Thermal Insulation Design for Container House Envelopes

In recent years, as natural disasters have become more frequent worldwide and mineral-resource development has intensified, demand has grown for temporary housing for disaster survivors and for on-site offices and living quarters at oil fields and mines. Sturdy, durable container houses that are easy to move and install have therefore developed rapidly. Following today's trends toward energy-efficient, environmentally friendly, and economical construction, strengthening research on thermal insulation of container-house envelopes has become an inevitable direction for the industry.

1 Goals and Standards for Envelope Insulation Design

1.1 Indoor Thermal Comfort

Indoor thermal comfort is the subjective sense of satisfaction with the indoor thermal environment. Beyond subjective factors such as metabolic rate and clothing, the main influences include indoor temperature, indoor humidity, air velocity, and environmental thermal radiation. Environmental thermal radiation consists of solar radiation that may enter the room and radiant heat exchange between the human body and surrounding surfaces such as walls, floors, and roofs. Indoor air temperature is the primary factor affecting indoor thermal comfort.

According to the national standard Code for Design of Heating, Ventilation and Air Conditioning, comfort requirements for residential indoor thermal environments are summarized in Table 1.

Raising the thermal insulation performance of the envelope and the insulation and airtightness of doors and windows can effectively improve the indoor thermal environment of a container house.

1.2 Difference Between Thermal Insulation and Heat Insulation of Building Envelopes

The thermal-insulation performance of a building envelope is its ability, under winter indoor and outdoor conditions, to resist heat flow from inside to outside so that an appropriate indoor temperature is maintained. Heat-insulation (summer) performance is the ability, under natural ventilation in summer, for the envelope's inner surface to stay at an appropriate temperature when acted on by the outdoor comprehensive temperature (from outdoor air and solar radiation) and by indoor air-temperature fluctuation. The main differences are:

(1) Different heat-transfer processes. Thermal insulation reflects winter heat flow from indoors to outdoors; heat insulation reflects summer heat flow from outdoors to indoors and from indoors to outdoors. In winter, outdoor air temperature varies little over a day, so heat transfer is mainly steady-state. In summer, outdoor air temperature and solar radiation change considerably with time—periodic unsteady heat transfer.

(2) Different evaluation metrics. Thermal insulation is usually evaluated by the heat-transfer coefficient K (unit: W/(m²·K)) or the thermal resistance RO (unit: (m²·K)/W). Heat insulation is usually evaluated by the maximum inner-surface temperature of the envelope under summer outdoor design temperature conditions. In current energy-efficiency design standards, heat insulation is measured directly by the envelope's thermal inertia index (D value); transparent glazing may be evaluated by the shading coefficient SC.

(3) Different construction measures. Winter insulation generally only requires higher thermal resistance of the envelope; lightweight porous or fibrous materials can meet energy requirements through composite or self-insulation. Summer heat insulation requires not only greater thermal resistance but also better thermal stability (a larger D value); exterior windows should also raise the glass shading coefficient or use shading to cut solar radiant heat.

2 Requirements for Envelope Insulation Design

2.1 Insulation Forms

When designing insulation for a container-house envelope, combine local climate, structural features of the container house, intermodal transport conditions, and cost requirements; choose the insulation form (external or internal); and determine insulation construction and thickness by calculating the minimum total thermal resistance. In hot-summer/cold-winter regions, both summer heat insulation and winter thermal insulation of the outer envelope must be considered.

Because of lifting, stacking, and securing requirements during intermodal transport, and limits of port and yard operations and sea/land transport environments, container-house envelopes usually use internal insulation.

2.2 Insulation Materials

Use high-efficiency insulation materials with low thermal conductivity wherever possible to reduce insulation thickness and increase usable indoor area.

By material type, insulation falls into inorganic, organic, and metallic thermal-insulation materials; by form, into fibrous, porous (microporous, cellular), and layered types. Performance depends on thermal conductivity—the lower the conductivity, the better the insulation. Thermal conductivity relates to composition, apparent density, internal structure, mean temperature during heat transfer, and moisture content. The most common envelope insulation materials include molded polystyrene foam, extruded polystyrene foam, rigid polyurethane foam, rock-wool board, and glass wool.

2.3 Fire Performance

Literature on insulation-system fire performance shows that for low-rise dwellings (under three stories), national codes generally require a fire-resistance rating of 0.5–1.0 h for building elements; China's rules on combustibility of building elements are among the strictest. For internal insulation design of container-house envelopes, follow relevant provisions of the Code for Fire Protection Design of Buildings GBJ16—1987, the Code for Design of Residential Buildings GB50368—2005, and the Code for Fire Protection Design of Building Interior Decoration GB50222—1995; use noncombustible or flame-retardant materials such as mineral-wool board (felt) and glass-wool board; if foam plastics are used, combustibility should reach Class B1.

2.4 Health and Environmental Protection

Insulation materials should be environmentally friendly products that do not pollute the indoor environment—free of radioactive, carcinogenic, or other substances that contaminate indoor air. At present, indoor environmental pollution control in China is covered by the Code for Indoor Environmental Pollution Control of Civil Building Engineering GB50325—2001 and the Indoor Air Quality Standard GB/T18883—2002.

