I am enclosing half my garage with what might be called a greenhouse to raise tropical fish. I was thinking of building studded walls inside my garage using treated 2 X 4's and 2" styrofoam insulation and then wrapping both the inside and outside of each wall with plastic to prevent air flow. Picture a box built within the garage not attached to the garage. 4 walls and a top!!

  • Don't use treated lumber except for the bottom plates. It'll warp badly and is an unnecessary expense. If you're using foam panels you don't need poly sheeting, and definitely don't sheet both sides.
    – isherwood
    Jun 14, 2017 at 14:39
  • @isherwood I'm guessing that the OP was going to use PT lumber, because the interior space will likely have high humidity.
    – Tester101
    Jun 14, 2017 at 14:45
  • 1
    Then a poly film will be appropriate, but only on the inside.
    – isherwood
    Jun 14, 2017 at 14:47
  • Depending on where you live, insulating the floor might be useful as well. You'll also want to make sure the door seals well, and is insulated. Since you're sealing the room so well, you'll also have to worry about ventilation. Likely a system that captures some of the heat from the air that's being vented out. To maximize insulation, you could use 2x6 framing instead of 2x4s.
    – Tester101
    Jun 14, 2017 at 14:51
  • I do just the opposite (a walk in cooler) don't forget about insulating the ceiling and floor. My first time worked but after adding insulation my room dropped enough that the modified AC unit ran about 1/3 as often with the floor in my case ceiling in yours being more important than I thought it would be.
    – Ed Beal
    Nov 21, 2017 at 0:32

2 Answers 2


Been there , done that. My room is a 13' X 13' corner of the garage . Mild climate ( Houston TX) I have 4" fiberglass in the walls, 2 walls of double glass windows , 8" of loose glass plus 4" of foam in the ceiling . Summer is fine with a small room AC unit. But in winter there is condensation from the Al window frames and glass. The paneled walls are damp and get moldy, the ceiling (drywall) has black mold. In spring I seal the aquariums and wipe everything down with bleach rags. I have 125,75,39 and 20 gals with about 80% glass tops. I would recommend concentration on your interior surface finishes , understanding they will tend to get mold. I have no heat in the room other than the aquarium heaters ; I believe an electric space heater would help by reducing the relative humidity . We enter the garage through the room so there is some air exchange. When I built it ( 14 years ago) the EPA required the mold inhibitors to be removed from caulk, so the caulk around all the windows is black and not cleanable. I understand new inhibitors have been developed , be sure to get "good" caulk.


In my opinion this is no different than any other room you might be heating. You want insulation, and you want a vapor barrier on the inside of the room.

I'd use fiberglass batts due to the ease of install. You could use faced batts and put the paper toward the outside of the room to act as a containment envelope, with poly sheeting on the inside of the room.

If you use rigid foam, consider fastening it right to the face of the framing with cap nails rather than fitting to stud spaces. Tape the joints with appropriate sealing tape.

  • Common misconception: “Install vapor barrier on INSIDE of room.” Actually, the vapor barrier should be installed on the NORMALLY WARM SIDE OF WALL. Those in Florida, Texas, etc. will install vapor barrier on outside of wall. (However, for OP, this will probably be inside of tropical room.) Remember, moisture (in the form of vapor) will move from warm to cool side of wall...not vice versa.
    – Lee Sam
    Jul 10, 2018 at 0:31
  • As you said, OP is building a greenhouse. I saw no need to elaborate on what a person would do in other scenarios.
    – isherwood
    Jul 10, 2018 at 12:12
  • Greenhouse...???
    – Lee Sam
    Jul 10, 2018 at 15:51
  • "Tropical room". Whatever. Perhaps I misunderstood. It's irrelevant.
    – isherwood
    Jul 10, 2018 at 15:58

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