Efflorscence can be brushed off once it has developed, certainly prior to rendering. But don't just take our word for it, find some advice on the FAQ page of the Brick Development Association site.

Efflorescence can be confused with lime staining, which produces much harder deposits which are very difficult to remove and may require acid treatment - best done by a specialist. The calcium hydroxide can come from the cement or from water in the blocks or ground water, which is why it is often seen on retaining walls.

On fair faced walls, there is a higher likelihood that rain may penetrate, for example, through the bed between brick courses. It is usual practice to provide weep holes for all cavity trays in fair faced walls. The weep holes do not usually show in the perps in brick walls. If weep holes were to be put into a rendered wall, everywhere that there is a cavity tray, the wall would look a mess with weep holes all over the shop!

The membrane will also ensure continued ventilation if the vents get blocked up by overenthusiastic stuffing of the insulation material into the eaves space.

Also, when a house is being built there is a lot of water about in the new plaster which gives rise to a lot of damp air. This is better dispersed from the roof space by over fascia vents.