Should I close off my fireplace damper in summer to keep Ottawa humidity and bugs out of the house?

Yes, you should close your fireplace damper during summer, but do it correctly — a closed damper is an important part of summer fireplace maintenance in Ottawa, not an optional convenience. Closing the damper prevents humid outdoor air from entering your home through the chimney, stops insects and birds from nesting in the flue, and prevents warm conditioned air from escaping up the chimney when your air conditioning is running.

Why This Matters in Ottawa's Climate

Ottawa's summer humidity can reach 70 to 80 percent, and that moisture-laden air flowing down through an open chimney into your living space adds real load to your air conditioning system and creates conditions for mold growth in damp basements and crawl spaces — especially problematic in older Ottawa homes with masonry foundations. The chimney acts as a giant ventilation shaft when the damper is open, creating stack effect that pulls humid air into your home and pulls cooled air out. Additionally, Ottawa's position near the Rideau Canal and Ottawa River means insects, particularly mosquitoes, wasps, and flying beetles, actively nest in chimneys during summer months. Birds — especially chimney swifts, which are protected under the Migratory Birds Convention Act — commonly nest in open flues from May through August. Once birds establish a nest, you cannot remove it until the nesting season ends, which can limit your fireplace use well into fall.

Closing the damper also prevents debris accumulation. Ottawa's high wind patterns during summer thunderstorms can drive leaves, twigs, and outdoor dust directly down into your firebox and onto your hearth, creating cleanup work and potentially introducing moisture and mold spores into the living space. An open damper on a hot summer day also allows warm, stale air to escape from your home while pulling in hot outdoor air — fighting your air conditioning system and wasting energy.

How to Close Your Damper Correctly

Most masonry fireplaces in older Ottawa homes have a simple damper operated by a handle or lever located inside the firebox at the base of the flue opening. To close it, simply push or pull the handle until you feel resistance and confirm that the damper plate has moved fully across the flue opening. You should be able to feel the seal with your hand — the damper should create an airtight closure. If the damper is stuck, difficult to move, or does not seal completely, this is a sign the damper mechanism needs professional inspection and possible repair.

Gas fireplaces with sealed combustion (direct-vent models that draw combustion air from outside) do not have operational dampers — the damper port is sealed, and there is nothing for you to close or open. This is one of the many advantages of direct-vent gas fireplaces in Ottawa's humid summers — there is no pathway for outside air, insects, or moisture to enter the home.

Wood stove chimneys cannot be damped from inside the stove the way a traditional fireplace damper works. Instead, if you have a wood stove, install a chimney cap with a damper mechanism or a spring-loaded top sealer that prevents downdrafts and insect entry while allowing any residual moisture to escape. These caps cost $200 to $600 and should be installed by a professional to ensure safe, weathertight fit.

Important Considerations

Before you close your damper for the season, have the fireplace and chimney inspected if you have not done so in the past year. A closed damper traps any moisture, creosote buildup, or debris that accumulated over the heating season inside the chimney. If there is significant moisture or organic matter inside, a sealed damper can actually accelerate mold growth or attract insects over time. A professional chimney sweep (WETT-certified recommended) will clean out debris and identify any moisture or ventilation issues before you seal the system for summer — this is a worthwhile $175 to $350 investment in spring.

One common mistake is closing the damper too early in spring or leaving it closed too late into fall. Ottawa's shoulder seasons (April, May, September, and October) are unpredictable — you may have warm days followed by cold nights where you want to use the fireplace. A good rule is to close the damper after the last time you expect to use the fireplace for the season and open it again about two weeks before you plan to light fires in fall. This allows any residual moisture in the chimney to dry out before you seal it and ensures proper ventilation when you resume use.

If you notice any difficulty operating the damper, rust or corrosion visible around the damper handle, or air leaking around the damper even when it is fully closed, have a professional chimney technician inspect and possibly repair or replace the damper. A properly functioning damper is essential for both summer comfort and winter safety.

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