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There are two main methods of making a concrete block cavity structure airtight:

1. Plastering

The first method is plastering. This is by far the most common method and a more
preferred option amongst Irish builders.

To begin, it is important to understand that concrete blocks are porous; this means they are not airtight in their own right. As a result the inner surface is parged (covered with plaster) to ensure airtightness. It is vital that the entire surface is thoroughly parged from the bottom of the wall to the very top. If the surface area is not fully covered there will not be an airtight seal.

Once parged, the wall is usually skimmed (covering with a thin coat of skim); again the skim needs to be applied to the total surface area of the wall. Providing that this is done adequately; the surface will be airtight and protected from any possible air leakage.

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2. Service Cavity

The service cavity is typically associated with a timber frame structure however many passive block constructions are using this method.

This method uses an airtightness membrane fitted against the inner leaf (blue). This will prevent any air penetration from the concrete block. 

There is then a service cavity; this can vary in width but is typically around 60mm. This service cavity is filled with services (e.g. conduits, pipes) and insulation.

Plasterboard is then applied and skimmed; ensuring to cover the entire surface area. This ensures an airtight structure.

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