Can I vent my kitchen hood through the roof or a wall? Yes, you can vent a kitchen hood through either the roof or an exterior wall. This detailed kitchen exhaust venting guide will show you the best way to set up your system for good air flow. Proper venting pulls smoke and grease out of your kitchen. It keeps your home clean and safe.
Why Proper Kitchen Ventilation Matters
A kitchen hood does more than just light up your stove. It cleans the air you breathe. Cooking creates smoke, grease, steam, and odors. Without good venting, these things fill your kitchen. They stick to walls and cabinets. They can also lower your indoor air quality. This is why proper kitchen ventilation sizing is so important. You need a system strong enough for your cooking needs.
Deciphering Ducted vs. Ductless Range Hood Systems
The first big choice you face is between two main types of range hoods: ducted vs ductless range hood models.
Ducted Range Hoods: The Best Choice
A ducted hood, often called a vented hood, moves air outside your home. This is the gold standard for kitchen ventilation.
- Pros: It removes all heat, smoke, grease, and moisture completely. This gives you the cleanest air.
- Cons: It requires installing range hood ductwork. This can be harder and costlier, especially in older homes.
Ductless (Recirculating) Range Hoods
A ductless hood, or recirculating hood, cleans the air and puts it back into the kitchen.
- Pros: It is easy to install. It does not need outside venting. This is good for condos or apartments where outside venting is not allowed.
- Cons: It only filters grease and some odors using charcoal filters. It does not remove heat or moisture. Filters need regular cleaning or replacement.
For the best results, always choose a ducted system if you can.
Determining Your Range Hood CFM Requirements
CFM stands for cubic feet per minute. This number tells you how much air the hood moves. Knowing your range hood CFM requirements is step one in getting the right size hood.
Calculating Needed Airflow for Kitchen Ventilation
The general rule for airflow for kitchen ventilation depends on the size of your cooking surface.
- For electric cooktops, aim for at least 100 CFM per 10 inches of cooking surface width.
- For gas cooktops, aim for at least 100 CFM per 10,000 BTUs of burner output. If you have high-powered burners (over 15,000 BTUs), you need more power.
Table 1: Minimum CFM Guidelines
| Cooktop Type | Cooking Surface Width | Minimum CFM Needed |
|---|---|---|
| Electric | 30 inches | 300 CFM |
| Gas | 30 inches | 300 CFM |
| High-Power Gas (over 40K BTUs total) | 36 inches | 400-600+ CFM |
If you have a large kitchen or high-heat cooking, add 100-200 CFM more than the minimum. Always check your hood’s manual.
Selecting the Right Ductwork for Your System
The pipe, or duct, that carries the air out is very important. Using the wrong size or material hurts performance. This section covers ducting materials for range hoods.
Duct Size Matters Most
The diameter of the duct must match the exhaust port on your range hood. Do not reduce the duct size, even if you think it will be easier. Smaller ducts cause back pressure. This makes the fan work harder and moves less air.
- Most standard hoods use 6-inch or 8-inch round ducts.
- High-power hoods often require 8-inch or 10-inch ducts.
Always use the largest duct size the hood manufacturer suggests.
Choosing Ducting Materials
You have a few choices for what the duct is made of.
- Rigid Metal (Galvanized or Aluminum): This is the best choice. It is smooth inside. This means less resistance to airflow. It handles grease buildup better than flexible ducts.
- Semi-Rigid Aluminum: This is better than flexible foil but still has slight ridges that slow air down a bit. It works well for short runs.
- Flexible Duct (Foil or Vinyl): Avoid this if possible. Flexible ducts have deep ridges. These trap grease and slow down air movement greatly. If you must use flexible ducting (for a very short, straight run), make sure it is metal foil and pull it as tight as possible. Do not use vinyl ducting for kitchen exhausts.
Steps for Installing Range Hood Ductwork
Installing range hood ductwork involves running the pipe from the hood up to the exterior. Keep the path as short and straight as possible.
Minimizing Turns and Bends
Every bend in the duct system causes a loss of airflow. This is often called “static pressure loss.”
- A 90-degree elbow can reduce airflow efficiency by as much as 25%.
- Try to use no more than two 90-degree turns in the entire run.
- If you need a turn, use smooth, gradual bends instead of sharp elbows where you can.
Kitchen Exhaust Duct Routing Best Practices
The path the duct takes dictates its efficiency.
- Straightest Path: Choose the most direct route through the attic, wall cavity, or above the ceiling cabinets.
- Vertical vs. Horizontal: Running vertically (up through the roof) is often better than a long horizontal run inside an attic space.
- Insulation: If the duct runs through cold spaces (like an unheated attic or exterior wall cavity), you must insulate it. Uninsulated ducts cause condensation. This moisture can drip, spoil drywall, and promote mold growth inside the duct.
Connecting Range Hood to Exterior Vent
This is the final step where the duct meets the outside world.
- Roof Vent Cap: If venting through the roof, use a high-quality roof cap designed for range hoods. It must have a built-in damper flap.
- Wall Cap: If venting through a wall, use a flat wall cap with a damper. Make sure the cap sits flush or slightly proud of the exterior siding.
- Damper Function: The damper is a small metal flap that opens when the fan is on and closes tightly when the fan is off. This prevents cold air, rain, or pests from entering your house when the hood is not in use. Make sure the damper moves freely. Do not let the duct seal tightly against the damper, or it won’t open.
