If waste mortar is allowed to drop down the cavity and collect at the bottom, there is a risk of the horizontal DPC becoming bridged by the build-up. Similarly, mortar droppings should not be allowed to collect on the wall ties, as this defeats the object of the integral drip and could eventually lead to damp patches on interior wall surfaces. Obviously this is not an issue where full-fill cavity insulation is concerned but partial-fill insulation leaves a residual open cavity and great care must be taken to ensure that mortar does not fall down it during the construction process. The bricklayer needs to be very careful when spreading mortar bed joints and should trim off excess mortar when laying brick and blocks.
Being careful does not solve the problem completely, but mortar can also be prevented from dropping into the cavity by placing a timber batten on to the last horizontal row of wall ties (see Fig 177). The batten is slightly narrower than the residual cavity and wire is attached to it, which allows it to be raised as the next row of wall ties is placed. Each time, it is cleaned off before being repositioned on to the new row of wall ties. Where the residual cavity is narrow, it may be too difficult to use a cavity batten and the bricklayer will have to make a judgement as to whether it is worth the bother, given that there is a reduced chance of mortar falling down into the narrower cavity. Good practice, however, dictates that using a cavity batten is the preferred course of action.
Fig. 177 installation of partial-fill cavity insulation.