Conduction - Revision history

Peter: Improve FSE wiki article: Conduction; source, clarity and calculation review (GPT-6)

2026-11-02T17:29:03Z

Improve FSE wiki article: Conduction; source, clarity and calculation review (GPT-6)

Sanne: Add diagrams fse-conduction-v1.svg, fse-conduction-v2.svg, fse-conduction-v3.svg (level-1 visual plan)

2026-11-02T06:25:28Z

Add diagrams fse-conduction-v1.svg, fse-conduction-v2.svg, fse-conduction-v3.svg (level-1 visual plan)

← Older revision		Revision as of 07:25, 2 November 2026
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	The direction of net heat flow is from hot to cold. A larger temperature gradient, meaning a larger temperature change over a given distance, drives more conductive heat flow under otherwise equal conditions. Cross-sectional area also matters because a larger area provides more parallel routes for energy transfer.<ref name="Rockett-Milke-1-28" />		The direction of net heat flow is from hot to cold. A larger temperature gradient, meaning a larger temperature change over a given distance, drives more conductive heat flow under otherwise equal conditions. Cross-sectional area also matters because a larger area provides more parallel routes for energy transfer.<ref name="Rockett-Milke-1-28" />
			[[File:fse-conduction-v1.svg\|thumb\|upright=1.5\|alt=Hot wall surface transferring energy through stationary material to the cool side.\|Conduction transfers energy through a stationary solid. Based on Rockett and Milke (2002), p. 1-28.]]

	Conduction does not require a visible opening or a flowing fluid. A continuous solid can transmit heat across its thickness even when it prevents smoke from passing. This makes conductive exposure different from direct flame contact and from smoke movement through gaps.<ref name="VR-5.2" />		Conduction does not require a visible opening or a flowing fluid. A continuous solid can transmit heat across its thickness even when it prevents smoke from passing. This makes conductive exposure different from direct flame contact and from smoke movement through gaps.<ref name="VR-5.2" />
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	Thermal resistance offers a convenient way to describe several layers in series. Heat passing from one side of a composite construction to the other crosses each layer in turn. Under the steady model, the resistances of the layers can be added, just as electrical resistances in series are added.<ref name="VR-5.2" />		Thermal resistance offers a convenient way to describe several layers in series. Heat passing from one side of a composite construction to the other crosses each layer in turn. Under the steady model, the resistances of the layers can be added, just as electrical resistances in series are added.<ref name="VR-5.2" />
			[[File:fse-conduction-v2.svg\|thumb\|upright=1.5\|alt=Three material layers with resistances adding between hot and cold surfaces.\|Layer resistances add in the steady series model. Based on Van Rossum (2027), §5.2.]]

	This approach makes the contribution of an insulating layer visible. A thin layer of low-conductivity material can provide substantial resistance, while a thicker layer of highly conductive material may provide less. Interface conditions and surface heat transfer still surround the conductive path.<ref name="VR-5.2" /><ref name="Atreya-1-45">Atreya (2002), p. 1-45.</ref>		This approach makes the contribution of an insulating layer visible. A thin layer of low-conductivity material can provide substantial resistance, while a thicker layer of highly conductive material may provide less. Interface conditions and surface heat transfer still surround the conductive path.<ref name="VR-5.2" /><ref name="Atreya-1-45">Atreya (2002), p. 1-45.</ref>
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	A steady state develops only after boundary conditions have remained constant long enough for the internal temperature distribution to stop changing. During the early stage, energy entering the solid changes both its stored energy and its temperature distribution. The surface can become hot while deeper material remains close to its initial temperature.<ref name="Rockett-Milke-1-29" />		A steady state develops only after boundary conditions have remained constant long enough for the internal temperature distribution to stop changing. During the early stage, energy entering the solid changes both its stored energy and its temperature distribution. The surface can become hot while deeper material remains close to its initial temperature.<ref name="Rockett-Milke-1-29" />
			[[File:fse-conduction-v3.svg\|thumb\|upright=1.5\|alt=Temperature profiles moving deeper into a slab at early, later and steady times.\|Transient conduction changes the temperature distribution through a solid. Based on Rockett and Milke (2002), p. 1-29.]]

