Comparative Analysis of the Thermal Conductivity Performance of Wall Energy-Saving Materials

Thermal conductivity is the decisive parameter that determines how effectively a wall energy-saving material limits heat flow and reduces building energy consumption. This article presents a professional comparative analysis of the thermal conductivity of mainstream wall insulation materials—including EPS, XPS, PUR/PIR, rock wool, aerogel and vacuum insulation panels—based on standardized test methods such as ISO 8302, ISO 8301, ASTM C518 and EN 12667. It also explains how designers, manufacturers and accredited third-party laboratories should interpret, report and apply the resulting data in building envelope engineering.

Technical Overview

Thermal conductivity, denoted λ and expressed in W/(m·K), quantifies the ability of a wall energy-saving material to conduct heat under steady-state conditions. The lower the λ value, the thinner the insulation layer required to achieve a given thermal performance. For a homogeneous layer, thermal resistance is calculated as R = d/λ (m²·K/W), where d is the thickness, and the thermal transmittance (U-value) of the whole wall assembly is the reciprocal of the sum of all layer resistances, as described in EN ISO 6946. Because national energy codes such as GB 55015 impose strict U-value limits on external walls, accurate determination and fair comparison of λ values are fundamental to material selection, thickness design and code compliance.

In practice, the thermal conductivity of wall insulation materials spans a wide range, from approximately 0.004 W/(m·K) for vacuum insulation panels up to 0.10 W/(m·K) or more for lightweight insulating mortars and foamed concrete. The measured value depends not only on the material itself but also on density, pore or fiber structure, mean temperature, moisture content, applied compressive load and, for closed-cell plastic foams, the long-term diffusion of blowing agents. A rigorous comparison therefore requires standardized specimen conditioning, standardized mean test temperature and standardized evaluation of the declared thermal conductivity λD at the 90/90 fractile, as specified in the relevant product standards such as EN 13162, EN 13163, EN 13164 and EN 13165.

• Thermal conductivity λ is an intrinsic material property; thermal resistance R and U-value are derived system properties.
• Declared thermal conductivity λD represents the value that 90% of production meets with 90% confidence.
• Design thermal conductivity applied in calculations may include corrections for moisture, aging and workmanship.
• Comparison between materials is only valid at the same mean temperature, typically 10 °C under EN standards or 25 °C under GB standards.

Testing Samples

Organic Foam Insulation Boards

This group includes expanded polystyrene (EPS, typically 18–30 kg/m³), extruded polystyrene (XPS, 28–45 kg/m³), polyurethane and polyisocyanurate foams (PUR/PIR, 30–45 kg/m³) and phenolic foam (PF, 35–60 kg/m³). Specimens are cut as flat, parallel-sided boards, commonly 300 mm × 300 mm or larger, with thickness matching the commercial product. Prior to testing, samples are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity until constant mass, following the conditioning requirements of the corresponding product standards. For PUR/PIR and XPS, aged specimens must also be prepared to evaluate long-term thermal performance, because the diffusion of cell gas progressively increases λ.

Inorganic Fiber and Granular Boards

This category covers rock wool boards (typically 80–160 kg/m³), glass wool boards (24–80 kg/m³), expanded perlite boards, foamed ceramic boards and calcium silicate boards. These materials exhibit open, fibrous or granular pore structures whose λ depends strongly on density and fiber orientation. Because they are hygroscopic to varying degrees, moisture content must be measured and controlled before testing, and the applied compressive load during measurement must reproduce the standard thickness-measurement condition defined in the product standard.

High-Performance and Composite Materials

Aerogel blankets (150–250 kg/m³) achieve very low λ through nanoporous silica networks. Vacuum insulation panels (VIPs) consist of fumed-silica cores evacuated and sealed in barrier films; their center-of-panel λ is exceptionally low, but thermal bridging along edges and long-term pressure increase must be characterized. Thermal insulation mortars, foamed concrete blocks and composite boards combining insulation cores with cementitious or mortar facings are also included in comparative programs, as they are widely used in specific regional wall systems.

Testing Items

A comprehensive comparative analysis of wall energy-saving materials evaluates the following items, each of which directly affects the comparability and engineering applicability of the results.

• Thermal conductivity λ at a defined mean temperature (10 °C or 25 °C) with a controlled temperature difference across the specimen.
• Thermal resistance R for products whose performance is thickness-dependent or non-homogeneous.
• Declared thermal conductivity λD and the statistical 90/90 fractile derived from at least ten measured values over a defined production period.
• Aged thermal conductivity for closed-cell foams (PUR/PIR, XPS) using accelerated aging conditioning specified in EN 13164 and EN 13165.
• Apparent density and thickness under standardized load, which are used to normalize and interpret λ values.
• Moisture content and moisture-dependent λ correction, since water in pores can raise λ dramatically.
• Temperature dependence of λ, measured at several mean temperatures between −10 °C and 40 °C where design conditions require it.

Testing Methods

Guarded Hot Plate Method

The guarded hot plate method is the primary absolute reference method, specified in ISO 8302, ASTM C177 and GB/T 10294. A centrally heated metering section, surrounded by a guard ring, creates one-dimensional steady heat flow through one or two specimen pairs. Because the apparatus directly measures heat flow from electrical power, calibration is not required, and measurement uncertainty can be limited to approximately 1–2%. It is the preferred method for certifying reference materials and for arbitration in disputed cases.

Heat Flow Meter Method

The heat flow meter method, specified in ISO 8301, ASTM C518, EN 12667 and GB/T 10295, sandwiches a specimen between two temperature-controlled plates and measures the heat flux with calibrated transducers. The apparatus must be calibrated using certified reference materials traceable to national metrology institutes, and the reference specimens must be similar to the tested products in λ, density and thickness. With a typical uncertainty of 2–3%, it is fast and highly productive, making it the standard tool for factory production control and routine third-party verification.

