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I am doing some minor renovations to my basement with the drop ceiling and found that one of the geniuses that owned the house before me, used fibreglass ceiling tiles to seal off an old vent for a gas fireplace. Obviously this will not do. I'm thinking of building a section of 2x4's around the old exhaust and then filling it with rigid foam and spray foam. I can't take the old exhaust off the side of the house, since it is a condo and I doubt that I'm going to convince them to replace the siding where this exhaust vent is, not to mention it will probably cost me. Open to any suggestions. I would post a pic but the settings don't allow it, kinda odd.

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spray foam might sound appealing as a quick fix, but will be a pain in the a** if you ever want to remove it!!! – diyaddict Sep 22 '14 at 21:42
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If it's really not being used, I'd cover the vent itself in plastic from the inside wall (staple and caulk it to make it weather tight) or some other seal. Then you can just place a standard piece of fiberglass insulation with the vapor barrier (paper backing) facing inside the home. The expanding spray foam can make a big mess, and should someone want to use this vent in the future, it would be a serious pain to undo.

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good point. Thanks for the tips! – Aaron Apr 11 '11 at 14:41

Not sure exactly where this vent is, but here in the UK it's pretty much essential to include a vent whenever a fireplace is blocked off. Otherwise the air inside the chimney goes stale and damp takes effect. Generally the vent is to the room in which the fire used to be situated. Just be sure that there is ventilation to the chimney before (more thoroughly) sealing the vent.

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True enough. However this vent is no longer in use, the fireplace was moved a while ago to a different location in the room. Thanks for the response though. – Aaron Apr 11 '11 at 2:04

You can probably buy a metal cap the correct diameter for the pipe inlet, you can screw it on with sheet metal screws. I would stuff a few handfuls of fiberglass insulation in there first. This is a lot nicer if anyone ever wants to use it in the future, and still seals it up nicely.

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Spray foam might sound appealing as a quick fix, but will be a pain in the a** if you ever want to remove it!

Here is an alternative solution:

If you are convinced that building a small section around the old vent will work if you could make it waterproof, then try this instead:

  1. Similar to the way for building a fish tank, go out and get some thick plastic, ideally polycarbonate.
  2. Next, begin building the structure you were going to make out of 2x4, instead out of the plastic.
  3. Thirdly, glue the edges together using the proper glue, and then seal it making it waterproof using a liberal amount of waterproof silicone.
  4. Lastly, install around the old vent, and using a liberal amount of waterproof silicone make it watertight all the way around.

This should do the trick. Let me know if this works, or if there are other factors that I need to elaborate further on.

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