Do I still need an air vent in my sealed-up fireplace? All manner of questions and solutions to problems and issues surrounding the subject of brickwork, blockwork and stonework

[danb5]

I've searched the forums and read the ones discussing air vents/bricks, and also looked in the projects page to understand the 'lie of the land' before posting.

I'm bricking up an unused fireplace in a house about 30 years old. It had an open flame gas fire in it previously (with a nasty brick fireplace!). It's worth noting that although the chimney stack was originally outside, there has subsequently been an extension built on the far side, so the stack is now internal for most of the way up until about 2/3rd of the way up the second storey.

I've got two quick questions:

a) Do I still need an air vent where the fireplace used to be, for airflow, if the chimney is now internal?

b) If I need to have an airbrick/vent, how do I know what size?

Thanks in advance!

[Perry525]

If you have a cavity wall home with no insulation, then a single leaf chimney breast is just the same as the rest of the home. (an express route for your heat to escape to the outside.)
If you have insulated or solid walls then the chimney breasts along with the windows are the weakest link and will attract condensation.
With a window you see the condensation.
With a chimney breast you may end up with a visible damp patch.
To stop damp patches forming one is encouraged to fit an air brick.
The down side of this is you then have a very large hole in your room, which lets all your expensive air out up the chimney. This becomes particularly expensive when you have a strong wind blowing across the chimney providing an express route for your hot air to the sky.
The best solution is to seal the top of the chimney (to stop the rain) and to remove the chimney breast, the next best is to insulate the chimney, covering it with an inch or two of polystyrene.