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Best way to build and insulate an exterior box?

Michael Bluejay | Posted in Green Building Techniques on

I’m cutting a hole in an exterior wall and building a box on the outside, so I can slide the bed into the box in the morning and have more space in the room during the day. Of course I’ll have a sufficient header above the opening. My plan for the box construction is:

FLOOR:

  • Concrete pad outside
  • Silicone paper on top of the pad as a moisture barrier
  • 3/8” ply on top of the paper
  • 4” of polyiso foamboard on the ply
  • 1/2” ply on top of the foam, screwed all the way through, into the concrete

WALLS:
All plywood, with no stud framing. From inside to outside:

  • 1/2” ply
  • Asphalt paper
  • 2.5” polyiso foam
  • Fiber-cement siding

SLOPED ROOF/CEILING, from bottom to top:
* 2×4 joists on 24” centers
* 1/2” ply
* 4” of polyiso foam
* Sheet metal roof

NOTES:

  • I’m in Austin, TX 78722.
  • Roof would be only R24, though I think the spec for Austin, TX 78722 is R30. However, the metal roof, plus the fact that it would be completely shaded by a second story deck, makes me think that R24 is enough. (?)
  • I hope to use polyiso instead of EPS because it’s greener, and because with a higher R-value per inch, I won’t need as much of it. In the other thread you helped me in, you said that EPS was safer on the exterior, but I wasn’t clear if that was just because on that project I originally planned to also have foamboard on the interior as well. I’m not sure whether I should be looking at EPS instead of polyiso in this application also.
  • If I run the wall foamboard all the way to the sheet metal roof, then the plywood would be vapor sandwiched between the 4” of ceiling polyiso and the 2.5” polyiso on the walls. Maybe a sandwich of only 4” of height around three walls would be okay, I don’t know.(?) Alternatively, maybe I could omit the 4” of exterior insulation around the perimeter at the roof, since the roof insulation would effectively insulate the top 4” of wall from the inside.
  • I guess it’s possible to just do regular 2×4 studs and add 2” as a Mooney Wall to get extra thickness and to avoid a thermal break for most of the stud area, and then tamp down cellulose from the top to get it dense-packed, but I’m attracted to the idea of all the wall insulation on the outside, since it seems simpler and more effective.

    Thank you in advance for your help. I signed up as a prime member. This site is invaluable.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Michael,
    If you are building an addition to your house, you will probably need a building permit. Your addition will need to comply with applicable building codes. So the first step (if you haven't done this yet) is to contact your local building department to find out their requirements.

    You haven't told us whether this "box" is only two feet tall, or whether it is 8 feet tall. Knowing the size of the "box" (especially its height) would help us visualize your plan.

    The concrete foundation for this addition needs to meet all the usual requirements for a foundation. I'm not sure how deep you have to go with your foundation to avoid frost-heaving problems in Austin, Texas. But in any case, make sure (a) that your proposed "concrete pad" had an adequate footing, and (b) that the slab won't heave in cold weather, and (c) that the slab won't settle, and (d) that the foundation for your addition meets code requirements.

    If you are pouring a new concrete slab, the usual place to put the rigid foam insulation is under the slab, not over the slab. For more information on this issue, see Polyethylene Under Concrete Slabs.

    The walls of your box need to support the roof load. You are planning to build site-built SIPs, but they don't appear to have been designed with structural loads in mind. You will either have to buy real SIPs from a SIP manufacturer, or include 2x4 studs. If you are uncertain about how to build a wall, consult an engineer or hire a builder.

    Finally, I urge you to at least meet minimum code requirements for your wall R-value and roof R-value. In most cases, your local building inspector will probably give you the same advice.

  2. Stephen Sheehy | | #2

    Can you skip all this and install a Murphy bed? Even the electric ones that slide up into the wall would be much cheaper than building an entire room just to accommodate a bed.

  3. Michael Bluejay | | #3

    Stephen, I'm sorry I didn't see your comment until just now. There's a window on the wall which I don't want to cover by folding the bed up against the wall. Also (still haven't built this yet), I'm thinking of attaching a sofa to the edge of the bed, so when the bed goes into the wall, the only thing that will be visible is the sofa. You know, a James Bond / Batman kind of feature.

  4. Steve Knapp CZ 3A Georgia | | #4

    Michael,

    It will likely cost almost as much to this "box" as a full-height addition. If the room is so small you need to hide the bed (Do you also occupy this space during waking hours?), you should add more space simply for resale value. I can guarantee you that no potential buyer will know what to make of the hide-hole.

  5. Michael Bluejay | | #5

    Steve,

    The box will be about 3' high, and the interior dimensions would fit a queen-size bed. I can't imagine it will be very expensive to build.

    If I built a full-height addition and kept the bed there, then I'd have to sleep in a claustrophobic cutout with walls on three sides of the bed, yuck.

    There's limited room outside the existing room, so I can't really put an addition there besides a bed box anyway, even if I wanted to, which I don't.

    No, the current room isn't small. Like I said, I just want to have a James Bond / Batman-type feature.

    I believe some buyers would be thrilled with this unique feature. This is Austin, with a unique population, and appraisers have told me the same thing about other unique features of my properties. I certainly can't imagine that this will *decrease* the value of the property. Even if it did, I wouldn't really care. And in any event, I don't intend to ever sell this house anyway.

  6. Expert Member
    Malcolm Taylor | | #6

    Micheal,

    Ive seen a few variations on this, including one very similar to what you are describing, in (I think) Dwell Magazine of a small rooftop apartment. Compared to ones that slide into an adjacent indoor space this seems like a lot of work, but it is doable.

    I'm less inclined to dismiss it as a solution that previous posters. Sometimes including a feature that isn't on first glance either a logical or easy solution makes up for theses deficiencies in how much delight it adds to the space.

    - At present your wall assembly doesn't appear to meet codes or be very build-able. The walls need framing, not only because of code requirements, but to provide something to attach the insulation, foam and siding to. 1/2" unsupported ply will not stay straight, can't be attached to the slab without framing and will look like a porcupine on the inside with sharp fastener-heads penetrating the sheathing. It also can't support the roof.

    - The roof needs strapping above the foam unless you can somehow figure out how to attach the panels and metal trim right through hitting the 2"x4's in all the necessary places. Our code would also preclude using 2"x4"s for a span that long.

  7. Michael Bluejay | | #7

    Thanks, Malcolm! I'll plan to add framing. The problem with that is that there might not be enough room for a framed roof, because the top of the roof would be higher than the windows. If I have to replace the windows with shorter ones, then this is gonna be a much bigger job...and I'll lose window space. I'll see what I can do....

  8. Expert Member
    Malcolm Taylor | | #8

    Michael,

    Do you need a sloped roof? As I recall, the one I saw in Dwell was really just an insulated metal box a bit bigger than the bed, covered in flashing. Flashing for a roof that size could easily be made up out of just two pieces.

  9. Michael Bluejay | | #9

    I guess in theory it doesn't have to be sloped, but at least a *little* slope would be nice. There's a deck above where the box will go. If I put a polycarbonate panel under the deck and sloped slightly away from the house, that should keep most water from getting on my bed box.

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