Can You Run Ductwork Through Floor Joists? (Explained)

Ductwork is generally floor ducts that pass through the basement or crawlspace. The main purpose of ductwork is to provide airflow from the heating or cooling system throughout the house. This airflow system is so convenient that people use this in their living spaces.

So, you might be confused about whether you can run ductwork through floor joists or not. Well, you have to know details about the mechanism of ductwork.

Moreover, knowing the consequences of running ductwork through the floor joist is also important. Thus you can decide whether you will run ductwork through the floor joist or not.

Can you run ductwork through floor joists?

You can run ductwork through floor joists. Floor joists are the spans between floors. These spans are like frames under the floor. So, you have to run ductwork according to the frame size and the cavities should be obstacle-free. Moreover, you should use the approved duct materials.

Generally, when we use ductwork, we have to make ducts in the basement or under the floor. These ductworks require drilling and attaching duct materials. If you run ductwork in the straight floor or basement, it will not create any confusion.

But floor joists are horizontal structural members. These members span an open space, often between beams. Moreover, these joists transfer the load to the vertical structural members. So, floor joists have important roles.

Generally, floor joists are pretty common and part of the floor system. They also carry the weight of everything inside a room. But these frames are sequenced one beside another leaving a narrow space. So, you might be confused about the size of the ductwork.

You have to install the ductwork according to the joist size. Moreover, the ductwork should not face obstacles between two frames. Thus you can run ductwork through floor joists. The process is not that difficult if you do it properly.

Can I drill through and cut a hole in my floor joist?

Yes, you can drill through and cut a hole in your floor joist. Even you can drill or cut holes in anywhere of the floor joists. You can drill through ½ to 1-inch diameter in the floor joist. But you should ensure the design should not be damaged.

While drilling or cutting a hole through the floor joist, you have to notice the width of the joist. You can’t drill up to half-width of the joist.

Because it might make the frame less durable. Moreover, if the effect of the penetration can ruin the design of the floor joist, you can’t drill through it.

So, you can drill and cut holes through your floor joist. But you have to keep the hole’s depth limited.

How big of a hole can you cut in a floor joist?

How big of a hole you can cut in a floor joist depends on the joist’s depth. You can cut a hole that is no larger than one-third of the depth of the floor joist.

Generally, the holes should not be more than 2 inches in depth in a floor joist. This depth is pretty suitable for inserting cables and other facilities like ductwork.

If you cut a hole or drill more than one-third of the depth of the floor joist, it can weaken the basic structure of the floor joist. So, you can drill through the floor joist, but you should also maintain the size limit.

When can and can’t you run ductwork through floor joists?

Running ductwork through the floor joist is pretty common. But you can’t always run ductwork through floor joists. You must follow some rules and regulations for doing this.

Generally, you have to ensure whether you can or can’t run ductwork through the floor joist according to your floor and measurement.

You can run ductwork through the floor joist if the empty spaces between the frames have enough space. Because, if the beams are too narrow, you can’t run ductwork. Moreover, the empty spaces must be free of obstacles.

Moreover, galvanized steel, aluminum, fiberglass duct board, and flexible duct should be used as duct materials. If you don’t use these materials or approved materials for ductwork, the ductwork might fail later.

But you can’t run ductwork through floor joists if the beams are too narrow from each other. Moreover, if the beams have obstacles between each other, you can’t install ductwork in the floor joists.

Besides, if the duct materials are not approved, the ductwork will not be durable. If the depth of the joist’s beam is too narrow, you can’t able to drill through it. So, installing ductwork will be difficult.

You also can’t run ductwork through the floor joist if there are electrical connections. Moreover, where water pipes are present in the floor joist, there will not be enough room for installing ductwork.

So, while running ductwork through the floor joist, you must ensure the above issues.

Rules to Follow When Running Ductwork through Joists:

If you want to run ductwork through your floor joist, you have to follow some rules. Let’s see some rules that you must have to follow.

Measure the space:

You must measure the space between floor joists. Because you have to know whether there is enough room for installing ductwork or not.

You can follow ACCA Manual D (ACCA 2009) for measuring the duct size. Moreover, you have to ensure there is no obstacle between beams.

Seal the ducts with mastic:

You have to seal all supply and return-duct connections with mastic or approved tape. When you will seal the ducts, they will be able to prevent loss of conditioned air. Moreover, they will also prevent thermal loss or gain from the ambient air.

Use appropriate duct materials:

You can’t use random materials to run ductwork through floor joists. You have to use approved duct materials. For example, you can use galvanized steel, aluminum, fiberglass duct board, flexible duct, etc.

Moreover, you have to choose materials that meet local code smoke and flame-spread criteria. Otherwise, the material might not be safe to use on floor joists. Besides, other materials might not be durable.

Test air-tightness:

You have to test airtightness with a duct-blaster test before installing the drywall. If you don’t test this, air and thermal loss might not be identified. This way, energy will be lost. So, you should test the airtightness of the ducts.

Use supports to the ducts:

You can apply supports to the ducts. You can use saddles or straps to support at regular intervals. If you don’t use supports, ducts can develop sags or kinks. Sags and kinks will reduce airflow. So, supports can make the ductwork perfect.

How to run ductwork through floor joists?

Running ductwork through floor joists requires some processes. You have to know them for installing the ductwork properly. So, let’s see how you can run ductwork through floor joists.

Make the measurements:

At first, you have to measure the room between the beans of floor joists. According to the size, start installing ductwork using sheet metal ductwork.

Then you have to secure the ductwork with sheet metal screws. Apply duct tape around the joint to make a tight seal.

Anchor the ductwork:

Use ductwork between floor joists wherever possible. Anchor the ductwork with galvanized hanging straps. Keep 4 feet intervals and attach the galvanized hanging straps into the joist on both sides.  

Apply ductwork:

Apply ductwork that branches off and approaches each vent. Then you have to register in the house.

You can apply sheet metal duct elbows that will work around bends and tin snips. Then apply a sheet metal ductwork collar that will attach the ductwork with the vents. You have to bend the metal tabs down to join it with the vent.

Install ductwork:

Then you can install the return ductwork. Do the same for outward.

Apply duct tape:

Finally, apply duct tape to prevent any leakage. Test the ductwork using a thermostat all the way up.

Can you run duct through floor trusses?

Yes, you can run duct through floor trusses. Generally, floor trusses are frames that withstand the load of the floor. Even these trusses can carry the weight of walls, furniture, appliances, and people. These trusses are like floor joists.

You can run duct through floor trusses if there is enough room. Because ductwork needs a minimum space to run through. If the trusses are too narrow from each other, you can’t run duct through it. Otherwise, you can install ductwork through floor trusses.

Final Thoughts

Ductwork is generally installed in the basement. So, if the basement has floor joists, you can still run ductwork through it. But you must ensure that the floor joists have enough room for installing ducts. Moreover, you should use approved materials and follow the rules of installing ducts.