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Cold air entering through a kitchen exhaust fan usually means the hood’s backdraft damper, the exterior wall or roof cap, or a duct connection is not closing or sealing correctly. The right fix is to identify where the air is moving, then repair that component without blocking the exhaust path. First establish whether the hood is ducted outdoors or merely recirculates air behind a decorative chimney.
A ducted hood should have a damper at the hood-to-duct connection and another at the exterior termination. They are designed to open when the fan runs, so they are not perfectly airtight, but they should greatly limit drafts when the fan is off. As Brian Wellnitz of Broan explained to This Old House, both flaps should keep cold (and hot) outdoor air from traveling through the hood.
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Before repairing anything, confirm the hood configuration. A recirculating hood sends filtered air back into the kitchen; its decorative chimney can look like an outdoor-vented system even when no duct reaches a wall or roof. A ducted hood has a physical exhaust route and an exterior termination cap.
- Find the hood’s model number and read the installation manual. The manual should identify whether the unit is configured for ducted or recirculating operation.
- Look for an exterior wall cap or roof cap that corresponds to the kitchen location. Only inspect a roof termination if you can do so without climbing onto an unsafe or icy roof.
- Trace the duct, where accessible, from the cabinet or chimney to the building-envelope penetration. If there is no outdoor termination, do not assume a hidden damper will solve the problem; the system may simply be recirculating.
If the hood is recirculating, cold air coming from the hood opening is not normally an outdoor duct draft. Investigate room pressure, gaps around cabinets and the appliance, or another nearby opening instead of adding a backdraft damper to a system that has no outdoor duct.
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Why does cold air come through when the fan is off?
Most winter drafts have one of four causes: a damper that is missing or stuck open, an exterior cap whose flap is damaged or obstructed, leakage at duct joints, or leakage where the duct passes through the wall or roof. These are different defects and require different repairs.
The This Old House explanation identifies two metal dampers on a properly installed range hood: one at the hood connection and one at the wall or roof cap. The flaps cannot make an airtight seal because the fan must open them, but they should close when the hood is idle. A flap bent by impact, held open by grease or debris, installed backward, or blocked by a fastener can admit outdoor air.
Air that enters around a seam is a separate problem. The Building America Solution Center’s kitchen-exhaust guide distinguishes duct leakage from airflow through an open damper and recommends sealing both duct joints and the building-envelope penetration.
How do you find the source of the draft?
Use this sequence to narrow the defect before buying a replacement part. Turn the fan off for the initial inspection, and disconnect electrical power before opening any housing or working near wiring. Stop if access requires an unsafe roof climb or if you find damaged electrical components.
- Read the model-specific instructions. Confirm the intended duct size, damper locations, and whether the manufacturer calls for a cold-weather accessory. A BSH hood manual, for example, specifies metal ductwork, a minimum 6-inch diameter for that model, as few elbows as practical, an additional backdraft damper, and a thermal break in cold-weather installations. Those requirements apply to that hood, not every product.
- Inspect the exterior termination from a safe location. With the fan off, look for a flap that is visibly open, bent, blocked by lint or nesting material, or unable to move. Some wall caps use a spring to hold the damper shut. If the cap does not close, a compatible replacement cap may be required.
- Check the hood-side damper when the manual permits access. Look for free blade movement and a complete closing position. A duct screw driven into the damper path can stop the blades; grease and debris can have the same effect. Cycle the fan only after reassembling guards and keeping hands clear, and verify that the damper opens while the fan runs.
- Inspect accessible duct joints. Look for gaps, disconnected sections, or tape that has peeled away. Leakage can occur even when both dampers close normally.
- Examine the wall or roof penetration. A visible gap between the duct sleeve and framing or sheathing allows cold air to bypass the damper. The penetration needs a durable air seal as well as weather protection outside.
- Note when the symptom occurs. A draft with the fan off points toward damper or envelope leakage. If the draft, door movement, flame disturbance, or combustion odors become worse when the exhaust fan runs, treat that as a pressure and combustion-safety warning and arrange a qualified assessment.
What should you repair when the exterior cap will not close?
Replace or repair the termination only after confirming that the cap is the source of the leak. Choose a wall or roof cap that matches the existing duct diameter, shape, weather exposure, and mounting arrangement. A new cap cannot correct a hood damper that remains open or a duct joint that leaks inside the building.
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Remove obstructions without bending the flap, and confirm that the damper swings freely and closes under its own spring or gravity action. Keep the outlet clear of screens or fasteners that restrict airflow unless the hood manufacturer specifically requires them. Seal the cap-to-wall or cap-to-roof connection against rain and air leakage, following the cap and siding or roofing instructions.
If the cap is inaccessible or the duct route is hidden, a ventilation or HVAC contractor can identify the termination and replace it without damaging the building envelope.
When is an in-line backdraft damper appropriate?
An in-line damper can help when cold air travels down an otherwise intact duct despite a functioning hood damper and exterior cap. It is not a universal cure: the damper must match the duct, fit the airflow direction, and be installed where its blades can close.
Trade-Wind’s 2024 installation instructions list 6-, 8-, and 10-inch models (DPR006, DPR008, and DPR010). Those dimensions describe those models only. The same instructions call for positioning the in-line damper just above the heated-space boundary, orienting it correctly, and keeping screws out of the blade path. An incorrectly oriented damper may stay open or impede exhaust.
Adding a damper also adds airflow resistance. Confirm the hood blower’s compatibility and the manufacturer’s permitted duct arrangement before installation. Do not insert a smaller damper to make an adapter fit; match the actual duct size and preserve the hood’s required airflow path.
