Renovation & Design

Sealing unused chimney need not be a complicated task

Robert Linder / The Associated Press Capping or sealing an unused chimney will prevent air intrusion and stop pests from entering the home.

Question: I'm looking for advice on how to cap off an unused chimney. Last season I replaced my extremely old oil furnace with a high-efficiency propane furnace. The oil furnace used the chimney to exhaust its combustion gases. It is obviously not in use anymore, as my new high- efficiency furnace uses PVC piping for intake and exhaust through the side wall of my house. The chimney is brick with two clay tile liners running through it. The chimney runs up along the exterior wall. I have noticed some debris like leaves and bird feathers sitting in the clean-out in my basement. I have also had some condensation enter the basement by way of the clean-out. I have read lots of different articles on capping off an unused chimney, however, I have received some conflicting information. Are you able to provide me with some guidance here? Your professional advice would be greatly appreciated.

Regards, Bill Piasetzki

Answer: It is true that myself and others have recommended capping off an older, unused chimney to prevent moisture and air-leakage issues, but we have been vague about specific methods. I will try to highlight the key considerations when doing this task, whether by yourself or with professional help.

As with many home-related modifications, common sense should dictate the methods employed in sealing your redundant chimney. The first thing to ask is; "Why am I doing this?" Your inquiry has identified the two main reasons for attempting this task. You will be sealing your chimney to keep debris and pests out of the flue and also to prevent condensation, which may lead to spring leakage into your basement. Both of these objectives can be achieved with the same modifications, but will require more than one component.

The most difficult part of sealing your old chimney is at the top, due to obvious concerns with height and safety. Depending on the style of your home, and the height of the top of the chimney above your roof, special equipment may be needed for this task. If your house is a bungalow, you may be able to reach the top of the chimney quite easily by standing on the roof, or with a relatively short ladder. In that situation repairs can be done fairly quickly while supplies are temporarily stored on the roof adjacent to the chimney. A moderately skilled homeowner may be able to tackle the exterior repairs, as long as proper safety precautions are taken.

If your home is a two storey or higher, professional help likely will be required. Even a bungalow with a very steep roof and a chimney top well above the eave may require temporary installation of scaffolding to ensure a safe worksite. This can be costly to rent and may require more time to set up and break down than the repair itself. Many masonry contractors or chimney experts already have their own scaffolding and safety equipment to minimize these costs. It would definitely be a good idea to call for a couple of estimates, even if you were thinking of doing the exterior sealing yourself.

The key to capping the top of the chimney is to install a barrier that will be moisture and tamper resistant and will not be affected by environmental factors such as snow, wind and sunlight. Squirrels and raccoons have been known to invade poorly sealed chimneys. The cap must be durable enough to keep these nuisances out. For that reason, installing a galvanized or painted-steel chimney cap is the most common option.

You may be able to buy a pre-made cap at a heating supply retailer if you have a standard size clay liner that protrudes above the top of the chimney. If not, one can be fashioned from sheet metal using standard tools such as tin snips and pliers. This cap should be large enough to fit easily over the top of the clay flue, but not too large that significant gaps appear between the sides and the clay liner. Once cut and bent to fit, this cap may be secured with a combination of fasteners and sealant to prevent movement and leakage. Using removable fasteners -- such as special galvanized or stainless steel screws -- will ensure the cap can be removed in the future. That may be necessary if it becomes damaged or access to the inside of the chimney for future repairs is required.

Installing a metal cap on the top of the chimney will solve the pest, debris and rain concern, but some air leakage may still be possible due to the imperfect fit. At the bottom of the chimney, a tighter fitting flue cap may be installed if you have a standard round metal clean-out protruding from the chimney. These caps can be easily found and should be secured only with short sheet metal screws, to allow for future removal. If you have an unevenly shaped clean-out, or an older cast iron one embedded in the masonry, capping may require a little more ingenuity. Before this is done, however, you must address the air leakage and condensation issue.

Air leakage may occur from warm house air leaking into the bottom of the chimney, rising inside due to the stack effect. Preventing this will be of primary importance, but cold air intrusion from the top of the chimney must also be eliminated. Both of these should become a non-issue if you properly air seal the former chimney openings top and bottom. This can be easily accomplished with a combination of batts or rigid insulation stuffed tight inside these openings and then sealed with a few inches of blown-in polyurethane foam. The expanding foam found at home centres or hardware stores in small aerosol cans should be sufficient for this purpose. Repeated applications -- with time to cure in between -- may be required to get a thick enough layer to fill all gaps above and below the insulation plugs.

Sealing your older, unused chimney should not be a complicated task as long as attention is paid to prevention of air intrusion, as well as environmental and pest control. Sealing and insulating the clay flue at the top and bottom of the stack will ensure the job will yield these desired results.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors -- Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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