Resistance to weather - Walls.

Monday, January 24, 2011

Between 1920 and 1940 it became more usual for external walls of small buildings to be constructed as cavity walls with an outer leaf of brick or block, an open cavity and an inner leaf of brick or block. The outer leaf and the cavity serve to resist the penetration of rain to the inside face and the inner leaf to support floors, provide a solid internal wall surface and to some extent act as insulation against transfer of heat.

The idea of forming a vertical cavity in brick walls was first proposed early in the nineteenth century and developed through the century. Various widths of cavity were proposed from the first 6 inch cavity, a later 2 inch cavity followed by proposals for 3, 4 or 5 inch wide cavities. The early cavity walls were first constructed with bonding bricks laid across the cavity at intervals, to tie the two leaves together. Either whole bricks with end closers or bricks specially made to size and shape for the purpose were used. Later on, during the middle of the century, iron ties were used instead of bond bricks and accepted as being adequate to tie the two leaves of cavity walls.

From the middle of the twentieth century it became common practice to construct the external walls of houses as a cavity wall with a 2 inch wide cavity and metal wall ties. It seems that the 2 inch width of cavity was adopted for the convenience of determining the cavity width, by placing a brick on edge inside the cavity so that the course height of a brick, about 65 mm, determined cavity width rather than any consideration of the width required to resist rain penetration. This was adapted to the 2 inch (50 mm) wide cavity for walls that became common until recent years.

In constructing the early open cavity walls it was considered good practice to suspend a batten of wood in the cavity to collect mortar droppings. The batten was removed from time to time, cleaned of mortar, and put back in the cavity as the work progressed. The practice, which was largely ignored by bricklayers as it impeded work, has since been abandoned in favour of care in workmanship to avoid mortar droppings becoming lodged inside the cavity.

With the increase in the price of fuels and expectations of thermal comfort, building regulations have of recent years made requirements for the thermal insulation of external walls that can best be met by the introduction of materials with high thermal resistance. The most convenient position for these lightweight materials in a cavity wall is inside the cavity, which is either fully or partially filled with insulation. A filled or partially filled cavity may well no longer be an efficient barrier to rain penetration so that, with the recent increase in requirement for the thermal insulation of walls it has now been accepted that the width of the cavity may be increased from the traditional 50 to 100mm to accommodate increased thickness of insulation and still maintain a cavity against rain penetration.

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