Best way to build and insulate an exterior box?
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:
- 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
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
- 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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