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Question DetailsAsked on 6/25/2012

What type of paint should we use for the outside of the chimney? The house is heated by gas.

Should we use a regular outside paint or some type of sealant?

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2 Answers


Paintingmasonry can prohibit the necessary evaporation process for masonry andespecially the chimney above roof line. Freeze/ thaw cycle damage can result incasing fractures, spalling, separations between brick and mortar. Moisturepresent in masonry fissures can exert 20,000 psi. against and between what ithas contact with. This causes cracks in the casing and the masonry crown thatseals the top of the chimney casing to the flue.

Usually when chimneys are painted there ismoisture in the masonry at some level. Condensation in the spring and humidseason produces moisture inside the system on the liner walls daily, that isabsorbed and processed through the system. Moisture procuced from gascombustion, which you have, is produced at a rate of up to 1 oz. H2O vapor /1000 btu input / hour. So a 50k btu log set makes 50 oz. vapor per hour. Thatsa lot of moisture absorbed during the condensing period in the chimney in thewinter. If you do not have a cap that is significant exposure to the system asa standard size flue 13x 13 has 169 sq. in opening for rain to enter.
So if you must paint the chimney, firstseal thecrown w/ Saver Systems Crown Coat, install a cap, wash exterior, let dry asmuch as possible, with air if needed, consult with a paint manufacturer likesherwin williams about the latest technology in masonry CHIMNEY paint, you want"vapor permeable" and not a sealant if possible. Perhaps a thin whitewash.
Once painted and or sealed, if you must, then wewill need to monitor the interior for sweating, effloresence, and softening ofthe mortar during regular servicing.

Thanks for the question.
David Lamm
The Classic Chimney Sweep

Answered 8 years ago by David Lamm


Really late on this response, but in case you are planning ondoing it this year:

Since you said gas heated, I suspect when you said "Chimney" you were referring to a galvanized steel exhaust stack. If this is the case, then the exhaust portion is supposed to be below 150 F by design, so you can just wire brush off any rust, use rusty metal primer on any rusty areas, then after dry paint with house trim paint. I painted mine (and all roof hoods and vents) 28 years ago with Sears Weatherbeater trim paint and they have never needed repainting. I would stay away from very light colors, though, as any rust or weather staining or bird droppings will show up easily - I would stay with dark colors.

Answered 7 years ago by LCD

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