R-Value (Thermal Resistance)

What is R-value and how does it differ from U-value?

R-value, or thermal resistance, is a material property measuring resistance to heat flow, expressed in m²·K/W. The higher the R-value, the better the insulation. Unlike U-value, which describes a complete building element's thermal performance, R-value refers to a single material layer or its thermal resistance component. In Slovak building codes and EU regulations, U-value dominates because it captures real-world performance of assembled elements. R-value appears on product sheets and in design calculations. This distinction is important: R and U are not simple inverses once air-film resistances are included in the calculation.

How is R-value calculated for a material layer?

For any homogeneous material, R-value is calculated using R = d / lambda, where d is thickness in meters and lambda is thermal conductivity in W/(m·K). Mineral wool insulation with lambda = 0.04 and 150mm thickness has R = 0.15 / 0.04 = 3.75 m²·K/W. Different insulation types have very different lambda values: expanded polystyrene (EPS) at 0.04, but timber at 0.12, and brick at 0.60. The same thickness of different materials produces wildly different R-values. Manufacturers provide both thickness and lambda on technical data sheets, allowing architects to verify R-value or calculate it independently. This transparency is why R-value is useful at the product level, even though building regulations use U-value exclusively. Understanding the R calculation helps architects select the right insulation thickness to meet target assembly U-values.

How do surface air resistances complicate the R-to-U relationship?

A complete building assembly's R_total must include air-film resistances at both surfaces. When heat flows through a wall, it passes through: (1) inside air boundary layer (R = 0.13 m²·K/W), (2) all material layers and their resistances, and (3) outside air boundary layer (R = 0.04 m²·K/W). Only when you sum all these resistances does R_total relate to U-value. A wall with material layers totalling R = 5.5 m²·K/W, plus surface resistances of 0.17 m²·K/W, gives R_total = 5.67 m²·K/W, yielding U = 0.176 W/(m²·K). Without surface resistances, you would incorrectly calculate U = 0.182 W/(m²·K). Standard calculation methods in EN ISO 6946 automatically include these surface resistances, so engineers use them without manual calculation in practice. Understanding this prevents confusion when comparing material R-values to whole-element U-values in regulations.

Where do R-values appear in actual construction practice?

Every insulation product carries R-value on technical documentation: mineral wool rolls, foam boards, fibreglass mats, and wood-fibre products all display it. When you order 100mm mineral wool, its R-value is immediately visible, helping you judge performance before installation. Engineers use product R-values to design building assemblies: sum R-values of all planned layers plus surface air-film resistances to calculate R_total, then verify U = 1 / R_total meets project requirements. For windows and doors, manufacturers quote frame and glazing thermal resistance separately, though whole-window U-value (Uw) is what matters for compliance. In Slovakia, where STN 73 0540 mandates U-value limits, the universal workflow is: specify materials using their R-values from data sheets, sum to R_total including surface resistances, calculate resulting U-value, confirm compliance. R-value is therefore the practical language of product selection and design calculation, even if U-value is the language of regulation and energy certificates.

What are typical R-values for common Slovak building materials?

This table shows R-values for common materials at standard thicknesses, calculated from typical lambda values. These are approximate and can vary by product and density:

How do R-values relate to building standards and energy classes?

Different standards require different total R-values to meet U-value limits. This table shows how to work backwards from regulations to required assembly R-values. When calculating a wall assembly, add surface air-film resistances (0.17 m²·K/W total) to material layer R-values to reach the target R_total. For example, a passive-house wall must achieve U less than 0.15, which requires R_total greater than 6.67 m²·K/W. A designer selects 180mm of insulation (R approx 4.5) plus other layers to reach this target.

Energy class of the final building depends on modelled energy demand using these U-values plus solar gains and ventilation losses.

Is R-value used anywhere in Slovakia or modern EU practice?

R-value is not a metric in Slovak building codes or EU directives. However, it remains the universal language of product specifications and design calculations. Every insulation manufacturer worldwide provides R (or its components: thickness and lambda) on data sheets. Engineers routinely use R-value calculations as an intermediate step: pick materials, sum their R-values, calculate resulting U, verify compliance with STN 73 0540. Some older European documents may reference R-value, but modern practice in Slovakia and the EU uses U-value exclusively for regulations. The USA is an exception: American building codes and the insulation industry primarily market products by R-value in imperial units. A Slovak architect working on international projects must be comfortable converting between American R-values and European U-values. The two systems coexist: R for material properties and design calculations, U for building regulations and energy performance documentation.