Guarded and Calibrated Hot Box Method

For evaluating complete wall assemblies rather than isolated materials, the guarded and calibrated hot box method (ASTM C1363, GB/T 13475, EN ISO 8990) measures the U-value of full-scale wall specimens including masonry, plaster layers, joints, anchors and thermal bridges. This method is essential for validating design calculations and detecting workmanship-related degradation of thermal performance.

Transient Screening Methods

Transient techniques such as the transient plane source method (ISO 22007-2) and hot-wire methods provide rapid λ estimates within minutes. They are valuable for research screening and in-process quality control, but because building product standards require steady-state values, transient results are not accepted for declaration of λD and must be confirmed by steady-state methods in accredited third-party laboratories.

Comparative Results and Discussion

Based on typical declared thermal conductivity values determined at 10 °C mean temperature in accordance with EN 12667 and the corresponding product standards, the tested wall energy-saving materials rank as follows.

• Vacuum insulation panels: λ ≈ 0.004–0.008 W/(m·K) at the panel center, rising at edges and with service life.
• Aerogel blankets: λ ≈ 0.018–0.022 W/(m·K), outstanding performance with flexibility for renovation.
• Phenolic foam and PUR/PIR boards: λ ≈ 0.020–0.026 W/(m·K), the lowest among conventional board materials.
• XPS boards: λ ≈ 0.028–0.036 W/(m·K), combining low λ with high compressive strength and low water uptake.
• EPS boards, including graphite-modified grades: λ ≈ 0.030–0.038 W/(m·K), the most widely used economic solution.
• Glass wool and rock wool boards: λ ≈ 0.032–0.042 W/(m·K), offering non-combustible fire performance (Class A1).
• Cellular glass boards: λ ≈ 0.038–0.050 W/(m·K), fully impermeable and dimensionally stable.
• Expanded perlite and foamed ceramic boards: λ ≈ 0.050–0.080 W/(m·K), non-combustible inorganic systems.
• Thermal insulation mortars and foamed concrete: λ ≈ 0.060–0.120 W/(m·K), used where thermal performance requirements are moderate.

Thickness Required for an Equivalent Thermal Resistance

To achieve a representative target thermal resistance of R = 5.0 m²·K/W for an external wall, the required thicknesses illustrate the practical significance of λ differences: approximately 100 mm of aerogel blanket, 110 mm of PIR board, 140 mm of graphite EPS, 180 mm of white EPS, 200 mm of rock wool board, and 350 mm of inorganic insulation mortar. Clearly, materials with lower λ save space and increase usable floor area, which is decisive in high-density urban renovation projects, while non-combustible materials with higher λ remain indispensable where fire safety regulations dominate.

When interpreting these comparisons, engineers must consider that λ of closed-cell foams increases over time due to blowing-agent diffusion; product standards therefore require aged values for declaration. Mineral wools are insensitive to aging but show stronger moisture dependence. VIPs deliver exceptional center-of-panel performance yet demand careful detailing to limit edge thermal bridging and panel damage. Consequently, material selection should integrate thermal conductivity with fire class, mechanical strength, dimensional stability, durability and cost, verified through type testing and periodic inspection by accredited third-party laboratories.

Applications

Comparative thermal conductivity data for wall energy-saving materials directly support the following engineering applications.

• Thermal design of external wall insulation systems (ETICS) and calculation of U-values for compliance with national energy codes such as GB 55015.
• Type testing and factory production control for manufacturers declaring λD under EN, ASTM or GB product standards.
• Independent verification of supplier datasheets by accredited third-party laboratories before large-scale procurement.
• Energy retrofit design of existing buildings, where low-λ thin materials such as aerogel or VIP preserve interior space.
• Hygrothermal and dew-point analysis of wall assemblies, for which moisture-corrected design λ values are required.
• Green building certification and building energy performance modeling using validated material properties.
• Arbitration and dispute resolution between contractors and owners, using absolute guarded hot plate measurements where necessary.

FAQ

Question: Which wall energy-saving material has the lowest thermal conductivity?

Answer: Vacuum insulation panels exhibit the lowest center-of-panel λ, typically 0.004–0.008 W/(m·K). Among conventional materials, phenolic foam and PIR boards are lowest at approximately 0.020–0.026 W/(m·K), followed by XPS, EPS and mineral wools.

Question: Why is the design thermal conductivity sometimes higher than the declared value?

Answer: The declared value λD is determined under dry, laboratory conditions at a standard mean temperature. Design values include safety allowances for moisture absorption, aging of blowing agents, installation quality and thermal bridging, and therefore are deliberately more conservative for long-term performance prediction.

Question: How much does moisture affect the measured thermal conductivity?

Answer: Water has a thermal conductivity of about 0.6 W/(m·K), roughly fifteen to twenty times that of insulation materials. Even a few percent of moisture by volume can increase λ by 10–30%, which is why specimen conditioning and moisture-content determination are mandatory parts of every standard test procedure.

Question: Can λ values measured at different mean temperatures be compared directly?

Answer: No. Thermal conductivity varies with mean temperature, generally increasing as temperature rises. EN standards commonly use a 10 °C mean temperature while GB standards frequently use 25 °C, and the difference can reach several percent. Valid comparisons must reference the same mean temperature and temperature difference.

Question: Which test method should be chosen for a formal comparative report?

Answer: The heat flow meter method per ISO 8301, ASTM C518 or EN 12667 is the standard choice for routine comparison, provided the apparatus is calibrated with traceable reference materials. For certification of reference values or arbitration, the guarded hot plate method per ISO 8302, ASTM C177 or GB/T 10294 should be used due to its status as an absolute method with the lowest uncertainty.