	The required initial condition includes temperature throughout the solid. Boundary conditions specify surface temperature or heat flux over time. These inputs distinguish one conduction problem from another and cannot be replaced by a material name alone.<ref name="Rockett-Milke-1-29" />		The required initial condition includes temperature throughout the solid. Boundary conditions specify surface temperature or heat flux over time. These inputs distinguish one conduction problem from another and cannot be replaced by a material name alone.<ref name="Rockett-Milke-1-29" />

Jan: New FSE wiki article: Conduction (written by Kimi Code CLI)

2026-11-01T22:27:14Z

New FSE wiki article: Conduction (written by Kimi Code CLI)

New page

'''Conduction''' is heat transfer through molecular interactions within a material or between materials in contact. Energy diffuses from a higher-temperature region to a lower-temperature region without requiring the bulk material to move with it. In buildings, conduction is particularly relevant to heat travelling through walls, floors, roofs and structural members.<ref name="VR-5.2">Van Rossum (2027), §5.2.</ref><ref name="Rockett-Milke-1-28">Rockett and Milke (2002), p. 1-28.</ref>

A fire can heat one side of a construction while the other side remains comparatively cool. Conduction through the construction then raises the temperature of the cooler side and can expose people, combustible contents or structural material there. The process also removes heat from hot smoke and flames into surrounding boundaries.<ref name="VR-5.2" /><ref name="IFEG-2.6">Australian Building Codes Board et al. (2005), pp. 2.6-1–2.6-2.</ref>

The rate of conduction depends on temperature difference, material properties, thickness, area and time. A steady calculation assumes temperatures no longer change, whereas a real fire produces changing surface and internal temperatures. Both views are useful, but they answer different questions.<ref name="VR-5.1">Van Rossum (2027), §5.1.</ref><ref name="Rockett-Milke-1-29">Rockett and Milke (2002), p. 1-29.</ref>

== How energy moves through a solid ==

Conduction occurs through microscopic energy transfer. In good electrical conductors, free electrons make an important contribution. In electrical insulators, lattice vibrations called phonons still allow substantial heat conduction. Imperfections scatter these carriers and diffuse their energy through the solid.<ref name="Rockett-Milke-1-28" />

The direction of net heat flow is from hot to cold. A larger temperature gradient, meaning a larger temperature change over a given distance, drives more conductive heat flow under otherwise equal conditions. Cross-sectional area also matters because a larger area provides more parallel routes for energy transfer.<ref name="Rockett-Milke-1-28" />

Conduction does not require a visible opening or a flowing fluid. A continuous solid can transmit heat across its thickness even when it prevents smoke from passing. This makes conductive exposure different from direct flame contact and from smoke movement through gaps.<ref name="VR-5.2" />

Heat can also be conducted within gases and liquids, although fire assessments often emphasise movement of those fluids as well. At a stationary surface, the fluid immediately adjacent to it has little or no bulk velocity, so molecular transfer remains part of the surface heat-exchange process.<ref name="VR-5.2" /><ref name="Atreya-1-44">Atreya (2002), p. 1-44.</ref>

== Conductivity, heat capacity and diffusivity ==

[[Thermal conductivity]] expresses how readily a material conducts heat. A higher conductivity allows more heat flow for the same temperature difference, thickness and area. It is commonly expressed in watts per metre-kelvin, linking heat flow with distance and temperature difference.<ref name="VR-5.2" /><ref name="Rockett-Milke-1-28" />

Conductivity is not necessarily constant. It can vary with temperature, moisture, density, composition and previous material treatment. Some laminates, fibrous materials and reinforced products also conduct differently along and across their grain or layers. Such materials require direction-specific properties rather than one assumed value in every direction.<ref name="Rockett-Milke-1-28" />

[[Specific heat capacity]] describes how much energy is needed to raise a unit mass by one degree. It affects how much thermal energy a material stores while its temperature rises. Conductivity concerns transmission through the material, while heat capacity concerns storage within it.<ref name="Rockett-Milke-1-29" /><ref name="Drysdale-1-91">Drysdale (2002), p. 1-91.</ref>

[[Thermal diffusivity]] combines conductivity, density and specific heat capacity. It describes how quickly a temperature disturbance can propagate through a material by conduction. Rockett and Milke identify square metres per second as its unit. A high diffusivity does not mean that the material stores more heat; it means that temperature changes penetrate more readily.<ref name="Rockett-Milke-1-29" />

The distinction matters during a fire. One construction may accept heat slowly at its surface but transmit temperature changes efficiently, while another stores considerable energy near the exposed side. Material selection therefore needs more than a single conductivity value when transient exposure is assessed.<ref name="Rockett-Milke-1-29" />

== Thickness, layers and thermal resistance ==

For a uniform layer under steady, one-dimensional conditions, a thicker layer provides more resistance to heat flow. Van Rossum explains that doubling thickness doubles this resistance when conductivity remains unchanged. Doubling conductivity instead halves the resistance. These relationships describe the simplified steady model, not every transient fire exposure.<ref name="VR-5.2" />

Thermal resistance offers a convenient way to describe several layers in series. Heat passing from one side of a composite construction to the other crosses each layer in turn. Under the steady model, the resistances of the layers can be added, just as electrical resistances in series are added.<ref name="VR-5.2" />

