Under our garage is a cement room (23' x 20', 8' high). We added it to the construction plans as a storage room, with the option of using part of it as living space. Everyone seems to think it would make an amazing home theater room... ^_^

Well, before we can even use it effectively for storage, I need to get more lights installed. Framing the room seems a good way to make the space more usable, and get a roof I can attach lights to. But this raises some unusual (for me) questions.

How do I frame the ceiling? Do I just follow the basic plan for framing corners so I can attach sheetrock, or are there other concerns?

We plan on putting a wall through the middle and dividing the room. So the final spaces will be approximately 19.5' x 11', with ceiling members across the 11' dimension. What dimension lumber do I need, for the ceiling? 2x4? 2x6?

There is a connection for the central air system in one wall: a "source" against the ceiling, and a "return" against the floor. I believe I can simply put a grate over the return once we finish the wall, but do I need a minimum depth to the ceiling to support running ducts across the room?

These are the only major concerns I have planning this, but if anyone sees anything likely to come up, feel free to warn me. ^_^

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    What is the existing ceiling of the space now? concrete? Is that the parking surface of the garage? Jun 16, 2011 at 23:38
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    On a side note: You'll want to check your local fire code, you may need to install an egress window if your converting the space to a living area.
    – Tester101
    Jun 17, 2011 at 3:19
  • You may also want to install drain tile and a sump pit, if the room is below grade (it would be a shame to lose all the fancy theater equipment to a flooded room). Talk to a local builder to determine if this is necessary in your area.
    – Tester101
    Jun 17, 2011 at 3:21
  • Before you frame your room get a theater expert involved. You will be suprised at all the details that make a decent theater room. 23'x20' is not too big for a theater room all by itself. Once you add a snack bar (or just bar) and a place for the equipment that room will seem small. It sound like you are thinking about using the space for something else. My advice is to build the theater room. I build one in my house and we love it! Jun 17, 2011 at 16:48
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    @ Testor101 That's starting to sound like the endless variations on how to calculate "square footage" when listing a home. ^_^
    – Scivitri
    Jun 17, 2011 at 18:51

3 Answers 3


If the ceiling is not structural and is only there to hold up the drywall, you can get away with using 2x4s. 2x4s at 16" on center should easily span 11', while supporting only drywall.

Basically your going to support the ceiling rafters with the walls, so you end up with a structure like this crude MS Paint image.

Crude MS Paint framing image

The basic idea is to build a room inside your room.

If you are planning on using recessed lighting or HVAC ducting in the space, you may want to go with 2x6s for the joists (to allow enough room to install everything).

If the ceiling joists are going to be load bearing (especially if they are supporting the parking surface of your garage), you'll want to consult a structural engineer (and you'll likely end up using I-beams tied into the concrete walls as joists).

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    "The basic idea is to build a room inside your room." Yo dawg, I heard you like rooms...
    – Doresoom
    Jun 17, 2011 at 13:47

I did a lot of research before finishing our basement. Traditional thought is:

  • vapor barrier
  • wood framing
  • fiberglass batts
  • sheetrock

And every basement I've been in using that method has smelled really musty.

Building Sciences Corporation has some good research on this:


Based on that, here's what I did, and what I recommend for all basement remodels:

  • eps or xps sheet insultation, glued and taped to wall
  • metal studs. baseplate should be separated from concrete via thin insulation.
  • paperless sheetrock (use the fiberglass faced stuff...itchy, but won't attract mold)

Note the lack of a vapor barrier. The idea is that this leaves the system slightly breathable. XPS and EPS are vapor permeable so moisture can eventually dry to the inside.

The added bonus is that metal studs are SO much easier to work with than wood. They're light, straight, can cut with a tin-snip and you don't even need to use screws (you can crimp them all together). Plus, they won't let mold grow.

  • There's definitely a lot of moisture concern around here. Thank you for the information.
    – Scivitri
    Jun 17, 2011 at 18:42
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    +1, lot of great points. Don't forget about the floor material. Something like tile wouldn't have any moisture issues, but you may want an area rug on top for comfort.
    – BMitch
    Jun 17, 2011 at 20:37

Tester gave you an excellent basic framed wall design which should work well for running wiring and hanging drywall. I'd use 2X6 min in order to have space for your HVAC duct work if you do not want it exposed. My greatest concern for a concrete under grade room is going to be moisture or excess humidity. Unless you are in the Arizona desert, I would consider a water proof coating on the walls before framing. Something like UGL hydroloc at 3 psi would be good. I would also have a 4mil vapor barrier installed on the back of the wall frame between the wall and any insulation. I would also use a moisture resistant, or paperless, mold proof sheetrock instead of standard sheetrock. My last concern, and maybe the most important is ventilation and fire ratings. Because this space is below an area used for vehicles, fire ratings, fumes and fuel spills are a concern. Check with your building contractor and building inspector to see if there are any special regulations that would effect the use of this unique space, or even any special requirements for the garage space above, especially the floor.

  • To the list I would add PT lumber for anything in direct contact with the concrete.
    – BMitch
    Jun 17, 2011 at 12:58
  • @B Mitch: Wrapping any lumber that touches concrete with Asphalt felt is another (arguably better) solution.
    – Tester101
    Jun 17, 2011 at 14:31
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    Or use sill gasket made for this, e.g. homedepot.com/h_d1/N-5yc1v/R-202080947/h_d2/… Jun 17, 2011 at 15:22
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    vapor barriers are less ideal underground. I'll write up an answer.
    – DA01
    Jun 17, 2011 at 15:29
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    @Shimon, the gaskets are important for energy efficiency, but wouldn't frame with untreated wood. DA01 has a lot of great points in his post.
    – BMitch
    Jun 17, 2011 at 20:33

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