2.5 Interior Finish Design

(1) Internal insulation design for container-house envelopes should work together with interior finish design, aiming for integrated finishing of insulation and interior materials, standardizing and industrializing interior work, raising labor productivity, and improving finish quality.

During intermodal transport, container houses are affected by lifting, forklift handling, stacking, and sea/land bumps and vibration, so the box inevitably deforms. To prevent cracking of interior finish materials and joints, finishes should have relatively high transverse and longitudinal flexural moduli and some toughness; brittle finishes such as ceramic tile or stone cladding must not be applied directly. Joints in interior panels should preferably use open joints or battens; if seamless treatment is required, follow jointing procedures strictly and choose joint compounds and tapes with strong self-adhesion and strength, some toughness, and good workability.

(2) When heavy objects must be fixed or hung on interior walls or ceilings of a container house, backing plates should be embedded in the box structure in advance.

(3) Insulation design must ensure that inner surfaces do not condense—that is, inner-surface temperature must not fall below the dew point of indoor air. Condensation makes inner surfaces damp, moldy, or even dripping, worsening indoor hygiene. For internally insulated envelopes, dew-point locations occur at cold (thermal) bridges near the inner face of the exterior wall, where large indoor–outdoor temperature differences often cause condensation. Internal insulation design should avoid structural cold (thermal) bridges.

3 Common Internal Insulation Systems for Container House Envelopes

3.1 Color-Steel Sandwich Panel Internal Insulation System

Color-steel sandwich panels combine color-coated steel face sheets and color-coated or galvanized steel back sheets with an insulation core by adhesive (or foaming). By core type they include rigid polyurethane sandwich panels, polystyrene (or extruded polystyrene) sandwich panels, and rock-wool sandwich panels. Panels are typically joined by interlocking; for damp locations, a PVC finish may be high-frequency heat-bonded to the face in advance. Color-steel sandwich internal insulation is generally used for dry-box structures and container-type buildings with modest insulation requirements.

3.2 Stud-Frame Internal Insulation System

The stud-frame internal insulation system works with interior wall boards such as gypsum board, fiber-cement board, calcium silicate board, and magnesium oxide board. The box structure follows light-steel residential detailing. Thin-gauge light-steel studs serve both as structural members and as the fixing base for interior boards; the outer skin may be dry-box corrugated plate, flat plate, wall profiled sheet, and other sections. Cavities between studs are usually filled with fibrous insulation such as rock wool or glass wool.

3.3 Composite Insulated Gypsum Board Internal Insulation System

Europe began applying composite insulated gypsum board internal insulation systems in buildings in the early 1970s. After nearly 40 years of large-scale practice, mature technical standards formed, and the system became a mainstream European envelope insulation approach. After foreign building-material firms led by Lafarge introduced it to China in 2006, the system passed a series of theoretical and testing reviews in the construction industry; related technical procedures and atlases were issued; and it has been used successfully on many projects. Composite insulated gypsum board combines the decorative quality of gypsum board with excellent insulation, integrating insulation and interior wall finish for container houses and raising interior finishing efficiency.

3.4 Polyurethane Internal Insulation System

The polyurethane internal insulation system adapts refrigerated-container foaming technology for container-house envelopes. Its main process feature is mold-injected polyurethane foaming of each envelope part in advance to form major assemblies such as roof, sides, and floor; after final assembly, secondary foaming is applied at junctions between overall parts. The system suits many box structures and has clear advantages over EPS, XPS, and similar materials:

(1) Strong insulation performance. The thermal conductivity of rigid polyurethane foam is ≤0.024 W/(m·K)—among the best thermal performance of EPS, XPS, and similar materials. Mold-injected rigid polyurethane foam fits complex sections and forms a continuous effective insulation layer, avoiding cold (thermal) bridges from air movement through gaps or cavities.

(2) Strong adhesion. Rigid polyurethane foam bonds firmly to concrete, wood, steel, asphalt, rubber, and other materials. Mold-injected foam combines effectively with the box and interior wall boards; even under the most adverse temperature and humidity, plus self-weight and normal impact, the insulation layer stays stable and does not separate or fall from the box or interior boards.

(3) Relatively good fire performance. With flame retardants, rigid polyurethane foam is a flame-retardant, self-extinguishing, charring material without the XPS/EPS defect of melting and dripping under fire or high heat that can cause wall finish to detach; fire performance can reach Class B2.

(4) Good waterproofing. As a closed-cell structure with about 90% closed-cell content and water absorption ≤4%, rigid polyurethane foam blocks water and vapor penetration well and keeps the insulation system in a good, stable insulating state over time.

(5) Good crack resistance. Rigid polyurethane foam is a relatively flexible material with strong resistance to external deformation; under force, temperature, and humidity deformation it is less prone to cracking, helping ensure system stability and durability.

(6) Good environmental performance. Rigid polyurethane foam has a three-dimensional thermoset cross-linked structure and high chemical stability; blowing agents may be fluorine-free or partially fluorinated, so it does not produce harmful gases or harm the indoor environment or the ozone layer.

(7) Convenient construction. Mold-injected work can be mechanized with high efficiency. Polyurethane application adapts well to container-house shapes and fully protects insulation at complex details.

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