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How do you seal leaking duct joints and the wall penetration?
Sealing is the remedy for air escaping around seams, not for a damper that fails to close. The PNNL retrofit guide recommends duct mastic or UL-181-rated tape for joints and connections. Clean dusty or greasy metal so the sealant can bond, then cover the full circumference of each accessible joint. Ordinary cloth “duct tape” is not the same specification.
At the building-envelope penetration, use an appropriate exterior-grade caulk, compatible foam, or a gasket to close the annular gap between the duct and surrounding construction. Protect the seal from weather and follow the penetration manufacturer’s fire, moisture, and clearance requirements.
Ducts routed through attics, crawlspaces, garages, or other unconditioned areas should be insulated in colder climates to reduce condensation risk, as PNNL advises. Insulation reduces heat loss and surface condensation; it does not replace a functioning backdraft damper.
Can you permanently block the exhaust opening?
No. A damper must close when the fan is off but open when the fan operates. Permanently stuffing the duct, taping a flap shut, or installing a solid obstruction can stop the draft while also preventing cooking moisture, smoke, and combustion by-products from exhausting. It can overload the blower, increase grease accumulation, and defeat the hood’s safety purpose.
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Use a properly sized, serviceable damper and verify its movement in both conditions: closed with the fan off and open while the fan runs.
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Does a powerful hood require makeup air?
Makeup air is a combustion-safety issue as well as a comfort issue. The PNNL guide’s summary of the 2018 International Residential Code says makeup air is required when a range hood can exhaust more than 400 CFM in a home with atmospherically vented combustion appliances. The guide also cautions that backdrafting can occur below 400 CFM, so 400 CFM is not a universal safe threshold.
Whether makeup air is required depends on the locally adopted code, the hood’s rated capacity and controls, and the type and location of fuel-burning appliances. A furnace, water heater, fireplace, or other atmospherically vented appliance can draw replacement air through unintended openings when the hood runs. If flames move, exhaust odors enter the room, a carbon-monoxide alarm activates, or the draft becomes markedly worse with the fan on, turn off the hood and obtain qualified HVAC or combustion-safety help. Do not infer safety from the fan’s nameplate rating alone.
How do duct size and layout affect the repair?
Match the duct diameter to the hood and damper rather than choosing a part by appearance. One BSH installation manual specifies a 6-inch minimum metal duct for its particular hood; another model may require a different size. The manual also recommends as few elbows as practical. Tom Silva, quoted by This Old House, summarizes the principle: “The shorter the run and the fewer elbows, the better.”
Do not reduce a larger hood outlet to a smaller duct simply to accommodate an available damper. Restrictions can increase noise and pressure, and they may prevent the damper from opening fully. Where the existing route is undersized, badly kinked, or filled with unnecessary bends, have the hood manufacturer or a qualified installer specify a compliant correction.
Which repair fits each finding?
| Finding | Likely next step | Limitation |
|---|---|---|
| Exterior flap is damaged or will not close | Repair or replace a compatible wall or roof termination cap. | It will not fix a leaking hood damper or duct joint. |
| Hood damper is stuck, damaged, or missing | Service or replace the hood damper using the model manual. | Access may require duct disassembly; the replacement must still open during operation. |
| Cold air travels through an intact duct | Consider a compatible in-line backdraft damper. | Confirm diameter, direction, location, and blower compatibility; incorrect orientation can keep it open. |
| Air leaks at seams or the penetration | Seal joints with mastic or UL-181 tape and seal the penetration with suitable caulk, foam, or a gasket. | Sealing does not repair a damper that fails to close. |
| Draft or combustion symptoms occur when the fan runs | Arrange qualified ventilation and combustion-safety assessment. | Makeup-air obligations vary by code, fan capacity, and appliance type. |
When should you call a professional?
DIY inspection is reasonable when all components are accessible from a safe location and the manufacturer permits it. Hire a qualified installer for hidden ductwork, roof access, electrical work, damper replacement that requires disassembly, major duct rerouting, or any makeup-air and combustion-safety question.
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Give the contractor the hood model, duct diameter and material, approximate route, exterior-cap location, and the conditions that trigger the draft. Ask for the defect to be identified before authorizing a replacement. A cap, in-line damper, and sealant solve different problems; replacing all three without locating the leak can add resistance and expense without stopping the draft.
What is the safest long-term solution?
The durable fix is a complete, compatible air path: a hood damper and exterior cap that close when idle, a duct sized and routed to the hood manual, sealed joints and envelope penetrations, and insulation where cold-climate guidance calls for it. Keep the exhaust opening unobstructed, and address makeup air whenever the home contains atmospherically vented combustion appliances or the fan creates pressure symptoms.
Frequently Asked Questions
Can a recirculating hood still have a cold draft?
A recirculating hood has no outdoor exhaust duct, so an exterior backdraft damper is not the answer. Check the installation configuration first, then investigate room-air leaks around the cabinet, appliance, or nearby building openings.
Will an extra damper make my hood quieter or stronger?
An added damper is intended to stop reverse airflow, not to increase blower capacity. Its blades and housing add airflow resistance, so the part must be approved for the hood and installed in the correct direction and location.
What diameter replacement damper should I buy?
Measure the actual duct and consult the hood manual before ordering. Trade-Wind lists 6-, 8-, and 10-inch models, but those sizes apply to its specific dampers; another hood may require a different diameter or shape.
Is 400 CFM always the point where makeup air is required?
No. The 400-CFM figure comes from the 2018 IRC provision summarized by PNNL for homes with atmospherically vented combustion appliances. Local code, appliance type, controls, and actual pressure behavior can require an assessment below that rating.
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