Detailing the envelope
There are a number of reasons why cold bridges should be avoided: 1 They cost the building owner money because they are an effective path for heat out of the building or, in hot countries, heat into the building, so requiring more money to be spent on the heating or cooling of buildings. 2 They can cause serious discomfort in buildings by creating cold (or hot) walls, windows or areas in a room. 3 They cause condensation. Because, when the outdoor air temperature is lower than that indoors, the cold bridges are colder than the internal surrounding wall and room air. So, they cause the adjacent wall and air to cool rapidly. As cooler air can hold less moisture than warm air, so moisture condenses out on the surface of the cooler wall. This moisture can damage the structure of a building and provide a substrate for the growth of mould. This is why mould is typically found on the edges of metal window frames or at wall corners and in patches on the wall in front of a cold bridge. Mould is very bad for health, as described in Chapter 6. In order to prevent cold bridging in new buildings care should be taken to ensure there are no metal or concrete elements that span from the inside to the outside leaves of walls: • Replace metal wall ties with nylon ones. • Do not return the brick work around doors and windows but use a timber sub-frame detail as shown in Figure 3.7 (see also Vale and Vale, 2000, pp. 162–166). • Eliminate the need for through-wall metal ducts by sensible design of opening windows and vents, and the use of passive stack ventilation systems. • Split lintels so there is an external lintel for the outer leaf of a wall, a timber sub-frame for the insulation and a separate internal lintel for the inner leaf of a wall. Single metal lintels across the wall should be avoided. • Choose the windows carefully to have a U (or R) value in keeping with that of the walls. If there is a single glazed window in a very well-insulated wall it will attract condensation owing to its internal surface temperature being much lower than that of the wall. Try to avoid metal window frames. One method of characterizing the severity of a cold bridge is the temperature difference ratio (TDR). This is a coefficient that specifies how cold a surface is relative to the inside and outside temperature (Oreszczyn and Littler, 1989; Oreszczyn, 1992). The TDR is calculated with the following equation:
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