Range Hood Venting Mistakes That Cause Moisture Problems

A range hood is supposed to pull cooking heat, grease, and steam out of your kitchen and send it outside. When it's installed right, you barely think about it. When it's installed wrong — which is surprisingly common — it quietly leaves all of that moisture and grease in your home, where it turns into condensation, odours, and eventually mould. Here are the mistakes we see most, and what proper kitchen venting actually looks like.

The biggest one: it doesn't vent outside at all

Plenty of range hoods never send a thing outdoors. They're recirculating — the air is pulled through a filter and blown straight back into the kitchen. That filter catches some grease, but it does nothing for steam, humidity, or heat. All of it goes right back into the room.

Recirculating hoods get installed because they're cheap and easy — no duct to run. But if you cook regularly and your kitchen fogs the windows or feels muggy after dinner, a recirculating hood is a big part of why. A hood that's actually ducted to the outside removes the moisture and heat instead of stirring them around. For a lot of kitchens, that switch is the single best improvement you can make.

Mistake: terminating in the attic or soffit

When a hood is ducted, the next mistake is where the duct ends. We regularly find range-hood ducts that stop in the attic, or that dump into a soffit — which often just recirculates the air right back under the roof.

This is a genuine problem, not a technicality. You're pushing warm, greasy, moisture-heavy air into a cold attic. The moisture condenses on the framing and insulation; the grease coats everything and, over time, becomes a fire concern. Wet, greasy insulation stops insulating and starts growing mould. Kitchen exhaust has to leave the building envelope entirely — through an exterior wall or the roof — with a proper cap. Nothing short of that counts as venting.

Mistake: the wrong duct

Range hoods, especially powerful ones, need smooth, rigid metal duct, sized to the hood, on as short and straight a run as possible. What we often find instead:

Ribbed flexible duct, whose corrugations create huge resistance and trap grease.
Undersized duct — a big hood necked down to a small pipe, throttling it.
Long runs with multiple tight elbows, each one stealing airflow.

Any of these means the hood can't move the air it's rated for. It's running, it's making noise, but it isn't clearing the kitchen. Smooth rigid duct, correctly sized, run short and direct, is what lets a hood do its job.

Mistake: a bad exterior termination — or none

Where the duct exits the house matters. A proper exterior wall or roof cap does three things: keeps weather out, keeps pests and birds from nesting in the duct, and includes a backdraft damper that stops cold outdoor air from pouring back in when the hood is off. Skip the cap, use an unscreened opening, or install one with a stuck damper, and you trade a moisture problem for a draft, a pest, or a whistling wall. It's a small part that's easy to get wrong and easy to get right.

Mistake: ignoring makeup air

This one catches people with newer, tighter homes and big hoods. A high-CFM range hood can pull a serious volume of air out of the house. In a tightly sealed home, there's nowhere for replacement air to come from easily, so the hood creates negative pressure — which can backdraft other combustion appliances and actually chokes the hood's own performance.

The fix is makeup air: a provision to bring fresh air in to replace what the hood exhausts. Not every kitchen needs it, but powerful hoods in tight homes often do, and it's the kind of thing that gets skipped and then causes head-scratching problems later.

Why kitchen exhaust is its own animal

Kitchen venting isn't just a bigger bathroom fan. It handles grease, which coats duct interiors and demands smooth, cleanable runs and proper materials. It moves more air, so sizing and makeup air matter more. And it deals with real heat off the cooktop. That's why a range hood deserves its own properly designed exhaust path, not whatever pipe happened to be nearby.

What good kitchen venting looks like

Done right, it's straightforward:

A ducted hood, sized to your cooking, that actually vents outside.
Smooth rigid duct, correctly sized, on a short, direct run.
A proper exterior cap with a working backdraft damper.
Makeup air where the hood's power calls for it.

Get those four right and your kitchen clears fast, the windows stay dry, and the grease and moisture end up outside where they belong.

Where we fit in

Kitchen exhaust is air-side, sheet-metal work — exactly our trade. We duct range hoods to terminate outside the way they should, fabricate the transitions and runs to fit your kitchen, and handle the makeup-air side on powerful hoods. If your kitchen fogs up, smells linger, or you suspect your hood just recirculates, that's a straightforward fix that keeps grease and moisture out of your home for good.

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Frequently asked questions

Is a recirculating range hood good enough?

It's better than nothing for catching some grease, but a recirculating hood pushes the air — with its moisture and heat — right back into the kitchen. It doesn't remove steam or humidity from the home. If you cook regularly or fight kitchen moisture, a hood ducted to the outside is a real upgrade.

Why can't a range hood vent into the attic?

Because it dumps warm, greasy, moisture-laden air into a cold space. The moisture condenses on the attic framing and insulation, and the grease coats everything it touches — a combination that leads to staining, mould, and ruined insulation. Kitchen exhaust has to terminate outside the building envelope.

What size duct does a range hood need?

It depends on the hood's rated airflow, but powerful hoods need larger, smooth, rigid duct and a short, direct run to perform. Reducing the duct size, adding elbows, or using long runs of ribbed flex chokes the airflow so the hood can't actually clear the air, no matter what its rating says.

What is makeup air and do I need it?

Makeup air is fresh air brought in to replace what a powerful hood exhausts. High-CFM hoods can pull enough air out of a tight home to create negative pressure — which can backdraft other appliances and make the hood underperform. Larger hoods often need a dedicated makeup-air provision to work safely and properly.

Does a range hood have to vent outside?

For real ventilation, yes — a hood ducted to the outside is the only way to actually remove steam, grease, and heat from the kitchen. Recirculating hoods filter a little grease but push the moisture and heat straight back into the room. If your kitchen fogs up, or cooking smells linger, venting outside is the fix.

Why isn't my range hood clearing the steam and smoke?

Usually the ducting, not the hood. A run that's too small, too long, full of tight elbows, or made of ribbed flex chokes the airflow so the hood can't move the air it's rated for — and a recirculating hood won't clear steam at all. Smooth, correctly sized, short duct to the outside is what lets it actually pull.