A wet/wet wall or a wet/dry wall? Cavity Trays discusses the difference.
Cracking and movement in exterior skin where conventional DPC interrupts bonding
Design and construction habits can obscure logic. There are two reoccurring examples that are frequently witnessed on building sites. Where cavity insulation terminates part way up an external gable end cavity wall, the top of the insulation requires DPC protection (PD 6697:2010.6.2.7.7.6. and BBA certificates stipulate DPC protection).
Cavity Trays’ Type CD Cavity Dropcloak
This is traditionally achieved by building the DPC into the inner skin, dropping and trans versing the cavity, and emerging through the outer skin. It does the job, but the vulnerable gable triangle is separated from the masonry beneath. Instead, it is resting on a slip-pane and the exposed masonry triangle is susceptible to wind pressure/suction. This short BBC video-link puts everything into perspective!
Gable masonry triangle separated from the masonry beneath it
Similarly, a parapet cavity wall traditionally adopts the same L-shaped DPC protection within. Again, the entire parapet masonry above the roofline is rising off a slip-plane, and the masonry can adversely move.
Type CD profile
What is overlooked in both instances is that the outer skin is wet above the DPC and below the DPC, unlike an opening or intersection that needs to be kept dry. Building the DPC through the outer skin achieves nothing, except it introduces a structural weakness because traditionally-used roll DPC needs to be supported so it is built into both skins. But there is no need to do so because it is a wet/wet wall.
Conventional DPC placement results in structural discontinuity
The Type CD Cavity Dropcloak from Cavity Trays eliminates this weakness. Read Protecting the Building Envelope, delivered to your door with compliments of Cavity Trays.