Caps, Dampers and Waterproofing for Goshen Chimneys

Wind is the design problem here

Two failures come out of that exposure. The first is downdraft: air moving fast across the top of a flue can push down it instead of pulling from it, and you get a room full of smoke in a fireplace that behaved perfectly last week. The fix is normally a cap with the right profile and, where the terrain funnels wind at the house, a wind-directional cap that uses the gust to help rather than fight the draft.

The second is caps that leave. A lightweight cap held on by friction and good intentions will eventually be found in the yard. We use stainless or copper, sized to the flue, mechanically fastened, and we would rather over-build it once than come back for it.

What the cap is also doing

• Keeping rain and snowmelt out of the liner, which is where flue tile cracking starts
• Keeping birds, squirrels and the occasional raccoon out of a warm vertical shaft
• Holding a spark arrestor screen over the flue, which matters in a dry summer surrounded by open ground and stubble
• Shedding blown grit and dust off the crown instead of letting it wash into the joints

The middle two of those are not abstractions in Goshen. Open farm ground carries a lot of starlings and swallows looking for a warm vertical shaft in spring, and by late summer the same fields are dry stubble lying downwind of the house, which is the stretch of the year when the spark screen stops being a formality and starts being the point.

Top-sealing dampers earn their keep on a heated house

The cast-iron throat damper at the bottom of an old fireplace never sealed well and after decades of heat it seals worse. It warps, it rusts, and the gap it leaves is an open hole from your living room to the sky. A top-sealing damper sits at the top of the flue with a silicone gasket and a cable down to the firebox, and it closes properly. On a house that is heating hard through a cold rural winter, that stops a genuine and continuous loss of warm air.

Waterproofing, and what it must not do

Sound brick benefits from a breathable masonry repellent that keeps liquid water out while letting trapped vapor escape outward. What it must never be is a glossy film-forming sealer. Those look like they are working and are quietly making things worse, because water that gets in behind them from the crown or a bad joint now cannot get back out, and the next freeze pops the face off the brick.