Insulating a Shipping Container in Minnesota: Condensation, Code and Snow

Start with how the container will be used

An unheated storage container is not meant to stay warm. Its problem is condensation: on a cold night the steel roof drops below the dew point of the air inside, and water forms on the ceiling and drips onto whatever is stored. Vents that let air move through the container, a dry floor and insulation on the ceiling all reduce it.

A heated workshop or office has a bigger version of the same problem. Warm air can hold more moisture than cold air, and when it reaches bare steel at winter temperatures, that moisture condenses or freezes on the metal. A heated container needs insulation and an air seal that run without gaps across the walls, ceiling, floor and doors.

A container that people will live in is a home, and Minnesota's residential energy code sets the minimum insulation for it.

What Minnesota's energy code requires for a home

Minnesota Rules, chapter 1322 is the state's residential energy code. The Department of Labor and Industry's fact sheet says the chapter adopts chapters 2 through 5 of the 2012 International Energy Conservation Code for residential buildings, with Minnesota amendments.

Table R402.1.1, as amended in Minnesota Rules, part 1322.0402, sets these minimums for the two climate zones found in Minnesota:

A container wall is steel, not a wood frame, so ask your building official how these values apply to your wall design. Ask early, because the answer sets how thick the walls will be.

The building official also confirms which climate zone your county is in. For a heated office or shop at a business, ask whether the commercial energy code applies. Minnesota Rules, part 1323.0514 places 23 northern counties, including St. Louis, Itasca, Beltrami and Polk, in climate zone 7 for commercial buildings, and every other county in zone 6A.

Insulation takes room

A 20ft container is about 7 feet 9 inches wide inside. Three inches of insulation on each wall leaves about 7 feet 3 inches.

In a home, height matters too. Minnesota's residential code, as amended in Minnesota Rules, part 1309.0305, requires a ceiling height of at least 7 feet in habitable space, hallways, bathrooms, toilet rooms and laundry rooms, measured from the finished floor to the lowest projection from the ceiling. A standard container has about 7 feet 10 inches of interior height, which leaves about 10 inches for the ceiling insulation, the ceiling finish and any layers added to the floor. A 40ft high cube has about 8 feet 10 inches, which leaves about 22 inches.

Insulation on the outside of the container keeps the full interior width and height and keeps the steel near indoor temperature. It needs cladding to protect it from sun, rain and snow, and it adds to the outside dimensions.

Four ways to insulate the inside

• Closed-cell spray foam applied to the steel fills the corrugations and blocks air movement in the same layer, which suits a heated container. It needs a trained installer. Ask the building official what covering exposed foam needs inside a heated space.
• Rigid foam boards fitted between furring strips are cleaner to install. The gaps behind the corrugations and the joints between boards need sealing, or warm air will reach the steel.
• A framed wall with batts gives room for wiring and a familiar interior finish. It needs a continuous air and vapor barrier on the warm side, because moisture that gets into the wall cannot dry out through the steel.
• Ceiling-only insulation in an unheated storage container reduces the dripping that forms under cold steel, with far less work than lining the whole container.

Whichever method you choose, seal the insulation around doors, vents and any openings cut into the steel. A small gap lets warm, moist air reach cold metal, and the water that forms there runs down inside the wall.

Frost depth and snow load by county

Two state rules set numbers by county. Minnesota Rules, part 1303.1600 sets the minimum footing depth for frost protection at five feet in 34 northern counties and three and a half feet in the rest of the state, unless an engineer competent in soil mechanics determines otherwise. Part 1303.1700 sets the ground snow load used to design buildings at 60 pounds per square foot in 29 northern counties and 50 pounds per square foot in every other county.

Here is how the two rules fall for ten Minnesota cities:

The Department of Labor and Industry publishes a frost depth map and a snow load map of the same county lines. The snow load map converts the ground snow loads into roof snow loads for homes: 42 pounds per square foot in the northern counties and 35 in the rest of the state.

Snow on the roof

If a container becomes part of a building, its roof design is checked against these loads. A flat container roof holds snow until it melts, and melt water can freeze again at the edges. A pitched roof built over the container sheds snow and leaves space for more ceiling insulation, and it is designed to the snow load like any other roof.

The floor and frost

A container floor sits on steel cross members, and cold reaches it from below. Insulation under the floor, or a layer laid over it, slows that heat loss. An open, ventilated space under the container lets the underside dry.

Frost can heave the ground under a container set on blocks or gravel, so check that it is still level each spring. Ask your local building official whether your container follows the footing depth rule in Minnesota Rules, part 1303.1600, subpart 1, or qualifies for subpart 2's slab on grade exception for detached one-story private garages, carports and sheds up to 1,000 square feet on soil other than peat or muck.

Fresh air, heat and wiring

An insulated container still needs fresh air. People, wet boots and snow-covered equipment all add moisture to the air inside, and so does an unvented propane or kerosene heater, because burning fuel produces water vapor. Use only a heater rated for the space, follow its venting instructions, and install a carbon monoxide alarm in any container where fuel is burned.

An electric heater adds no moisture. In an unheated storage container, vents placed high and low on opposite ends let air move through and carry moisture out.

Wiring lights, outlets or a heater into a container is a new electrical installation. Minnesota Statutes, section 326B.36 requires every new electrical installation to be inspected, by the state or by a local government that runs its own electrical inspection.

Choosing the container

Insulation needs a sound, dry surface. Dents in the roof hold standing water and rust holes let it in, so inspect the roof and walls before insulating. The container condition guide explains the grades, and the 40ft high cube page covers the taller container. For a container fitted out before delivery, see modified containers.

The Minnesota permit guide covers the permit and zoning rules in Minneapolis, St. Paul and Rochester. A home built from containers has more to plan, covered in container homes in Minnesota.

Sources

• Minnesota Rules, chapter 1322, Residential Energy Code and part 1322.0402, Building thermal envelope
• Minnesota Department of Labor and Industry, Residential Energy Code fact sheet
• Minnesota Rules, part 1323.0514, Climate (commercial energy code)
• Minnesota Rules, part 1309.0305, Ceiling height
• Minnesota Rules, part 1303.1600, Footing depth for frost protection and the frost depth map
• Minnesota Rules, part 1303.1700, Ground snow load and the snow load map
• Minnesota Statutes, section 326B.36, Electrical inspections

This guide is general information about insulation and public rules, not engineering or legal advice. Your building official and your designer decide what your project needs.