Thermal conductivity

Fibre Reinforced Facade Encyclopedia: Properties

Thermal conductivity is the rate at which heat passes through a material, expressed in watts per metre kelvin (W/m.K). It is a material property, independent of thickness; the thermal resistance of a build-up is the thickness divided by the conductivity. For facade panels the figure matters in two ways that are often confused. It is almost irrelevant to the overall thermal performance of the wall, because a 15 mm panel contributes negligible resistance next to the insulation behind it, and it is very relevant to thermal bridging, because the brackets and rails that hold the panel pierce the insulation and their conductivity is what drives the heat loss.

At a glance

What the number does and does not tell you

Conductivity describes the material. Resistance describes the layer, and it is thickness divided by conductivity. A 15 mm panel of glass fibre reinforced concrete (GRC, GFRC) at 0.39 W/m.K gives about 0.04 m².K/W. That is close to the resistance of a still air film. It is not a thermal layer in any meaningful sense, and no facade panel of this thickness in any material is.

The consequence for specification is simple. Selecting a facade panel material on conductivity alone is a category error. Two panel materials that differ by a factor of two in conductivity produce a difference in wall U-value that is lost in the rounding, once insulation is included. The panel’s real contributions to thermal performance are the shading of the wall behind it, the ventilated cavity in a rainscreen build-up, and the substructure that penetrates the insulation.

Why the substructure dominates

Aluminium conducts at roughly 205 W/m.K, about five hundred times the conductivity of a GRC panel and about six thousand times that of mineral wool. Every aluminium bracket that passes through the insulation to reach the structure is a short circuit around it. In a poorly detailed ventilated facade the bracket penetrations can degrade the effective U-value of the wall by twenty to forty percent against the naive calculation that ignores them, which is why the National Construction Code and equivalent energy codes require thermal bridging to be accounted for rather than ignored (check the edition adopted in your jurisdiction).

The mitigations are well established: fewer and larger brackets rather than many small ones, stainless steel or glass fibre reinforced polymer isolators at the structure interface, thermal break pads under the bracket, and continuous insulation with the rails outboard rather than interrupting the layer. Lightweight panels help here, indirectly but substantially. A panel at 30 to 50 kg per square metre needs a smaller and less frequent bracket than a precast panel at 360 kg per square metre, so there is less metal through the insulation to begin with.

What changes the figure

Published conductivity values are measured on dry material at a reference temperature, and real panels are neither.

• Density: conductivity rises with density in cementitious materials, because solids conduct better than the air in the pores. A lower density mix conducts less
• Moisture content: water conducts around twenty five times better than air, so a saturated cementitious panel conducts considerably more than a dry one. This is one reason water absorption is worth specifying
• Temperature: conductivity varies modestly with mean temperature; published figures state the test condition
• Fibre content and orientation: in a polymer laminate the glass conducts better than the resin, so conductivity rises with fibre volume fraction and is slightly anisotropic
• Coatings and finishes: negligible at these thicknesses

For design calculations, use the figure the manufacturer publishes together with the condition it was measured at, and use a design value with a moisture allowance where the material will be wetted in service.

Where panel conductivity does matter

There are cases where the panel itself is thermally significant, and they are worth naming because they are the exceptions that justify quoting the number at all.

Surface temperature and thermal expansion. A panel with low conductivity and low thermal mass heats up fast on a dark colour in direct sun. Surface temperatures of 60 to 80 °C are reached on dark facades in Australian conditions, and the resulting thermal expansion drives joint width and the movement capacity the anchors must provide. A fibre reinforced polymer panel, which has both lower conductivity and a higher coefficient of thermal expansion than a cementitious panel, moves more.

Condensation risk. In a wall where the panel sits close to or against the insulation rather than across a ventilated cavity, the position of the dew point within the build-up depends on the resistance of each layer, and the panel layer has to be entered into the calculation even though it is small.

Interior linings. Where a gypsum or cementitious panel is used as an interior lining rather than an external skin, it is one of very few layers in the build-up, so its properties carry proportionally more weight.

Fire. Low conductivity slows heat transfer through a section, which affects how a build-up behaves in a fire resistance test, but fire resistance level is a system property established by test to AS 1530.4 and is never inferred from conductivity.

Standards and references

• ISO 8302 and ASTM C177, guarded hot plate determination of steady-state thermal transmission properties
• ASTM C518 and ISO 8301, heat flow meter method
• ISO 22007 series, transient methods for thermal conductivity of plastics and composites
• ISO 6946, building components and building elements, calculation of thermal transmittance
• ISO 10211, thermal bridges in building construction, detailed calculations
• NCC 2022 Volume One Section J, energy efficiency, including thermal break requirements (check the edition adopted in your jurisdiction)
• AS/NZS 4859.1, thermal insulation materials for buildings, material properties

Related terms

• Thermal bridging
• Thermal expansion
• Density
• Water absorption
• Rainscreen
• Substructure
• Lightweight cladding

Questions specifiers ask

Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.