The photograph below was taken by one of LABC's building control surveyors when carrying out a site inspection, and shows how attention to detail (or a lack of!) can lead to future problems. This extension will suffer from moisture problems, but there is not one reason why – there are three.
Look closely at the DPC arrangement.
A membrane is built into the wall to be joined to the damp proof membrane in the floor slab at the junction between the wall and floor. As damp proof membranes are generally unsuitable for use as a DPC in loadbearing walls, the contractor has rightly built an additional DPC into the wall to protect the membrane. But the DPC spans the full width of the cavity and so all moisture within the cavity will pond on the DPC. The result? The blockwork above the DPC will become saturated and cause damp problems within the extension, the skirting boards will rot and the mould will occur on the plaster. This could have been avoided by building the membrane on to the inner leaf only and protecting it with a 100mm DPC, or by protecting the wide horizontal DPC with an additional cavity tray over.
Now look at the blockwork in the reveal for the door.
The full picture shows that the contractor has built in a vertical DPC where the two wall leaves meet to prevent moisture tracking across. But in our ‘cropped’ part of the picture this is not visible – mortar has been smeared over the joint, making the vertical DPC useless at this junction.
The reveal has no insulation at all...
Just solid masonry separated by a vertical DPC. This means that this part of the wall will be colder than the rest of the wall, resulting in condensation forming around this part of the structure – a case of ‘cold bridging’. Judging by the standards of workmanship, it’s quite likely that there is no insulation below the DPC either, meaning the same problems will happen around the skirting too. It’s important to think about cold bridging through any part of the structure but especially where junctions occur. Use of an insulated cavity closer would have prevented these last 2 problems.