It’s that time again...’spot the defect’!

In this study we take a look at where a gas membrane is installed, a cavity tray is required to be formed across the external walls. Typically the gas membrane will also form a cavity tray as illustrated below:

So what’s wrong with this picture?

What you should have spotted was that the cavity tray must be at least 225mm in vertical height and lap fully across each leaf of masonry unlike the example above.

The correct type of membrane must be used and correctly lapped with the DPC.

Care must also be taken to avoid excessive debris building up in the tray or allow the insulation to become dislodged and bridge the cavity. Water penetration and cold bridging to the inner leaf is likely to occur.

The build-up of mortar in cavity trays must be avoided and applies at all levels; horizontal DPC / DPM junctions and at upper levels where lintels, fire stopping and roof abutments occur.

Full details on cavities below ground can be found in section 6.2.3 of the LABC Warranty Technical Manual.

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By Frzana Ferguson