Heat Conduction

What is heat conduction and how does it differ from convection and radiation?

Heat conduction is the transfer of thermal energy through solid materials via direct molecular contact. Molecules vibrate faster when heated and collide with neighboring molecules, passing kinetic energy along a chain. No material movement occurs; the heat simply diffuses through the solid from the warm side to the cold side. This is the dominant heat transfer mechanism through building envelope components like walls, roofs, and floors.

Conduction differs fundamentally from the other two universal heat transfer mechanisms. Heat convection moves heat via the flow of fluids (air or water), which is why air spaces and ventilation play such different roles in thermal performance. Thermal radiation transfers heat as electromagnetic waves, requiring no material contact at all; this is why reflective surfaces and low-emissivity coatings matter. In a real building assembly, all three mechanisms occur simultaneously. Conduction dominates through solid layers; convection dominates in air pockets and at surfaces; radiation influences glazing, high-emissivity finishes, and thermal comfort indoors.

What is thermal conductivity (lambda) and how do materials compare?

Thermal conductivity, denoted by the Greek letter lambda (λ), is the rate at which heat conducts through a material. It is measured in watts per meter-kelvin (W/(m.K)) and represents heat flow (in watts) through 1 square meter of material, 1 meter thick, with a 1-kelvin temperature difference across it. Thermal conductivity is an intrinsic material property, independent of thickness, shape, or how much of it you use. Copper conducts heat rapidly (lambda ~400 W/(m.K)); mineral wool conducts slowly (lambda ~0.035-0.040); air is even slower (lambda ~0.026), which is why trapped air in insulation is so valuable.

Common construction materials in Slovakia show a wide range of thermal conductivity values. Dense materials like concrete and brick conduct heat faster than lighter, porous materials. The table below shows typical values for materials used in Slovak residential building and renovation.

Why does material thickness matter as much as thermal conductivity?

Thermal conductivity alone does not determine insulating value. Insulation resistance, called R-value, depends on both the material's lambda and its thickness. The relationship is simple: R = thickness (in meters) divided by lambda. A thin layer of high-performance material can outperform a thick layer of mediocre material, or vice versa. A 5 cm layer of aerogel (lambda 0.015) has R = 0.33/(0.015) = 3.33 m2.K/W, equivalent to 12 cm of mineral wool (lambda 0.040) at R = 0.12/(0.040) = 3.00. This is why the R-value appears on building product datasheets and in renovation specifications, not lambda alone.

Building codes and retrofit standards in Slovakia (based on STN norms and the EU Energy Performance of Buildings Directive) specify minimum R-values for different envelope layers, not lambda. A designer selects materials and thickness combinations that achieve the required R-value within budget and space constraints. In a passive house or low-energy retrofit, wall layers might combine 5 to 15 cm of insulation depending on the material chosen, the existing structure, and the desired annual heating demand.

How do conduction, convection, and radiation work together in real building assemblies?

A single wall is never pure conduction. Consider a typical external wall in a Slovak renovation: warm indoor air contacts the interior plaster by convection, heats the surface, then heat conducts through plaster, concrete, insulation, and exterior sheathing. At each material boundary, conduction continues; in air gaps, convection and radiation supplement or replace conduction. The exterior surface radiates heat to the cold night sky and exchanges heat with wind-driven convection from outdoor air. The system integrates all three mechanisms into one overall thermal transmittance, expressed as the U-value (W/(m2.K)). A well-designed wall minimizes all three pathways: thicker insulation (reduces conduction), continuous insulation layers (reduces convection in gaps), and correct material sequencing (prevents condensation that would short-circuit insulation by wicking).

All three heat transfer mechanisms operate in real buildings, often simultaneously. The table below compares how each mechanism works and where it dominates in a typical building envelope.

Where do building owners encounter heat conduction specifications?

Building owners meet heat conduction numbers in several practical contexts. When buying insulation products, datasheets list lambda (thermal conductivity), R-value per cm or per product, and U-value for standard assemblies. Renovation quotations specify the insulation type, thickness, and expected U-value post-retrofit. Window specifications include the Ug-value (glazing U-value, dominated by conduction and radiation through glass) and Uf-value (frame U-value, dominated by conduction through frame materials and convection in cavities). Energy audit reports (mandatory for large renovations under STN 730545 and EU regulations) calculate heating demand based on U-values of each envelope layer, directly tied to conduction rates. Passive house certifications require measured U-values (often supplied as thermal resistance certificates from manufacturers) and careful checking of thermal bridge details.

In practice, a homeowner in Bratislava or Banská Bystrica commissioning an insulation retrofit receives a quote specifying exterior wall U-value goals (perhaps 0.15 to 0.20 W/(m2.K) for a low-energy retrofit, down from 0.6 to 1.0 in typical 1970s-1980s panel housing). The contractor selects mineral wool or EPS thickness to achieve it. Higher-performance materials allow thinner layers; cheaper materials require more thickness. The choice balances cost, space (important in renovations where exterior dimensions matter), and vapor permeability (essential in Slovakia's humid continental climate to avoid interstitial condensation). Understanding conduction helps owners ask the right questions: Why is this thickness specified? How does the proposed material's lambda compare to alternatives? Are thermal bridges being addressed in the design?

What are common misconceptions about heat conduction in buildings?

A widespread misconception is that thicker materials always insulate better. A 20 cm layer of dense concrete (lambda 1.8) gives R = 0.20/(1.8) = 0.11, barely better than 1 cm of mineral wool (lambda 0.040) at R = 0.01/(0.040) = 0.25. Thermal mass is conflated with insulation; thermal mass stores and slowly releases heat, but it does not reduce average heat loss; it delays and smooths it. Heavy concrete walls feel warmer in evening because mass releases stored solar and internal gains, but they conduct heat away in winter just as readily as any other material of the same U-value.

Another myth is that conduction can be stopped. It cannot, only slowed. Every material conducts heat to some degree. The goal is to minimize it via thick insulation layers and to prevent shortcuts via thermal bridges. Air is an excellent insulator (lambda 0.026) only when trapped; moving air (convection) defeats insulation. This is why ventilated air gaps in roofs are cold air spaces that help drain wind-driven rain but do not insulate; convection inside them carries heat upward. Similarly, some designers assume that reflective foil on insulation reduces conduction significantly; it primarily reduces radiation on the foil's surfaces, a separate mechanism. The conduction through the insulation layer itself remains governed by its lambda and thickness.