Closing up fireplace

[DIYamateur87]

Confused with variety of answers on this subject..
I have removed a fireplace and gas fire, precast unlined flue raincapped at chimney, terraced property with flue on internal wall.

Questions are
1. Is an additional vent required?
2. If required can I knock through side wall of recess (vent sized hole only) and vent to under floor void rather than into living room?
3. Is it ok board up with insulated plasterboard or is brick better?

Many thanks for any advice.

 

[Wayners]

Vent on front or side is fine. Just need air blowing through..
I used Thermal blocks and bags of pre mixed sand and cement.
Cut blocks with old saw.
Bung in and board or skim over.
You could use insulation and board if you want

 

[alex.bennett]

If the flue is capped at the top, you still need some airflow. So yes, in most cases a vent is recommended. It helps stop moisture build-up inside the chimney.

I wouldn’t vent it into the underfloor void. That space needs to stay dry too. Better to vent into the room with a small grille so air can move through the flue naturally.

You can board it up with insulated plasterboard, but leave a vent in place. Brick is fine too. The key thing isn’t the material. It’s making sure the flue can breathe so you don’t get damp later on.

 

[DIYamateur87]

@alex.bennett not quite sure how venting into the underfloor foundation would cause damp as there is airflow throughout from airbricks on both sides of the house, was more concerned about a draught into living area causing cold spots. Thanks for the advice on this.

 

[JohnD]

It is OK to make a vent into a disused chimney into the underfloor void, but the airflow will be up the flue, from the void, and out of the opening you will need at the top. It will not flow downwards into the void.

When doing that, you can shovel out the rubble, below the old hearthstone, in the base of the chimney, which is a reservoir for ground damp and carries it into the chimney.

 

[mrrusty]

Just be aware that any vented air moving up the chimney will do one of two things. If the top of the chimney stack in the loft or the exposed bit is at a temperature lower than the dew point of the air going in the bottom, you will get condensation on the inner surface which will probably get absorbed in to the brick/soot/salts

If the top of the chimney is warmer than the dew point of the room air going up the chimney, then that air will evaporate any liquid moisture on the surface of the flue.

Ideally use a hit and miss vent, and only open it on warm days, and not when the room air is nice and warm and humid and the top of the chimney is freezing!

There are lots of conventions on what is best to do with unused chimneys, but it does help to understand the physics of why ventilation can be both a help and a hinderance depending on the temperature differentials and the humidity of the air.