Yes to both. A bare-steel container in direct sun can hit 130°F inside on a 90°F day. In winter, the interior tracks within a few degrees of outdoor temperature. Insulation, shade, or active climate control is required for anything heat- or cold-sensitive.
Steel is a great conductor of heat. Direct sun on a corten roof in July gets the surface to 150°F+; the interior heats with it. Conversely, in January, an uninsulated container in the Northeast or Midwest tracks within 5-10°F of outdoor lows. Containers don't moderate temperature — they amplify it.
The fixes, in order of cost: paint the exterior a light color (cuts solar gain ~15%); shade the container (50% reduction in peak interior temps); add closed-cell foam insulation on walls + ceiling (huge improvement — interior stays within 10°F of outdoor temp); add active climate control (mini-split or window AC, runs on standard 110V).
For container homes and workshops, the standard is: insulation (R-13 to R-30 depending on climate), a vapor barrier, interior wall finish, and a small mini-split. Total conversion cost: $5,000-15,000 for a 20ft container, depending on finish level. Worth it for daily-use spaces; overkill for occasional storage.
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