Is there a right or wrong way to face the plywood when building a soffit for ductwork? I was following a tutorial from Family Handyman http://www.familyhandyman.com/basement/how-to-finish-a-basement-framing-and-insulating/view-all

which has the plywood facing on the "inside" side of the duct.

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Seems like almost all of the other tutorials I've seen, they have it on the outside, and they seem to use OSB instead. Is there a prefered side, and does it matter if using plywood vs OSB?

2 Answers 2


Putting the sheathing to the outside makes hitting backing easier when hanging drywall, etc. On the other hand, it may be easier to hang the verticals pre-assembled if the sheathing is to the inside, as you can then screw through the 2-by into the joists unobstructed. From a structural standpoint it does the job equally well inside or outside. Do whatever makes sense in your situation.

OSB is more dimensionally stable than plywood (so you're not fighting warp while you build), and it's usually quite a bit cheaper. For this purpose 7/16 or 1/2" is more than adequate as the drywall adds support as well.


I think you have misinterpreted the picture in your posting. The OSB is not really on the "inside" of the soffit. Instead that part of the soffit in the picture was constructed using some I beam structural members that are made with OSB material notched into upper and lower 2x4s (or possibly a 2x3s).

The fact that they show the flat 2x4 over the splice point of two of these structural members implies that upper and lower members are 2x4s with half inch OSB.

So as can be surmised, there is no real preferred side for the OSB/plywood. It can be on the outside, inside or in the "middle" as in the picture. Material choice is really one of preference with OSB being a good choice if cost is a factor. If you elect to match the picture and use the structural I beam you will normally get OSB material anyway.

  • I think you're referring to I joists (or TJI joists), and while it's a good guess I think you're mistaken for two reasons. First, the joists look to be about 12" high. That's uncommon, since most builders go to truss joists at that point. Second, the splice has a 2x4 block over it, which appears flush with the top and bottom chords of the "joist". That wouldn't be the case if it were an actual joist. Plus one for good info otherwise.
    – isherwood
    May 17, 2016 at 13:18
  • I looked more closely at the posted picture source and saw another picture that specifically shows building the side panel of the soffit out of 2x2s and plywood. So I was wrong and the 2x4 splicer just sits between the 2x2s.
    – Michael Karas
    May 18, 2016 at 0:26

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