Thermal Insulation Design of 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 victims and for on-site offices and living quarters at oil fields and mines. Sturdy, easy-to-move, quick-to-install container houses have therefore developed rapidly. In line with today's trends toward energy-efficient, environmentally friendly, and economical construction, strengthening research on thermal insulation for container-house envelopes has become an inevitable direction for the sector.

1 Envelope Insulation Design Goals and Standards

1.1 Indoor Thermal Comfort

Indoor thermal comfort is a subjective sense of satisfaction with the indoor thermal environment. Factors that affect it include not only subjective ones such as metabolic rate and clothing, but also objective indoor conditions: air temperature, humidity, air speed, and environmental thermal radiation. Environmental radiation comprises solar radiation that may enter the room and radiant heat exchange between the body and surrounding surfaces—walls, floor, and roof. Indoor air temperature is the main factor affecting thermal comfort.

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

Improving the insulation performance of the envelope and the thermal performance 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 Shielding of Building Envelopes

The thermal insulation performance of a building envelope is its ability, under winter indoor–outdoor conditions, to resist heat flow from indoors to outdoors so that an appropriate indoor temperature is maintained. Heat-shielding performance is the ability, under summer natural ventilation, for the envelope under outdoor comprehensive temperature (outdoor air plus solar radiation) and indoor air-temperature fluctuation to keep its inner surface at an appropriate temperature. The main differences are:

(1) Different heat-transfer processes. Insulation reflects winter heat flow from indoors to outdoors; heat shielding reflects summer heat flow from outdoors to indoors and from indoors to outdoors. In winter, outdoor air temperature varies little over a day, so steady-state heat transfer dominates; in summer, outdoor air temperature and solar radiation change substantially over the day—periodic, unsteady heat transfer.

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

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

2 Envelope Insulation Design Requirements

2.1 Insulation Forms

When designing insulation for a container-house envelope, local climate, structural characteristics of the unit, multimodal transport conditions, and cost must be considered when choosing the insulation form (external or internal). The insulation build-up and thickness are then set by calculating the minimum total thermal resistance. In hot-summer/cold-winter regions, both summer heat shielding and winter insulation of the external envelope must be addressed.

Because of lifting, stacking, and securing requirements in multimodal transport, and limits from port and yard operations and sea–land shipping environments, container-house envelopes usually use internal insulation.

2.2 Insulation Materials

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

By composition, insulation materials fall into three groups: inorganic, organic, and metallic; by form they include fibrous, porous (microporous or cellular), layered, and similar types. Performance depends on thermal conductivity—the lower the conductivity, the better the insulation. Conductivity relates to composition, bulk density, internal structure, mean temperature during heat transfer, and moisture content. Common insulation materials in building envelopes include expanded polystyrene foam (EPS), extruded polystyrene foam (XPS), 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 (three stories or fewer), national codes generally require a fire-resistance rating of 0.5–1.0 h for building elements; China's codes are the strictest on combustibility of building elements. For internal insulation design of container-house envelopes, follow 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 non-combustible or flame-retardant materials such as mineral-wool board (felt) and glass-wool board; if foam plastics are used, combustibility must reach Class B1.

2.4 Health and Environmental Protection

Insulation materials should be environmentally friendly and not pollute the indoor environment—free of radioactive, carcinogenic, or other substances that contaminate indoor air. China currently controls indoor environmental pollution with 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 also serve interior finish design, aiming for integrated installation of insulation and finish materials, standardizing and industrializing interior fit-out, raising labor productivity, and improving finish quality.

During multimodal transport, lifting, forklift handling, stacking, and sea–land vibration inevitably deform the box. To prevent cracking of finish materials and joints, finishes should have relatively high flexural moduli in both directions and some toughness; do not apply brittle materials such as ceramic tile or stone cladding directly. Joints in finish panels should use open joints or cover strips; if seamless treatment is required, follow jointing procedures strictly and select joint compounds and tapes with strong self-adhesion and strength, adequate toughness, and good workability.

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

(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 envelope inner surfaces damp, moldy, or even dripping, harming indoor hygiene. For internal insulation, dew-point risk occurs at structural 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 bond color-coated steel face sheets and color-coated or galvanized steel back sheets to an insulation core with adhesive (or foaming). By core type they include rigid polyurethane, polystyrene (or XPS), and rock-wool sandwich panels. Panels join by interlocking edges; for damp locations, a PVC finish can be high-frequency heat-sealed onto the face sheet in advance. This system is generally used for dry-box structures and container houses with modest insulation requirements.

3.2 Stud-Frame Internal Insulation System

The stud-frame internal insulation system is an internal insulation approach used with gypsum board, fiber-cement board, calcium-silicate board, magnesium-oxide board, and similar interior wall finishes. The box structure follows light-steel residential detailing. Thin-gauge light-steel studs act both as structural members and as fixing bases for interior boards; exterior sheeting may be dry-box corrugated plate, flat plate, wall profiled sheet, and other sections. Stud cavities 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 and verification, a complete set of technical standards has formed, and the system has become a mainstream European envelope insulation approach. Since foreign building-material firms such as Lafarge introduced it to China in 2006, the system has passed a series of industry theoretical and testing reviews, related technical regulations and atlases have been issued, and it has been successfully applied on many projects. Composite insulated gypsum board combines the finish quality of gypsum board with excellent insulation performance, integrating insulation and interior wall finish for container houses and raising fit-out efficiency.

3.4 Polyurethane Internal Insulation System

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

(1) Strong insulation performance. Rigid polyurethane foam has thermal conductivity ≤0.024 W/(m·K)—among the best thermal performance of EPS, XPS, and similar materials. Mold-injected rigid PU foam conforms to complex section shapes, forms a continuous effective insulation layer, and avoids cold (thermal) bridges caused by air flowing through gaps or cavities.

(2) Strong adhesion. Rigid PU foam bonds firmly to concrete, wood, steel, asphalt, rubber, and other materials. Mold-injected rigid PU foam bonds effectively to the box and interior finish boards; even under unfavorable temperature and humidity, self-weight, normal impact, and similar factors, the insulation layer remains stable and does not separate or fall from the box or finish boards.

(3) Better fire performance. With flame retardants, rigid PU foam is a flame-retardant, self-extinguishing, char-forming material and lacks the melt-drip wall-detachment failure of XPS and EPS under fire and high temperature; fire performance can reach Class B2.

(4) Good waterproofing. Rigid PU foam has a closed-cell structure (closed-cell ratio about 90%, water absorption ≤4%), which effectively blocks liquid water and vapor so the insulation system stays in a good, stable insulating state over time.

(5) Good crack resistance. Rigid PU foam is relatively flexible and resists external deformation; under force, temperature, and humidity effects it is less prone to cracking, helping keep the system stable and durable.

(6) Good environmental performance. Rigid PU foam has a three-dimensional thermosetting cross-linked structure and high chemical stability; blowing agents may be fluorine-free or partial-fluorine types, so harmful gases are not produced and indoor air and the ozone layer are not harmed.

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

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