Damp-proof courses in cavity walls.

Sunday, January 2, 2011

A cavity wall is built as two leaves separated by a cavity. The purpose of the cavity is to act as a barrier to the penetration of rainwater to the inside of buildings. It is practice to build a cavity wall directly off the foundation so that the cavity extends below ground. A requirement of the Building Regulations is that the cavity should be carried down at least 150 mm below the level of the lowest dpc. 

A dpc in external walls should ideally be at the same level as the clpm in the concrete oversite for the convenience of overlapping the two materials to make a damp-proof joint.

Where the dpcs in both leaves of a cavity wall are at least 150 mm above outside ground level and the floor level is at, or just above, ground level, it is necessary to dress the dpm up the wall and into the level of the dpc. This is a laborious operation which makes it difficult to make a moisture tight joint at angles and intersections.

The solution is to lay the dpc in the inner leaf of the cavity wall, level with the dpm in the floor, as illustrated in Fig. 37.

Where the level of the foundation is near the surface, as with trench fill systems, it may be convenient to build two courses of solid brickwork up to ground level on which the cavity wall is raised, as illustrated in Fig. 38. As little vegetable top soil has been removed the floor level finishes some way above ground and the dpm in the floor can be united with the dpc at the same level.

The cavity insulation is taken down to the base of the cavity to continue wall insulation down to serve in part as edge insulation to the floor construction.

It is accepted practice to finish the cavity in external walling at the level of the dpc, at least 150 mm above ground, where the wall is built as a solid wall up to the dpc, as illustrated in Fig. 38. This form of construction may be used where the inner leaf of the cavity wall was built with light weight concrete blocks, used for their insulating property. These blocks fairly readily absorb moisture, expand when wet and might be affected by frost and deteriorate, whereas solid brickwork below ground will provide a stable base.

With this arrangement the requirements of the Building Regulations recommend the use of a cavity tray at the bottom of the cavity. This tray takes the form of a sheet of a flexible, impermeable material such as one of the flexible dpc materials which is laid across the cavity from a level higher in the inner leaf so that it falls towards the outer leaf to catch and drain any snow or moisture that might enter the cavity. The cavity thus acts as both tray and dpc to the cavity wall leaves.

In this detail of construction the under concrete insulation is below the lowest level of the cavity and should be turned up against the outer walls as edge insulation. 

Fig. 37 Damp-proof course at different levels.

Fig. 38 Damp-proof courses in cavity walls.

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paukel said...

I agree with you that the dpc in the inner leaf of the wall should be at the same level as the dpm in the floor, but when I posted a comment to this effect on a building forum I got slated and told that I knew nothing about building.
I'm sure that putting the dpc in at finished floor level and trying to bend it down to overlap the dpm is a recipe for disaster. What do you do at the corners? Surely you would have to cut it in order to bend it down both sides of the corner so there would be no overlap at all.
I can't understand why architects show it this way. Even in the case where ground level is a problem I don't see why the dpc in the outer leaf can't be at a higher level than the dpc in the inner leaf. After all there's a cavity between them.

paukel said...
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