What is the difference between a hat and a cap?
Masonry chimneys without mortar caps (that would be the mortar installed around the flues at the top of the chimney) and hats (installed on the top of the flues) can let lots of water into the flue and structure of the chimney. Over time, chimneys can be destroyed by this water infiltration. One of the most obvious signs of moisture infiltration into a chimney is efflorescence. Efflorescence is the whitish powder that forms on the face of the chimney or in the fire-box of the fireplace. As moisture in the brick evaporates from the surface of the brick it leaves salt deposits we call efflorescence.
Efflorescence itself doesn’t represent more than a cosmetic issue but it does tell us how much moisture is being introduced into the chimney. Right now I am only talking about “outside” sources of moisture. There can also be interior sources of moisture that create or add to the efflorescence. Inside sources are gas and oil heating appliances—-especially gas furnaces. These internal sources can actually add more water to a chimney’s structure than outside sources if the chimney isn’t properly lined for the combustion appliance.
The chimney pictured below has no mortar cap and one can easily imagine how much rain it could collect and channel into the chimney structure. Of course the open flues obviously collect a lot of water too.
The efflorescence on the chimney in this next picture almost looks like failed paint—especially on the left side. This chimney is near my house, so I get to watch it annually. The owners frequently clean off the efflorescence and sometimes it looks much worse than it does in this picture. While it would appear to have a mortar cap, there is obviously a great deal of moisture finding its way into the structure—-and of course there is no hat on the flue itself. So while a mortar cap is essential to protect the chimney, we must make sure that it is functional, with no cracks or ways for moisture to get through it. They often crack and have gaps around the flue.
If you don’t keep the moisture out you will sometimes—-under very severe conditions—-find efflorescence on the inside of the home as well—and not just in the fire-box where it is common.
The flue itself should have a proper hat and spark arrestor and vermin screen. These “hats” can keep a lot of water out of the chimney structure.
Sometimes homeowners get very creative about keeping moisture out of the chimney and flues. This next picture shows some of this ingenuity.
My favorite “hat” of all time has to be this functional hat on the chimney for an oil fired furnace.
Charles Buell, Seattle Home Inspector
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