I’m remodeling a room on my first floor and will need to move a HVAC duct, make it run across 4 joists of the second floor. Room is gutted, and I can see second floor has a 3/4" OSB subfloor on dimensional 2x10 joists, 16" OC, 13’ in span:

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Ducting is 6" round so I cannot drill through the dimensional lumber I have in place without compromising it. But I could if I had i-joists (a 9-1/2" i-joist allows up to 7" roles per manufacturer). So I'm considering this:

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Assuming this is my only option, but being happy to be proven wrong, my problem is: a 2x10 is actually 9-1/4" but the i-joist is 9-1/2", so I see two options to make up for the 1/4":

  1. Align the top of the i-joist to the top of the current joists: that would bring my ceiling 1/4" lower only on those 4 joists so I'd need to shim every other joist to match. I'd also need to notch the beam and top plates so the i-joist can sit lower

  2. Align the bottom of the i-joist to the bottom of the current joist: same issue, and even greater pain because it would mean ripping the subfloor upstair, shim the top of the other joists and re-install it

So #1 seems easier since ceiling is already gutted but I'd need to bring the whole first floor ceiling down.

Yet another option is go with #2 but route a 1/4" rabbet on the underside of the subfloor for the top of i-joist to tuck into. The subfloor is 3/4" which would leave 1/2" and I was planning on adding a 1/2" underlayment for hardwood floors on top of it anyway, so doesn't seem to me there would be a problem:

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Looking for any ideas besides the ones mentioned here, or to be talked out of my hacky third option

Notes: Shaving 1/4 from i-joist flange is out of question, per manufacturer's manual. And I read i-joist manufactures make it a pain on purpose so builders are not mixing dimensional and engineered lumber due to differences on expansion rate, warping, etc. but that doesn't seem to be an issue on my case (I thought about sistering the current 2x10s with i-joist but after reading that I changed to replacing it altogether)

2 Answers 2


I think an I-joist is a good solution.

Your existing 2x10 floor joists have dried out over time and are stable enough to mix with I-joists.

However, I question the 7” allowable hole size. I’d double check with your I-joist manufacturer and follow their guidelines for size AND distance from bearing point.

There are many sizes and grades of I-joists, so be sure to coordinate all that with the manufacturer. I have no doubt the 9 1/2” I-joists will be strong enough for your 13’ span.

I’ll give you a third option: Option 3: Use the 9 1/2” I-joists at 12” on center. Align the lower chord with your existing 2x10’s (this allows you to use the existing bearing walls, beams, etc.) so no adjustment for replacing the ceiling. Then, use 1/2” plywood subfloor instead of replacing your 3/4” subfloor. (1/2” plywood on 12” centers is as “stiff” as 3/4” plywood on 16” on center.) You won’t notice the difference.

The underlayment can be reinstalled or the existing underlayment or it can be removed throughout the space and new installed for uniformity.

  • You mean rip off the limited subfloor section right above there the i-joists and replace only that part with 1/2” OSB? That’s a good idea and could be simpler than routing a rabbet. Just to correct myself, and make sure I got it right, maximum allowable hole seems to be 6-1/2”. From TJI literature: fp-supply.com/cmss_files/imagelibrary/I%20Joists/…
    – R. Liu
    Commented Mar 4, 2021 at 2:19
  • Yes, but I like 1/2” plywood instead of 1/2” OSB board. Remember, don’t align the joints in the new underlayment over the joints in the subfloor. Stagger both ways as much as possible.
    – Lee Sam
    Commented Mar 4, 2021 at 2:27

Split the duct into 4 3" pipes and put those through 4 3" holes in the existing joists.

  • Thanks, that crossed my mind by a) that would be hard to find 3” ducts and fittings to make it work though and b) airflow would likely be impacted. Another option in those lines would be to use joist/wall cavity without any splitting of duct, just a few holes on the joist and sheet metal with mastic
    – R. Liu
    Commented Mar 4, 2021 at 15:17

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