The installation of a range hood has many parts to it. One of the most important parts is the ductwork that will be used, and with so many options, it is important to be aware of the “Do’s” and “Don’ts”.
This article will provide you with your best option for a range hood duct. Additionally, we’ll go in-depth into why certain ducts should be avoided and the special cases that exist that you may not know about yet.
Copper, galvanized steel, or stainless steel rigid metal ducts with smooth interiors can be used for range hood ductwork. Stainless steel is the best option. Flexible and semi-flexible ducting is not permitted. PVC can sometimes be used for down-draft range hood exhaust systems.
Use Rigid Metal Ducting for a Range Hood
As dictated by International Residential Code (IRC) Section M1503.4:
“Ducts serving domestic cooking exhaust equipment shall be constructed of galvanized steel, stainless steel or copper.”
Local codes may differ, so make sure you check those, even though they are based on the IRC.
A range hood duct must be made of one of the metals listed above. There are some exceptions, like PVC can be used in certain situations, but we will look at this in more detail in later sections.
Even with the exceptions, there is a common factor: these are all rigid ducting materials.
Another regulation that supports this is found in Section M1503.3, which talks about the materials requiring a smooth interior surface. Flexible ducting has a ridged interior surface, so it is not appropriate.
In stores or online, you can ask or search for “rigid metal ducting for a range hood”, and many suitable results will be offered. These materials can be purchased online or in person at various hardware stores.
Stainless steel rigid metal ducting is considered the best option for range hoods. This is because of its durability and ability to remain clean. Stainless steel also has a strong resistance to rust and can be easily maintained.
For steel rigid metal ducting, you can expect to pay between $4.06-$5.74 per linear foot.
Why Not Flexible Ducting?
Flexible ducts should not be used for venting a range hood for health and safety reasons.
While cooking, grease and grime can build up within the unit because of all the creases that are found on the interior surface of flexible ducting. This buildup can lead to fires because grease is flammable.
Flexible ducting is also much more difficult to clean. This in turn contributes to the accumulation of dirt and residues, which again increases the potential for fires, but it also acts as a breeding ground for bacteria and mold.
The ridges also slow down the rate of airflow, which means the efficiency of venting is impeded. This increases the likelihood of moisture, debris, grease, etc., condensing or sedimenting out of the air to accumulate in the ducts.
Additionally, flexible ducts tend to be made of plastics or aluminum. This means that the durability of the flexible ducts is fairly low, especially when compared to the durability of the acceptable materials listed by the IRC.
Not only will the inside of the flexible duct be a health and safety risk, but the outer portion of the duct will also suffer.
The creases and ridges here can also trap debris. Heavy or large amounts of accumulated dirt can compress the ducts, which would interfere with adequate venting.
Ultimately, flexible ducts do not meet the criteria set out by the IRC (normally made of aluminum) and do not have smooth interiors. So, besides not being as safe, they are simply not permitted.
What About Semi-Rigid Ducting?
Semi-rigid ducting is like a middle ground between rigid and flexible ducting.
Typically made of aluminum, these types of ducts can be formed into the desired shape and tend to hold said shape. These types of ducts can also be extended or compressed.
However, semi-rigid ducting poses similar problems to flexible ducting.
This ducting also has ridges and creases throughout (i.e., no smooth interior) that allow for greater accumulation of dirt and grease and less efficient exhausting.
Additionally, semi-rigid ducting is made of aluminum. Aluminum is not a material that the IRC has allowed for range hood ducts.
Semi-rigid ducting is commonly used for bathroom vents and general exhaust vents. Some people use them for their dryers, but this is also against the IRC regulations.
PVC Can Sometimes Be Used
As mentioned, PVC is the one potential exception to the IRC’s list of materials that a range hood duct must be made of, but only for down-draft exhaust systems, and only under certain conditions.
Section M1503.4 of the IRC clearly outlines that schedule 40 PVC can be used rather than copper, stainless steel, or galvanized steel when specific criteria are met.
For PVC to be used, five different regulations must be complied with. These five regulations that must be adhered to if PVC is to be used are as follows:
- The PVC must be installed beneath a slab of concrete on ground level.
- The trench dug for the PVC must be filled with gravel or sand.
- The duct cannot protrude more than 1″ (25 mm) above the concrete inside the house.
- The duct cannot protrude more than 1″ (25 mm) above the concrete outside of the house.
- The ducts must be solvent cemented.
I have tried to research why PVC can be used in these cases and I just cannot find a clear answer. Here are my best guesses:
- Certain metals, even if they are rigid, may not be able to handle the weight of a concrete slab.
- Some metals corrode in the presence of concrete.
- If a fire were to start in the ducts and spread, then it’s less of a risk underground than it is running through the house.
Should the Ducts be Insulated?
Range hood ducts only need to be insulated when going outside of the home because these ducts are exposed to extreme temperatures that exist outside of the insulated building envelope.
Insulation is not allowed when ducts go through floors and walls that are fire blocked. The reason is that the insulation can act as a fire conductor, negating the function of the fire blocking.
Although insulation is not always mandatory, it does offer benefits like minimizing energy loss, mold growth, dripping, and noise.
Insulation also helps protect against corrosion. When ducts are exposed to moisture (think steam from cooking), corrosion is more likely to happen. Insulation helps reduce the amount of condensation that accumulates in the ducts.
Additionally, the accumulation of condensation can lead to mold growth, so insulation is also helping to prevent mold.
Also, the reduction of noise, although it may seem insignificant, is very beneficial. I grew up in a house where every shift and shudder of the vents could be clearly heard and I had my fair share of sleepless nights because of the constant noise!
When deciding to install ducts for a range hood, insulation is an important factor to be aware of. While adding insulation to the duct is rarely required, it is highly recommended for the various benefits it offers.
Sources
https://airflowacademy.com/what-type-of-duct-is-best-for-venting-a-range-hood/