Is my chimney vent causing my downdraft problems?

[newatthis]

I have posted before about my ongoing downdraft problem. The fireplace, and now stove (Oslo F500), just has a terrible cold draft that makes cold starts in the stove tricky (and smoky). We almost have to run the stove continuously to keep the cold air out. The chimney pipe has 1/4" insulation I just realized. Chimney pipe is at least 16' tall. Exterior chimney on a north wall in the shade.

The chimney guy is here now, and he is pointing out that the attic vent is just next to the chimney pipe, and maybe cold exhaust from the attic is just going directly down the chimney pipe. The intake for the attic is a gable vent.

He said that either covering the attic vent or setting up a metal divider in between the two holes might solve the downdraft problem. The photos will help clarify the problem. Attic vent is on the left, and stove pipe is on the right. What does Hearth.com think?

 

[stoveliker]

I think the attic is always warmer than the outside air in winter. Even for a vented attic.
So I don't think that's the cause.

A divider is good nonetheless to avoid smoke going into the attic vent instead.

And air sealing the boundary between living space and the attic will help a lot

 

[newatthis]

Stoveliker, can you clarify how to air seal the boundary between living space and attic?

 

[stoveliker]

I went in the attic, took all insulation out from between the joists, uses spray foam (cans from the box store) to seal all gaps between top plates of walls (don't forget outside walls) and use silicone caulk for any holes where wires go through top plates down to outlets etc. and around ceiling fixtures (silicone is not flammable).

Then I put the original insulation back (if good enough) and I added unfaced batts in top, 90 deg rotated. The latter step doesn't air seal but did increase my R value from the original R19 to now R57 (I added R38).

I did this before I started heating with wood. My heating bill was cut by 2/3.

See also here

Attic Air Sealing Project

www.energystar.gov

Air sealing helps because your home functions like a chimney too. Warm air rises. If it can escape up top, you create underpressure at lower floors.
Air then has to leak in to compensate (just like air has to enter your stove to replace what went up the flue).
You'll always have leaks at doors, bathroom fans etc. So the best first step is to stop the exhaust of air thru the top. Make sure your windows upstairs seal well too.