Puzzling Draft at Mantle

[Jeff Spencer]

I ran into a strange situation I haven't seen before and would appreciate some educated speculation.

I ran into this fireplace this morning. The firebox and chimney (brick veneer) project off the rear wall of the house. The ghosting over the mantle got my attention. Looking closer, I could detect a heavy draft rising from between the mantle and the wall and the ghosted area was damp to the touch. In fact, you could hear the wind whistling through the gap. We had moderate wind today, but nothing that I'd expect to create the velocity I encountered.

No air was escaping from the firebox or between the firebox and front wall. Apparently the previous owners thought air was leaking through the damper, because it had been sealed with silicone caulk. No signs of voids in the chimney chase or at the chimney cap.

What is/are the possible cause(s)? Short of removing the mantle, I haven't a clue.

And yes, the mantle is too close to the firebox opening.

Jeff

[Bob Harper]

House severely depressurized. When the fireplace was sealed up, path of least resistance became through gaps in wall. The cold air infiltration will result in condensation of moisture on the wall there. Probably even wetter inside the wall.

If you were to open a basement window wide open, the cold air infiltration there should cease.

fix the source of negative pressure in the home then fix the air leaks at the fireplace.

Any more pics of the fireplace?
Nice touch with the IR! Great applicaton.

[Eric Barker]

I tend to wonder if the firebox is actually fully sealed. An area where I commonly find gaps is between the lintel and the damper throat. I also agree that there's significant depressurization in the home.

[Jeff Spencer]

Eric and Bob,

This house is a slab-on-grade in coastal southeast TX -- no basements (we call them interior swimming pools here).

HVAC was off, temps in mid-60s, wind about 5 mph with gusts maybe to 10 mph. Coastal location, hence omnipresent humidity.

I spent 15 minutes with 1/2 my body in the firebox and I could detect absolutely no drafts between brick veneer/lintel and the damper throat. I had a small toy flycerine smoke generator that I actually used for a change and within the firebox, the plume was absolutely undisturbed.

The top of the mantle is about 16" above the firebox, and the draft at the gap was essentially even across its width.

I referred to a fireplace specialist, but I've never seen this before and it bugs me that I can't even give a SWAG. My thoughts are that maybe sheetrock has been removed behind the mantle and there is communication with the chase or that the flue line has been breached near the top of the throat. Of course, since the damper was sealed shut I couldn't verify either guess.

The top of the mantle is about 16" above the firebox, and the draft at the gap was essentially even across its width.

Jeff

[Bill Hetner]

with out distructive investigation it is really hard to tell where the air and mositure is getting in. sounds like it could even be coming from the chimmy. leaking back in. the wind might be channeled thru the chimmy to the break where it comes in the house. seeing something like that I wonder about flashings and if there is a downdraft at the top part of the chimmy. My thoughts are see something like that is there a downdraft with your smoke maker if you were to put it inside the top of the chimmy as far as you can reach. if the smoke is going down maybe a cap to the chimmy is needed to seal it off there if the fireplace is no longer being used

[Mike Schulz]

Any pictures of the exterior in that area and top?

[Evan Wray]

Sounds like there are gaps at the exterior where the exterior finishes meet the brick chimney allowing wind and moisture to enter . The roof flashing could also be a concern. If the flu is large and there is no rain cap in a area where there is a lot of rain the smoke shelf would suck up alot of water but to leach into the drywall would mean the rough in is to close to the drywall.That would all be figured out when removing the rotten drywall.