If you are experiencing cold drafts and heat loss with the flue capped, you'll need to check two things:
- Does the fireplace have a combustion air intake? This air intake is usually visible from the inside of the fire box and resembles a small sliding door. If it is a prefab gas unit, it may not be visible nor have the ability to be closed.
- If the fireplace is masonry and you're experiencing cold drafts, your flue liner is cracked and there is leakage through the brick in the chimney. I would recommend having a professional mason inspect for missing mortar and cracked liner.
Sealing off the fireplace
You can seal off an unused fireplace by installing wood or light-gauge metal framing, insulation and drywall over the entire wall or opening.
If you intend on using it in the future, install an insert constructed out of 2" rigid foam board glued to MDF.
- Seal around the insert using neoprene backer rod insulation.
- If it is a gas unit, make sure the gas has been turned off prior to sealing the opening.