This approach makes the contribution of an insulating layer visible. A thin layer of low-conductivity material can provide substantial resistance, while a thicker layer of highly conductive material may provide less. Interface conditions and surface heat transfer still surround the conductive path.<ref name="VR-5.2" /><ref name="Atreya-1-45">Atreya (2002), p. 1-45.</ref>

Real constructions can depart from the simple layered model. Studs, fixings, joints and cavities create parallel paths and local thermal bridges. Direction-dependent conductivity and changing properties further alter the temperature field. A single one-dimensional resistance is therefore an approximation whose suitability depends on construction detail.<ref name="Rockett-Milke-1-28" /><ref name="Rockett-Milke-1-29" />

== Heating over time ==

A steady state develops only after boundary conditions have remained constant long enough for the internal temperature distribution to stop changing. During the early stage, energy entering the solid changes both its stored energy and its temperature distribution. The surface can become hot while deeper material remains close to its initial temperature.<ref name="Rockett-Milke-1-29" />

The required initial condition includes temperature throughout the solid. Boundary conditions specify surface temperature or heat flux over time. These inputs distinguish one conduction problem from another and cannot be replaced by a material name alone.<ref name="Rockett-Milke-1-29" />

[[Thermally thin and thick materials]] describe different responses to heat penetration. A thermally thin object can be approximated as having relatively little internal temperature variation, while a thermally thick object retains important gradients through its depth. This is a response classification, not simply a statement about geometric thickness.<ref name="Rockett-Milke-1-29" /><ref name="VR-5.1" />

Physical and chemical changes add further transient effects. Moisture can evaporate in one region, diffuse and condense elsewhere, removing and releasing energy at different locations. Decomposition and other internal changes can also act as heat sources or sinks. The simple conduction equation then needs additional terms and data.<ref name="Rockett-Milke-1-29" />

== Conduction and fire resistance ==

[[Fire resistance]] concerns the ability of a construction or member to perform required functions during fire exposure. Conduction influences the temperatures that determine separation and structural response. A barrier can remain intact against flames yet still transmit enough heat to affect its unexposed side.<ref name="IFEG-2.6" /><ref name="VR-5.2" />

For separating constructions, conductive heat flow can raise the temperature of adjacent rooms and combustible objects. For load-bearing members, the evolving internal temperature affects material properties and structural behaviour. These are different performance questions, even when both use the same heat-transfer physics.<ref name="IFEG-2.6" />

Conduction also removes energy from the fire environment. Hot gases transfer energy to walls and ceilings, and the solid then conducts part of it away or stores it. This boundary loss can influence gas temperature while simultaneously creating an exposure problem elsewhere.<ref name="VR-5.1" /><ref name="Rockett-Milke-1-29" />

An assessment should identify the quantity of interest. Heat flow through an entire area, heat flux per unit area, internal temperature and unexposed-surface temperature are not interchangeable. The required performance criterion determines which result matters and how detailed the temperature calculation must be.<ref name="VR-5.2" /><ref name="IFEG-2.6" />

== Interpreting conductive calculations ==

A conductive [[Heat transfer]] result begins with a defined temperature difference. For differences, a change of one kelvin equals a change of one degree Celsius, so either scale can express the same driving difference. Absolute temperature must nevertheless be used where another relationship, such as gas density or radiation, requires it.<ref name="VR-5.2" />

Heat flux and total heat flow answer different questions. Heat flux describes energy per unit area, while total heat flow adds the contribution over the whole exposed area. Comparing two constructions only by total watts can therefore mislead when their exposed areas differ. The relevant area and orientation must accompany the result.<ref name="VR-5.2" />

The surface temperatures used in a conduction model are not automatically equal to adjacent gas temperatures. Convection and radiation connect each gas environment to the surface, after which conduction acts within the solid. Surface resistance and solid resistance therefore belong to different parts of the wider heat-flow path.<ref name="Atreya-1-45" /><ref name="VR-5.2" />

Layer resistances provide insight into where temperature changes occur under steady conditions. A larger share of the total resistance produces a larger share of the temperature drop in the simple series model. This can help identify whether a finish, insulation layer or structural layer controls transmission, without requiring every interface temperature as an initial input.<ref name="VR-5.2" />

The model becomes less certain when properties vary with position or direction. Wood and reinforced plastics can conduct differently along and across their fibres, and laminate directions can create several paths. Selecting an average conductivity is then an additional approximation whose effect needs review.<ref name="Rockett-Milke-1-28" />

== Changes within heated materials ==

Moisture changes both energy storage and transport inside porous solids. Evaporation absorbs energy at a heated location, while condensation can release energy at a cooler location after moisture diffuses through the material. The resulting heat-flow pattern is not captured by dry, constant-property conduction alone.<ref name="Rockett-Milke-1-29" />

