A properly insulated chimney is a key component of an energy efficient building. Effective insulation also contributes to safe chimney operation. Unfortunately, chimney insulation has never been a top priority for builders, so you may need to install an insulated chimney liner into your flue to make your system safe and efficient.
Many houses and commercial buildings are designed with the chimney on an outside wall. Modern prefabricated fireplaces with metal flues are usually adequately insulated between the outside of the flue and the chimney chase. Older masonry chimneys, however, are not insulated. A bricks and mortar chimney with a clay tile flue liners offer very little thermal resistance, so the chimney on an outside wall sucks heat out of the building even if the flue damper is closed.
A chimney that rises inside the interior of a building is adequately insulated by being inside the structure, up to the point that it protrudes from the roof. This chimney arrangement has less negative impact on the building's energy efficiency than a chimney on an external wall, but it can cause problems with draft and creosote buildup. Having the top of the chimney exposed to cold air will make combustion products condense inside the flue near the top, leading to dangerous creosote buildup. A cold chimney top above a warm chimney bottom also leads to downdrafts, difficulty lighting fires, and smoke entry into the building.
If your chimney is not insulated, the easiest and most economical solution for you is to install a metal chimney liner inside your existing flue. You can wrap the liner with fireproof insulation before you insert it into the flue, or you can install the liner first and then pour insulating material into the space between the liner and the inner chimney walls. Be sure the liner you choose is big enough in cross sectional area for your fireplace and is made of the proper material for your application. Stainless steel should be used for wood, oil, or coal burning and high efficiency appliances, while aluminum may be adequate for low efficiency gas burning applications.
You should see immediate benefits from your newly insulated chimney liner. Fires will light easier and your wood will burn cleaner. The combustion products will stay hot all the way out of your house, rather than cooling as they rise and condensing to form creosote and soot inside your flue. Your room should feel warmer even when you are not burning a fire, especially if you have a top damper closed over your flue. Your annual chimney cleaning job will also be easier. You will not only have less creosote buildup, but a cylindrical flue is easier to clean than a square flue.
Retrofitting your chimney with an insulated chimney liner is a relative small investment that will pay for itself over time. You will use less fuel, have a more comfortable house, and will be at ease knowing your chimney system is safe and efficient.