A chimney cap is a wise investment that can save you trouble and expense over the long haul. Here's a simple rundown of the advantages:
This is the most obvious reason to add a chimney cap. Birds can enjoy the warmth and protection of your chimney and decide to build their nests there. Small branches and leaves can get stuck in the crown of the chimney and block it as well. Even raccoons can decide to set up house in your uncapped chimney! The result, of course, is a lot of smoke billowing into your living room, a messy cleanup job both inside and out, and maybe even a couple of disturbed children when they figure out what happened to the poor birds.
This is the other obvious reason for a chimney cap. Rain falls from the sky and goes down your chimney. It can seep into the walls and ceilings, damaging them or setting up a breeding ground for mold and mildew. In colder regions, the water can seep into the chimney brick itself where it freezes. Since freezing water expands, the result can be cracked and "spalled" brick and flue tile. Moisture buildup is also one of the leading causes of chimney odor that can make your home unpleasant in warmer weather. With metal chimneys, the water can even get between the layers of metal, destroying the insulation and causing premature failure of the apparatus.
This benefit is less obvious. It may even seem that putting a chimney cap on will decrease the draft you get by obstructing your chimney's access to outside air. That's not the case. A properly sized and installed chimney cap will not do anything to hurt the air flow. And especially in windy areas, the cap can protect the chimney from disruptive gusts that can cause the draft to ebb and flow rather than continue at a steady rate.
Choose a chimney cap that doesn't have too small a screen or it can become clogged up with soot and creosote, adding to your problems rather than solving them. And make sure that your chimney cap is securely installed, especially in areas with high winds. You don't want your cap blowing off, endangering the neighbors and necessitating another trip to the hardware store.
If you have a metal chimney, it may even have come with a cap. If you didn't select this option, you can probably still order that cap now. This cap is the best option since it was designed with your specific model of chimney in mind (or the chimney was designed with the cap in mind, which amounts to the same thing).
If there isn't a manufacturer specified cap available, a generic retrofit will probably work fine. If you have draft problems, you can even get a draft increasing chimney cap that uses the power of the wind to create a stronger updraft. There are also electric draft increasing caps that can solve larger draft issues, allowing you to set the amount of draft with the touch of a dial.
Masonry chimney caps come in standard sizes of 8 inches or 12 inches on a side (or 8 x 12 for a rectangular opening). Stainless steel is the best material since it will be impervious to rust. Most models will come with screening to help keep the critters out. A cap with a damper mechanism can add to airtightness of your home when the fireplace isn't in use.
Many home improvement stores will carry a couple of types of chimney caps. Others will be available via catalog or can be ordered. These are not typically stocked in large quantity by the big retailers. However, local fireplace distributors will offer ways for you to get many differnt chimney cap styles. Here are a few places to look online for chimney caps.
Chimney Supply: Carries a large line of chimney caps along with other chimney accessories. Also view several articles on how to measure and install a chimney cap.
Chimney Caps: Maker of speciality design chimney caps that can help to prevent draft problems.
Chimney Cap Design: Online chinmey cap retailer offers a variety of metal and broze colored chimney caps. Also read the chimney cap sizing guide.