Caps, Dampers and Waterproofing in Eagle Mountain

What wind actually does to a chimney

People assume a downdraft means wind blowing down the flue, and sometimes it is. More often it is subtler than that. Wind moving across a roof separates over the ridge and creates a zone of positive pressure on the downwind side. If your termination sits in that zone, the pressure at the top of the flue is higher than the pressure in the room, and the chimney runs backwards. You get smoke rolling out of a wood fireplace, or a gas pilot that will not stay lit, and it happens only on windy days, which is why people describe it as intermittent and unpredictable rather than as a wind problem.

Two things fix it. The first is height, because getting the termination up out of the pressure zone solves the physics rather than fighting it. The second is the right cap. A cap with a generous overhang and open sides lets crosswind pull across the flue opening instead of into it, which actively helps draft. There are also dedicated wind-directional caps that rotate to keep their back to the wind, and on a badly exposed Eagle Mountain roof they are sometimes the honest answer.

Chase covers, which are not the same thing as caps

Most homes here have a framed chase, and the flat lid on top of it is the chase cover. It is doing a roof's job on a small scale, and a lot of builder covers were not built to do it well. Flat instead of sloped, so water stands. Galvanized instead of stainless, so it rusts. No hem at the edge, so runoff clings to the underside and tracks back into the framing. A stainless replacement, cross-broken to shed water and hemmed to drip clear, is the single highest-value thing most homeowners here can do to their chimney, and it costs less than most people expect.

Waterproofing, and when it is the wrong answer

Where there is real masonry we do apply a vapor-permeable water repellent, which lets the wall dry outward while stopping liquid water getting in. What we will not do is spray a sealer over a chimney with a cracked crown or failed flashing and call it fixed. Sealing the face of a wall that is taking water in at the top traps the water inside, and freeze-thaw then does more damage than it would have done otherwise. We find the entry point first. Sealing is the last step, not the first.

Animals

An uncapped flue is an attractive cavity in an area with a lot of open ground and not many nesting options. Birds get in and cannot get out, and a nest in a flue is a blockage, which is a carbon monoxide problem before it is a smell problem. A screened cap ends it permanently. If something is already in there we clear it first, then cap it, in that order.