ASK THE INSPECTOR: Here’s a concrete solution to patching wall

Hey there, time traveller!
This article was published 18/09/2010 (5585 days ago), so information in it may no longer be current.

QUESTION: Since my home does not meet the clearances required by the insurance industry, I have disconnected my wood stove fireplace insert in my rec-room. I am planning to take down the remains of the chimney this year. Since the stove-to-chimney connection goes through the concrete basement wall, and is below ground level, I would like to know what material I can use to plug this hole securely so that no water leaks incur.

William J. Balacko

ANSWER: Many homeowners discover that older wood stoves installed in basements do not have the required clearances to be safely operated. While your insurance provider many demand you not use your unit or require its removal, it is the National Building Code and fire safety codes that determine the requirements for all solid-fuel burning appliances in homes.

Hopefully, this did not come as a major surprise after purchasing your home, which would likely have been identified by a competent home inspector or WETT-certified inspector.

There is also another major consideration for anyone planning to use a wood stove or fireplace in a basement. All wood-burning appliances require a large amount of air for combustion of the fuel. On the main floor of a large open home or cabin there may be sufficient combustion air to operate a wood stove without causing any issues. In basements, however, fresh air supplies may be limited and wood burning can often cause air to be drawn from the upper levels of the home. Dryers and naturally aspirated furnaces and water heaters can also vent a large amount of air from the basement when in operation.

In some cases, this large draw of air from the basement will cause a significant depressurization, which can lead to serious indoor air-quality issues. The largest potential problem in this regard is backdrafting of the wood stove or furnace, which allows products of combustion to spill into the living area. Dangerous levels of carbon monoxide and carbon dioxide, as well as other toxins, can build up in the home, with major health and safety consequences.

The solution to the negative-pressure problem is to install a fresh-air intake duct near the unit to equalize the air pressure and provide adequate combustion air. An even simpler solution is to stop using the wood stove, or to remove it completely. The only drawback is the problem you’ve identified — plugging the hole that the old chimney occupied in the building envelope.

Unfortunately, the amateur that installed your unit incorrectly also made a bad decision about where to punch a hole though the outside of the house for the chimney. This opening is much better located above grade, often between the floor joists of the main floor, which eliminates a couple of issues. The higher location prevents damage to the metal chimney from moisture and also makes it easier to seal around the opening to prevent heat loss and moisture intrusion. Your opening is through the foundation below grade, making sealing the area difficult.

In your situation, the only proper way to fill a hole that size in any concrete below grade is with more concrete. This can be accomplished by digging down around the exterior opening and removing any wall coverings or insulation from the inside of the foundation wall. Once the area is exposed, temporary plywood forms should be made to close the opening and hold the wet concrete in place until cured. Hopefully, the original installer made the opening right to the top of the foundation wall, as is normally the case, which will allow you to pour wet concrete into the form. The patching material can be mixed from scratch, or done more easily by purchasing a few bags of dry redi-mix concrete and adding water according to the instructions.

You also may want to consider drilling a couple of holes in the existing foundation for installation of short sections of reinforcing steel rods. This may only be necessary if you feel the original opening compromises the strength of the concrete walls, or of there are any significant cracks emanating from the hole.

Once the concrete patch has sufficiently set and the forms are removed, there is one final step to ensure a successful repair. Concrete is porous and must be damp-proofed or sealed at the exterior to prevent leakage. This can be done with a bitumen-based foundation coating or membrane. Your original foundation likely had a bitumen coating, which should look like black tar on the concrete in the excavated area. This inexpensive coating can be purchased in small quantities and trowelled over the patched area to damp-proof it. Alternatively, self-adhesive rubberized foundation membranes can also be found in small rolls and easily applied over the opening, sometimes after painting the area with a primer, depending on the manufacturer’s recommendation.

No matter which method you chose, proper installation of damp-proofing is as critical to leak prevention as a good concrete mix.

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

trainedeye@iname.com