David Wrightson on

an apparent threat to windows

in historic buildings

MANDATORYD OUBLE GLAZINGQ)

HEAT LOSS VERSUS HERITAGE

In this, the first of two articles, David Wrightson looks at the

implications of the revised Part L of the Building Regulations (Gonservation offuel and power): 1995, (Part J, Scottish Building Regulationsj

“Windows are vital to the history and architectural integrity of all domestic buildings, particularly those of the 18th and 19th centuries. They are now under serious threat, from both PVCu double-glazed units, and well intentioned people repairing windows with inappropriate copies”, wrote Charles Brooking recently.

English Heritage’s Framing Opinions campaign aims to make the public more aware of these issues and to promote sympathetic repairs rather than unsuitable replacements. John Fidler, the campaign’s director says that “the threat to historic towns and villages in alterations to windows can be summarised as a lack of knowledge among home owners”.

A new threat appears to be looming over the horizon, that of ‘mandatory’ double glazing. The revised Part L of the Building Regulations (Conservation of fuel and power) comes into force in July this year; manufacturers of glass and other insulating products are suggesting that double glazing is to become mandatory. This disinformation may well be more of a threat than the new regulations themselves which, when studied in detail, cannot be said rigidly to demand double glazing. Difficulties could arise in a few cases, and the information which follows may be useful as a guide. For a fuller picture, reference should be made to the legislation itself and supporting documents.’

A word must be said first about the technical aspects of double glazing (in the form of hermetically sealed insulating glass units held in position within a window rebate by means of putty or beads). Failures have occurred for a number of reasons. The durability of a correctly manufactured sealed unit

depends on several factors: the compatibility of the edge seal of the unit with the glazing materials, the degree of protection of the seal against sunlight, the satisfactory prevention of moisture ingress, and the quality of workmanship on site.

BS6262: 1982 Glazingforbuildings laid down the basic principles for installation of sealed units. Neither the later BS 8000 Part 7: 1990 Code of practice for glazing (workmanship on building bites) nor the Glass & Glazing Federation Manual now recommends the use of putty in double glazing, substituting instead solid timber or other beading. The GGF Glazing Manual recognises that “however well they are glazed, insulating glass units cannot be expected to have the same life expectancy as single glazing”. Hence the need for increased protection of the edge seal in the form of very wide rebates, and drainage/ventilation of the support systems (see figures 1, 2 and

3).

Figure 1: Plan of a traditional box sash window of mid-i8th centu?y to early 19th-century. The astragal profile shown is one of many variants. All dimensions are in nillimetres.

Figure 2: The minimum recommended dimensions according to BS 6262 Glazing for buildings, using 11 mm sealed units. The use of putty is not recommended by the more recent BS 8000 Part 7.

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