family in front of their container home

Shipping containers are becoming increasingly attractive as dwellings. They have been traditionally used as emergency housing in disaster areas worldwide but are gaining popularity as a sustainable housing option.

One problem is that they are steel boxes and conduct heat too well for comfort. So how can we insulate container homes to improve their habitability and also save money on energy? Read on for a list of insulating materials you can use and some considerations for applying them.

Fiberglass and Rockwool

The first type of insulation many people think of is the most common: fiberglass. It is installed as batts in walls or ceilings or blown-in as loose-fill. You attach 2×4 studs to the walls and fill the spaces between these studs with fiberglass, then cover the studs and insulation with drywall, paneling, or any other finish you fancy.

A picture of pink fiberglass in the wall, with a man installing it

Fiberglass typically has an insulation value of R-3.5/inch. Some batts have a paper backing that extends beyond the edges for attachment to the narrow edges of the 2×4. 

The paper can serve as a vapor barrier if correctly installed, although many builders prefer installing a separate, specifically-designated barrier.

The fiberglass must contact all six sides of the cavity you put it in to be fully efficient; any voids will reduce its R-value.

Rockwool is fibers spun from minerals instead of glass but is similar. It gives R-15 for a 3.5-inch thick batt and has a higher melting point than fiberglass, offering slightly better fire resistance. Also, the batts are more rigid than fiberglass, so it is more likely to be installed better.

Cotton or Denim

A popular sustainable insulation is denim, usually in the form of recycled jeans or denim waste. This insulation is similar to fiberglass with an R-13 insulation value and is installed the same way as batts between 2×4 studs.

It is also usually treated with a non-toxic, borate-based flame retardant that helps percent the growth of mold and bacteria.

A denim insulation batt

Since denim doesn’t have the irritating fibers of fiberglass, it is more suitable for a DIY project. All you need is a basic facemask, not the half-face respirator suggested for respiratory protection with fiberglass.

Denim is more rigid to work with than fiberglass. But that means it will be more efficient when installed since installers often stretch the fluffy fiberglass insulation to fill the cavity, diminishing its insulation value.

Denim is recyclable, whereas used fiberglass generally is not. However, it costs at least 10% more than fiberglass, up to double the price. And there may be availability issues of some kind.

But denim is more soundproof than fiberglass and can earn LEED credits for its sustainability.

Wool

Wool from sheep is another natural alternative. Some wool harvested from sheep is too coarse to make fabrics and provides excellent insulation. It has a slightly higher R-value than fiberglass, and you install it similarly.

It is naturally mold and fire-resistant, although it is usually treated with non-toxic flame retardants, and other more toxic additives may be present for pest control.

Wool is hygroscopic and can absorb and release water to help regulate your container’s humidity, which is a definite benefit. In addition, because of its density, it has superior sound-deadening properties, something always helpful with a metal structure.

Expanding Spray Foam (Polyurethane)

Both open-cell and closed-cell spray foams are excellent options. Closed-cell spray foam is more expensive than open-cell but is more moisture-resistant. Both provide an air barrier when adequately applied and can be sprayed onto the interior or exterior side of the walls.

One benefit of these spray foams is that they can cover the potentially toxic paints used on shipping containers and prevent toxic chemicals from escaping.

Closed-cell foam has a higher R-value, and installers will usually apply a two-inch-thick coat for R-13 or three-inch-thick for roofs to achieve R-19. 

On the interior, it is sprayed between 2×4 wall framing but doesn’t fill the depth of the cavity; you have to be careful that the insulation is continuous with no gaps. When applied to the exterior, you also have to protect it from the elements with suitable cladding.

Open-cell foam on the interior completely fills the openings between the 2×4 wall framing and is more likely to form an intact air barrier. It costs about $0.45 to $0.65/board foot, according to Bob Villa, but you need almost twice as many board feet versus the more expensive closed-cell insulation at $1.00 to $1.50/board foot. But, again, prices vary considerably from one source to another.

