Q: We have two chimneys, both painted white. The one for the furnace is fine, but bricks keep popping out of the other chimney, which is for two fireplaces that we do not even use. The flues for these are closed tight. Last summer, I paid a large sum of money to have this chimney scraped, prepped with a special sealant to keep out moisture and then painted. But just a few weeks ago, I noticed the bricks are starting to crumble and fall out again. Why is this happening, and how can we stop it?

A: Most masonry problems start with moisture intrusion, and that is almost certainly the culprit in your case.

Moisture is somehow soaking into the bricks, probably because of a leak at the top of the chimney. There might also be leaks at the top of the chimney that you’re still using, but it probably has enough heat and air movement to allow the bricks to dry out without causing immediate damage. Any moisture that gets into the chimney you aren’t using, though, is likely to remain trapped in the bricks or mortar, which can create havoc, especially in cold weather.

Water expands about 9 percent as it freezes, so if a soaked chimney freezes, the expansion could create enough force to crack the bricks and eject the pieces. If freezing weather isn’t a factor, it’s possible that water pressure alone could do the damage. Heat from the sun on the chimney could cause water in the bricks and mortar to expand, for example, and that could cause some of the bricks to crumble. Why do certain bricks stay intact while others break? Perhaps it’s differences in manufacturing, the consistency of the clay in the bricks or the thickness of mortar in joints.

Whatever the explanation, the cure is to keep water from getting into the bricks. Because the exterior is waterproofed, the place that probably needs attention is the very top. All chimneys should have a cap that deflects rain and snow, but these caps can’t be watertight because there needs to be a way for hot air to escape. On an unused chimney, however, you can provide a better seal. If you don’t plan to use the fireplace chimney again, you can install tightfitting, solid caps over the flues. Or if you, or a later owner, might want to use the fireplaces at least occasionally, you can install top-sealing chimney dampers. This type of cap has a flap that you can pull open or closed via a cable that drops down the flue and links to a control handle in the firebox. One example is the Lyemance Top Sealing Chimney Damper, which costs about $150 to $240, depending on the flue size; one retailer is Chimney Direct (chimneydirect.com).

Making sure you have good flue caps is just the first step, though. Masons fill gaps between a flue and the surrounding chimney with mortar, creating a “crown” that not only keeps out pests but also sheds water. However, this mortar can crack over time, allowing water to get through. And even intact mortar soaks up moisture like a sponge. If the mortar is severely cracked, you will probably need help from a mason to repair it. But if there are only hairline cracks, or if you are dealing with porous mortar, there is a relatively easy solution – one you can tackle yourself if you are comfortable working on the roof and if the roof isn’t too steep or too high. Seal the mortar with elastomeric paint, similar to what you’d use to waterproof basement walls. This paint is thick enough to fill small cracks. But be sure to use paint fortified to withstand ultraviolet rays, as the top of a chimney usually gets plenty of sun. One suitable paint: ChimneyRx Brushable Crown Repair, $54.37 a gallon on Amazon.com.