Foam insulation best bet to seal unused chimney

Hey there, time traveller!
This article was published 24/06/2023 (817 days ago), so information in it may no longer be current.

Question: What’s the best type of insulation for stuffing a chimney pipe that is being decommissioned? I’ll put a foam cap on the top and bottom, too, but just wondering what I should use to stuff in the pipe between top and bottom?

Thanks, W. Dyck.

Answer: Sealing the top and bottom of your redundant chimney vent pipe with foam insulation will be the right choice, but may not require additional insulation. If the rest of the pipe is not damaged, there may be no additional insulation required, as long as the sealing job is done thoroughly.

There are two main reasons to seal an unused older furnace vent pipe, whether it is installed inside an older masonry chimney, or not. The top of the vent, which should protrude slightly above any surrounding chimney or chase above the roof, should be well sealed to protect it from natural elements. The first of these will be blown rain and snow that could enter the top of the pipe, even with a rain cap intact. Any precipitation that enters could run down the inside of the vent and leak out the bottom into the basement or fireplace. The second reason for closing the top is to keep out unwanted pests. Wasps, bees, hornets, squirrels, or birds may enter the unused opening and build a nest. The stinging insects and birds may find the location ideal for a summer location, partially sheltered from the elements by the rain cap. Squirrels may seek a warmer, dryer nesting area for use in the colder months.

While precipitation and critters may be worthwhile items to prevent entering the unused flue, a potentially more problematic factor should also be kept out. Air infiltration of the older flue pipe in the colder months can lead to a problem that will only be noticed when the weather warms. Cold air leaking into the large, metal pipe, which may extend all the way through the heated home into the basement, can cause a serious problem. When that cold air hits any warm, heated indoor air inside the flue, condensation will occur. In moderate weather, it may be insignificant. But, in cold winter weather, the condensation will freeze and cause a buildup of frost and ice inside the open pipe. This may form only near the top, or inside the entire unit within the attic and exposed to the outside temperatures. When there is a sudden winter warm spell, or during the spring thaw, the frozen condensation will melt and run down the pipe. Water will surely pour out the bottom of the vent, but may also leak out any small holes or seams higher up, if the pipe is corroded. That could cause serious damage to walls or ceilings inside your home.

The solution to this potential moisture issue is to stop both cold air from the outside and warm interior air from entering the unused flue. This can be accomplished by properly air sealing and capping both the bottom and top of the metal pipe. In theory, and in most practical cases, if you air seal both the top and bottom of the vent pipe, there will be no way for air to infiltrate the inside area. Warm air from the home normally enters from the bottom and rises due to normal convective forces. Cold air from the exterior environment, which is much denser, will drop down and enter from the top opening. When these two air masses contact each other inside the flue, condensation is almost a certainty, given the right weather conditions.

Unless the inside of the metal flue pipe is damaged, typically from flue gasses and moisture over its lifetime, it should not need to be completely filled. The extent of any damage may be visible from the bottom or top with a strong flashlight and mirror, or with an inspection camera. You should ensure you inspect the pipe, at least from the basement cleanout or opening, to make sure it is not rusted through. If it is, then the remediation may require removal and discarding, or other more thorough repairs.

To seal the top and bottom you will first require the properly sized metal caps. These are inexpensive and should be available at any home centre or heating supply store. Measuring the diameter of the pipe, normally to the nearest inch increment, will tell you which size of cap to purchase. To properly seal the flue before installing the cap, there may be two acceptable methods. The first will require cutting a few pieces of extruded polystyrene insulation to the exact inside diameter of the flue pipe. These should be tight-fitted inside and sealed around the edges with Styrofoam adhesive or caulking. Once in place, the metal cap can be installed and secured with metal screws or duct tape. While this method should provide a good, waterproof air seal, it may not be nearly as easy as the following air sealing technique.

The other alternative method is to push a small piece of insulation, polystyrene or batts, part way into each open end of the old metal pipe. Once securely in place, blow in a can of expanding spray foam against the insulation plug and let cure. Use additional spray foam, as needed, to fill the rest of the pipe, leaving some room for it to expand. Any excess could be trimmed with a utility knife after curing, followed by the cap. Or, the cap can be screwed in place before the foam is fully expanded, making sure to leave enough room for it to fill the void without putting excess pressure on the cap.

Completely filling your old, unused metal flue pipe with insulation will be unnecessary as long as it is in reasonable condition and you properly air seal both ends with foam insulation to prevent air leaking inside.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)(cahpi.ca). Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at trainedeye.ca.

trainedeye@iname.com