You can get enough polyurethane spray foam to cover the sides and top of your 20-foot container to 2” (about R-12.5) for about $4,000 at Home Depot. But this is not a DIY project. It should be done by a professional for better results at a significantly lower cost and possibly a warranty. You should use this spray foam to fill gaps where pieces or areas of the insulation meet; if you want a DIY project, consider installing foam boards.

Non-Expanding Spray Foams

These are water-based compositions. The most common comprises cellulose from shredded recycled paper products with adhesive binders and a fire retardant. Another type is a sprayed-on foam of certain minerals mixed with water that hardens and looks like concrete. Since these are not expanding foams, they have to be applied carefully to obtain 100% complete coverage.

Cellulose insulation has an R-value of about 3.5 per inch and is sprayed into the space between studs on an interior wall, where it hardens into place. Cementitious spray foam has somewhat less R-value and may be crumbly after curing.

Expanded Polystyrene (EPS, Styrofoam, or Equivalent)

Styrofoam (expanded polystyrene) is one of the least-expensive insulations. It has an R-value over four per inch, and as a closed-cell foam, it has tiny trapped air bubbles that conduct heat poorly—it’s about 98% air. However, it is good at blocking sound.

Polystyrene blocks used for container home insulation stacked in a warehouse

It comes in 4’x8’ sheets in various thicknesses that you can glue to the walls of your container. Sheets are available that conform to the contours of the container wall.

Extruded Polystyrene

Extruded polystyrene (XPS) is similar to EPS but made through different manufacturing processes. As a result, the surfaces of the slabs have a smoother texture and a higher R-value of about five/inch.

Polyisocyanurate 

Polyiso insulation has an R-value of eight/inch and costs around $0.50 per board foot. It is the most expensive of the foam boards but allows you to save space because of its high R-value.

Cork

Cork is another sustainable insulation that can be applied either on the interior or the exterior. Since it grows on trees and grows back after being harvested, it is one of the most sustainable materials. 

Closeup of a cork board insulation texture

The sound-deadening properties of cork make it particularly desirable to offset the undesirable acoustic properties of the metal container.

Hay Bales

These are only possible on the exterior and are not practical if your container has windows and doors. However, they may make good insulation for the roof if adequately tied to other insulation with a sound vapor barrier.

Green Roofs

A living roof is one of the most natural choices since it is a slice of nature itself. 

This substrate should be 2 inches to 8 inches deep, depending on the roots of the types of plants you want to have, and be 20%-30% organic material and 70%-80% inorganic material like crushed brick.

Closeup of a container home with a green roof

First, you must install various layers on top of the container and add the substrate. Of course, you may have to water and otherwise care for the roof, depending on the climate and plant type.

Radiant Barriers

Radiant barriers have potential use in combination with other materials; a radiant barrier would not provide much insulation value alone. However, at least one product (Prodex) combines a radiant barrier with 0.4-inch closed-cell polyethylene foam; it claims to provide both a thermal barrier and a moisture barrier with up to R-22.

You could install a rigid radiant barrier above the container home’s roof to reflect heat, with an air gap of at least an inch. This installation would reduce the need for ceiling insulation but would only be practical if weather conditions permit it.

Interior Insulation Considerations 

Since shipping containers are only eight feet wide, adding interior insulation reduces the internal width significantly. For example, installing 2×4 studs covered with ½” drywall minimizes the width of the container home’s interior by 8 inches.

It makes more sense if you are joining two or more containers together since some interior finish is usually desirable, and the framing provides a place for electrical and plumbing infrastructure.

Exterior and Mixed Exterior + Interior Insulation Considerations

Adding insulation to the container home exterior changes the rugged look that some people prize.

Mixing insulation is an option, but raises the possibility of thermal bridges reducing the insulation’s effectiveness and complicating installation of thermal and moisture barriers.

Summary

A wide variety of container home insulation types to reduce your energy bills are available for every taste and budget.

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