Special Considerations: Venting a Microwave Hood
Many people opt for a microwave with a built-in hood, also known as venting a microwave hood. These units often have lower CFM ratings than dedicated range hoods.
Checking the Microwave Hood’s Venting Type
When you buy a microwave hood, it often comes set up for ductless use. You must change the internal blower motor configuration to vent outside.
- Review the Manual: The manual will show you where the pre-drilled holes or removable plates are for converting to ducted.
- Duct Size: Microwave hoods usually use 3 1/4 x 10 inch rectangular ducting or a 6-inch round duct. Do not use the small 4-inch duct often found on older installations; it restricts air too much.
- External Vent Hood: Even when venting through the wall, you must use a proper exterior termination cap. Do not just poke a hole in the wall. The cap keeps weather out.
If your microwave hood has a low CFM rating (under 200 CFM), it might not handle heavy cooking well, even if vented properly.
Fire Safety and Grease Management
Grease buildup in the ductwork is a major fire hazard. Proper venting minimizes this risk.
Regular Cleaning is Crucial
For maximum safety and performance, clean your ductwork periodically, especially if you do a lot of frying.
- Hood Filters: Clean the metal grease filters monthly (or as needed).
- Blower Wheel: If you can access the blower fan, clean the squirrel cage or fan blades. Grease buildup here throws the fan off balance and reduces power.
- Duct Cleaning: For very heavy cooks, consider professional duct cleaning every few years. This is more common in commercial kitchens but wise for residential kitchens with powerful gas ranges.
Fire Codes and Clearances
Check local building codes. Some codes require specific fire-rated materials or clearances around the duct if it passes near combustible materials like wood framing in the attic. Always seal joints securely with metal foil tape (not standard cloth duct tape) to prevent grease leaks inside the wall.
Addressing Common Venting Challenges
Sometimes the perfect straight run isn’t possible. Here is how to manage tricky situations when installing range hood ductwork.
Dealing with Long Duct Runs
If your distance to the exterior is over 25 feet, you lose significant CFM.
- Rule of Thumb: For every 90-degree elbow, subtract 5 feet from the maximum allowed run length.
- Solution: If the run is very long, you must upgrade your hood to one with a much higher CFM rating. A 300 CFM hood venting through 30 feet of duct and two elbows might only move 150 CFM effectively. A 600 CFM hood might still deliver adequate airflow in that same run.
Preventing Air Pressure Problems (Make-Up Air)
Powerful exhaust systems (usually those over 400 CFM) can pull too much air out of your home too fast. This creates negative pressure.
Negative pressure can cause issues:
- Backdrafting of fuel-burning appliances (like water heaters or furnaces). This pulls dangerous carbon monoxide back into the house.
- Doors sticking or windows difficult to open.
If your hood exceeds 400 CFM (check your local code, some jurisdictions set the limit lower), you likely need a Make-Up Air (MUA) system. An MUA system brings fresh outside air into the home to replace the air the hood exhausts. This keeps your home pressure balanced.
Summary Checklist for Perfect Venting
Use this checklist to confirm you have followed all the best practices for your system.
- CFM Selection: Is the CFM adequate for my cooktop type and BTU rating?
- Duct Sizing: Does the duct diameter match the hood exhaust port exactly (e.g., 8-inch hood needs 8-inch duct)?
- Material Choice: Is rigid metal ducting used for the entire run?
- Run Length: Is the duct run as short and straight as possible?
- Bends: Are there only a few 90-degree bends? If so, are they smooth sweeps?
- Exterior Cap: Is a high-quality cap with a working damper installed at the exit point?
- Sealing: Are all seams sealed with metal foil tape, not cloth tape?
- Microwave Hood Check: If using a microwave hood, was it properly converted from ductless to ducted mode?
Frequently Asked Questions (FAQ)
Q: What is the minimum duct size I can use for any kitchen hood?
A: Generally, the minimum recommended duct size for residential range hoods is 6 inches round. Some very low-CFM recirculating models might specify 4 inches, but this severely limits performance if you try to vent it outside. Always match the duct size to the hood outlet.
Q: Can I vent my kitchen hood into the attic space instead of outside?
A: No, you should never vent a kitchen hood into the attic. The moist, hot, greasy air will condense on the attic structure, leading to moisture damage, mold growth, and potential rotting of the wood framing. Always vent directly to the exterior wall or roof.
Q: Do I need to use the same size duct for my gas range as my electric range?
A: Not necessarily based on the fuel type alone, but based on the heat and smoke output. Gas ranges often produce more contaminants, so they usually require higher CFM and thus potentially larger ducts (like 8 inches instead of 6 inches) compared to a small electric cooktop. Always base the duct size on the required CFM rating for the hood you select.
Q: What happens if I use a duct that is too long?
A: A duct that is too long causes high static pressure. This friction slows the air down. Your fan might be rated for 400 CFM, but if the duct run is excessive and has many bends, the actual airflow reaching the outside might only be 150 CFM. This leaves smoke and grease trapped near your cooking surface.
Q: How do I seal the ductwork joints?
A: Use aluminum foil tape (HVAC tape). Do not use standard grey cloth duct tape, as the adhesive breaks down quickly from heat and grease exposure. Apply the foil tape securely around every joint, overlap the seams, and make sure the tape covers the entire connection point to prevent grease leaks.