Party walls separating buildings and thermal bypasses

Cavity walls separating buildings (hereafter referred to as party walls) may not have zero heat loss because a thermal bypass is created where there is air movement in the cavity. Fully filling the cavity may have implications for sound transmission through the party wall. Where edge sealing is adopted to address the party wall bypass, it is essential that the edge sealing is effective in restricting air flow into the cavity and that it is aligned with the thermal envelope. Sealing is required at the top, the bottom and vertically. Although effective edge sealing may be incorporated as part of a cavity barrier which is provided as a fire stop, a cavity barrier on its own may not be effective in restricting air flow into the cavity. In order to use the reduced cavity wall U-values in Table 2.4 (0.2 or 0.0 W/m2.K) it will be necessary to demonstrate that the design adopted is likely to be robust under normal site conditions. For example, in a room-in-the-roof design, the insulation layer is likely to follow the sloping roof to a horizontal ceiling and then continue at ceiling level. In such a case it is important that the party wall cavity seal follows the line of the insulation in the sloping roof and horizontal ceiling sections. In considering heat losses via party walls it is important to be aware that wherever the wall penetrates an insulation layer, such as when the blockwork of a masonry party wall penetrates insulation at ceiling level, a thermal bridge is likely to occur. This will be the case even when the party wall U-value is zero. The evaluation of thermal bridges should ensure that any bridging at the party wall is taken into account along with other thermal bridges. It is important also to be satisfied that any solution to the party wall bypass does not contravene other parts of the regulations, in particular Part G (Resistance to the passage of sound) and Part E (Fire safety).