So I'm trying to finish my basement. I have a conundrum as to how to build a wall under a soffit. Here's my situation - I have an HVAC duct that is running really close to the wall - like maybe 1.5" or less. I need to build a soffit around the duct AND build a wall under that soffit. Is it possible to attach a wall to the soffit? I was thinking that I could build a U, with a PT 2x4 on the back side (if it fits) and attach it to the joist at the top and to the wall at the bottom. I could then use regular 2x4 to build the rest of the U and attach the wall to the soffit. That doesn't seem like it'd be super stable How would I insulate behind the duct given that I might have less than 1.5" to work with?

Update: I measured on the open end and it looks like I have about 1", maybe 1 1/4" max, between the foundation wall and the duct.

Update 2: Not sure what pictures you wanted added, so here's 2. The first is the end of the duct line, the second is looking back at the duct running all the way back to the unit.

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    Ae you planning on adding more insulation, or reinsulating the wall? Consider using these guidelines if so. It will help with the answer. Also I will need another picture of the other end of the duct where it goes to a corner or ties into the HVAC unit. Depending on your insulation options and how easy it is to move, which looks like it is REALLY easy so far... will help tailor an answer – Jack Jan 6 '16 at 5:48
  • I forgot to add the link for the guidelines, got it from another post, handy to keep. buildingscience.com/documents/information-sheets/… – Jack Jan 6 '16 at 5:58
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    I didn't see a mention of the gas line or wire next to it. Definitely have the gas line moved back so it's touching the duct. Otherwise you're going to waste a lot of space going around it. Also, do the world a favor and take that contractor out back behind the woodshed, please... :) – pbarranis Jan 6 '16 at 14:10
  • I am trying to do the minimal amount of modifications possible. According to the inspector "you are not required to do anything else insulation wise to finish your basement". So I really don't want to incur the cost, time to install new insulation, and honestly, it's below grade and the temperature seems fine as it is right now. – 2 Left Thumbs Jan 7 '16 at 1:27
  • What did you end up doing? I’m in the exact same situation, and am thinking of affixing a furring strip to the top of the insulation, just under the duct, then securing the top plate of my frame (steel studs) to this furring strip with tapcons that go through the insulation and about 1.5” into the concrete wall. – Charlie Schliesser Mar 12 at 0:55

Possible options I see:

1 - Take the duct down temporarily or as needed.

2 - Build a flat wall in front of the duct, top plate to the left of the gas line & that wire, to then do closets under the duct.

3 - Do an open four or square four (not this pointy 4) without the top right back leg & of course a double long bottom leg.

I favor #3 & it should be plenty stable enough, good place for the TV or a bar. Use Fir studs instead of the cheap pine studs, much stiffer. You can also double the top plate for the left turn & double the front for the up-turn around the duct. I see the far wall that will also help. And, just insulate the face of the duct behind your wall.

If building-down the ceiling then I'd do that first, so you know if you're coming down enough & if anything has to be moved, but it doesn't really matter if you can measure.

  • I had a bear of a time understanding Iggy's #3, but that pretty much sums up your options. I'd favor #2 and use the space for closets, built-in bookcases, and other useful storage. I did some built-in bookcases using just cheap Ikea Billy bookcases recently, and I'm thrilled with how upscale it looks yet how fast it was to do. But I totally agree that #3, done with 2-3 screws per joint (NOT nails), will absolutely be strong enough. – pbarranis Jan 6 '16 at 14:14
  • Ha-ha & sorry, yeah I was going to say zigzag around the duct, but I didn't want him thinking I meant go behind or above the duct. I like #2 the best as well, but I don't know if he needs storage or just wants as much floor space as possible. Closets are meant to conceal stuff. – Iggy Jan 6 '16 at 14:35
  • What do you mean by "double long" bottom leg? I had thought about doing #2, but it didn't really seem to solve the problem - what would I attach the drywall at the back of the closet to? I can't picture how #3 is stable. It seems like if you pushed on the wall it just flex as there's nothing behind it to keep it from moving. – 2 Left Thumbs Jan 7 '16 at 1:33
  • The "double long" was just to illustrate the height up to the duct. Option 3 is just putting a couple of kinks in a straight up wall. There's really no stability or stiffness difference from a straight up wall. Option 2's closets would have their own ceiling studs & back wall studs (an upside down L), bottom plate & top plate (if you desire). You can also do the 2x4's on-flat below the duct for a tiny bit more finished height. – Iggy Jan 7 '16 at 1:52
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    I still can't wrap my head around how #3 is stable. If you attached stud only at the top/bottom and pushed laterally, it would flex like crazy. The horizontal board is just acting as the pushrod between the two so it should flex quite a bit. With a standard wall, if you pushed on it, the top and bottom would be anchored, so instead of the board rotating like a fulcrum, you'd have to apply enough force to snap the board. So for option 2, it's essentially an upside down 4 / lower case h? – 2 Left Thumbs Jan 8 '16 at 2:49

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