Closing off fireplace - we have a cowl, do we need to cap off?

hello camilla 94076, if you have closed down the fireplace im assuming you have boarded it off? what you are trying to prevent is moisture stagnating within the void i would advise you keep the cowl at the top and remove a brick above the damp coarse ( if there is one dependent upon the age of the building ) i would go one whole brick above the damp coarse and add an air vent there to ensure its ventilated from bottom to top to allow a draught through the chimney and you shouldn't have a problem, the only other consideration is ensuring rain water doesn't enter from the top and sit within the void to do this make sure the cowl is of a larger diameter than the pot if rain water can and does get in then cap the chimney off, capping off really is a last resort because bricks which have been exposed to excessive temperatures such as a fire become porous so allowing air flow is a much better option