Q: I am planning to make a chimney top with aluminum covered by copper. Does there need to be any type of barrier between the two because of heat, ash, or wood smoke? What material do you recommend? – Pat
A: For the longest lasting chimney cap, the recommended materials are stainless steel or copper. Caps made with these usually come with a lifetime warranty. Galvanized steel and aluminum may rust out within a few years unless you are in an arid area such as the U.S. Southwest. So I’m not sure why you want to place a copper cap over aluminum.
Metal caps are naturally fireproof, so no heat barrier is needed.
Chimney caps are a good idea that perform a number a functions. Caps with screening keep squirrels, birds, and other critters out of the flue. The screening also keeps fireplace embers from flying out of the flue and potentially causing a fire. All caps keep out rain, snow, and other moisture that can promote deterioration of the flue, whether metal or traditional clay tile.
A full-size, multi-flue design that covers the entire chimney top can also help protect the concrete chimney crown from deterioration. Concrete caps tend to develop cracks over time allowing water to deteriorate the brick. Caps made from masonry mortar fail very quickly.
A properly designed cap, with a minimum 5-inch clearance from the top of flue, should not interfere with chimney draft. However, if the draft is inadequate this should be addressed before considering a chimney cap. There are many possible causes of poor draft including a blocked flue, wrong flue size (too small or too large), equipment problems, poor fireplace design, or a cold flue. Some caps are designed to increase the draft, but these require expert installation.
Other issues to keep in mind:
- Do not install caps on metal chimneys unless the cap is designed and approved for the specific system.
If you have a gas-fired water heater, boiler, or furnace, only use a cap that is UL-listed and code-approved.
- Inspect the cap periodically to make sure the cap has not shifted or the screening is not blocked, interfering with the draft.
- Use Tapcon screws or other masonry fasteners to secure the cap. Don’t rely only on an adhesive. This prevents the cap from shifting over time or blowing away in a strong wind.
- Consider hiring a chimney sweep or other chimney specialist to install the cap.
- Make sure you have working smoke and CO detectors on every floor.
— Steve Bliss, Editor, BuildingAdvisor.com
See Also Metal Flashing Materials