Temperature-dependent conductivity creates a related difficulty. A value obtained near ambient conditions may not represent the same material during a severe fire. Density, specific heat and internal structure can also change, altering both the resistance and the energy needed to raise temperature.<ref name="VR-5.2" />

Char formation and decomposition introduce moving boundaries and internal heat effects. Newly formed layers can have different conductivity and thickness from the original material. Treating the original section as unchanged throughout exposure can therefore miss both insulation and consumption mechanisms.<ref name="Rockett-Milke-1-29" /><ref name="Beyler-Hirschler-1-110">Beyler and Hirschler (2002), p. 1-110.</ref>

Conductive loss to boundaries also feeds back into the fire environment. Energy entering walls and ceilings is no longer available to heat the room gas at that moment, but the stored energy can later affect surfaces and contents. Boundary conduction is thus part of the compartment energy balance, not merely an external detail.<ref name="VR-5.1" /><ref name="Rockett-Milke-1-29" />

Construction details require the same care as bulk properties. Joints can open, fixings can form local conductive paths and cavities can change the effective transport mechanism. A calculation based on an ideal continuous slab should not be presented as a complete prediction of a built assembly without checking these differences.<ref name="Rockett-Milke-1-28" /><ref name="IFEG-2.6" />

== Limits and common misunderstandings ==

Conduction is not the movement of warm material from one room to another. It is energy transfer within or between materials. Smoke crossing an opening transports mass as well as energy, whereas a wall can conduct heat without any wall material crossing to the other side.<ref name="VR-5.1" />

A low-conductivity material is not automatically a suitable fire barrier. Performance also depends on thickness, joints, thermal stability, decomposition, structural behaviour and exposure duration. Conductivity alone says nothing about whether the construction will remain intact.<ref name="Rockett-Milke-1-29" /><ref name="IFEG-2.6" />

Nor does a cool surface prove that negligible heat has entered the construction. Energy may be stored within a thick section or consumed by moisture evaporation and decomposition. The full temperature field and time history can matter more than one surface reading.<ref name="Rockett-Milke-1-29" />

Steady calculations can estimate an order of magnitude, but Van Rossum emphasises that real fires are transient. Fire power, smoke temperature and receiving-surface temperature all change. Using a steady result as a complete fire-duration prediction would exceed the model's stated purpose.<ref name="VR-5.1" />

A reliable calculation also preserves the direction of heat flow. The hot and cold surface labels must correspond to the scenario, and a negative intermediate value should not be hidden by taking an unexplained absolute number. Direction matters when assessing whether a boundary gains or loses energy.<ref name="Rockett-Milke-1-28" />

Material data need the same dimensional check as the result. Conductivity is per unit length and temperature difference, whereas heat flux is per unit area. Mixing these quantities, or entering thickness in millimetres while the property is expressed per metre, changes the answer even when the physical description is correct.<ref name="VR-5.2" />

The selected output should finally be compared with an appropriate criterion. Heat flux through a separation, temperature at a structural point and total energy crossing a boundary are different measures. None should be substituted for another only because it is easier to calculate.<ref name="IFEG-2.6" /><ref name="Rockett-Milke-1-29" />

Where practical, simplifying assumptions can be tested by varying thickness, conductivity and exposure conditions. This shows whether the conclusion depends on one uncertain value. Such a sensitivity check is especially useful when a steady model is used to represent a changing fire.<ref name="VR-5.1" /><ref name="VR-1.7">Van Rossum (2027), §1.7.</ref>

Conduction should finally be interpreted alongside the construction's purpose. Insulation can reduce heat transmission, but structural and separating functions still have to be maintained during exposure. A favourable heat-flow result is therefore only one part of the complete fire-performance assessment.<ref name="IFEG-2.6" /><ref name="VR-5.2" />

The same principle applies during review: the calculated thermal path must remain connected to an actual construction detail and a stated fire exposure. Without those two anchors, even a numerically correct conduction result has limited engineering meaning.<ref name="Rockett-Milke-1-29" /><ref name="VR-5.1" />

== See also ==

* [[Biot number]]
* [[Heat transfer]]
* [[Thermal conductivity]]

== Notes ==

<references />

== References ==

* Atreya, A. (2002). Convection heat transfer. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). ''International fire engineering guidelines'' (2005 ed.). Australian Building Codes Board.
* Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* Rockett, J. A., & Milke, J. A. (2002). Conduction of heat in solids. In P. J. DiNenno (Ed.), ''SFPE handbook of fire protection engineering'' (3rd ed.). National Fire Protection Association.
* Van Rossum, E. (2027). ''Introduction to Fire Safety Engineering''. Fire Safety Academy.

[[Category:Physics & Heat Transfer]]