Active job

Capping two chimneys

I have two identical chimneys that need capping. The fireplaces have already been closed off but a gap has been left in the fireplace to allow for ventilation. Just the exterior chimney to cap. I have been told that the chimney needs to still have a flow of air through it to prevent dampness problems in the chimney stack itself, the chimney breast where it passes through the loft space, and in the rooms below. I am looking for a simple cost effective chimney cap installation to achieve this. Both chimneys are on a one storied building.