purchased for installation by a joiner, or alternately there are companies included in the Building Conservation Directory which specialise in overhauling, repairing and draught-proofing sash windows. Thorough treatment using purpose-made fittings has the added benefit of making the sashes slide more easily and stops them from rattling with every gust of wind.

Secondary glazing also provides an alternative where the exterior of an historic building needs to be protected, although the double reflection caused by the second pane of glass can be seen externally. This approach may be unacceptable internally, as the effect of even the most sensitively designed secondary glazing on the character of the windows from the inside can be excessive, and they cannot be used where there are shutters on the inside.

In the 19th century some houses were constructed using secondary glazing in the form of a second pair of sash windows which drop down together into a pocket below the window, covered by a hinged extension of the window cill, and fronted by an attractively moulded panel. Although rarely used today, this approach provides an interesting solution where there are no shutters to obstruct. Metal-framed vertical sliding sashes are commonly used, which fit tight against the window. The small, simple sections are not too obtrusive during daylight hours, when they are seen against the light, but at night they can be glaringly obvious as an entirely alien element in a fine interior.

A cheap and simple method of secondary glazing is to fit a single frame of glass over the whole window within the reveal which can be removed and stored in the summer. The only limitation on this system is the size of glass which can be handled easily without breakage, and it is really only suitable for small windows and windows divided into smaller lights by mullions and transoms.

Secondary glazing allows the originals to be retained and is generally accepted by planning authorities for use in listed buildings. Double glazing on the other hand can rarely be added to existing windows without major modification due to the need for thicker glazing bars to hide the spacer bars (see diagram), the additional thickness of the sealed units and the additional weight of glass on fine timber members.

Replacement Windows
Where a building is listed, planning authorities usually require existing windows to be retained and repaired where possible, unless they are clearly falling to bits and cannot be repaired, or are later additions which are obviously the wrong design for the building. A fine antique piece of furniture is prized not only for its completeness but also for its imperfections, and so may a window; early glass may display the radiating ripples and tiny air bubbles indicating that it was hand blown; and sash windows in particular are highly intricate and sophisticated pieces of furniture, superbly constructed to last, and deserving careful restoration.