BUILDING PHYSICS / ENGINEERING GUIDE

Thermal Conductivity of Common Building Materials: How k Affects R and U

Reviewed by WattCostLab Editorial Team · Updated September 2, 2026

Thermal conductivity k tells the wall model how readily a material conducts heat. At the same thickness, lower k produces a larger layer resistance R = x/k.

What thermal conductivity means in a wall calculation

Thermal conductivity, usually written k, is the material property that appears in the plane-layer resistance equation:

If thickness x stays constant, decreasing k increases thermal resistance. If k stays constant, increasing thickness increases resistance. This simple relationship is the reason insulation layers can contribute a large share of total wall resistance despite being thinner than structural materials.

Illustrative conductivity presets used by the WattCostLab calculator

The calculator includes editable presets to make scenario setup faster. They are not universal design values. The exact conductivity of a real product or construction can differ, so replace them with the value from the documentation you are using.

Example: why thickness and k must be considered together

Take two hypothetical 50 mm layers. A layer with k = 0.040 W/mK has R = 0.05/0.040 = 1.25 m²K/W. A layer with k = 0.20 W/mK at the same thickness has R = 0.25 m²K/W. The first layer provides five times the conductive resistance in this simplified calculation.

Now keep k = 0.040 W/mK and double the thickness from 50 mm to 100 mm. R doubles from 1.25 to 2.50 m²K/W. That linear relationship is the basis of the target-U insulation-thickness calculation.

Why generic conductivity values vary

Values quoted for broad material categories can differ because real products have different composition, density and structure. Test conditions and moisture state can also matter. That is why the calculator treats material names as convenient labels and allows conductivity to remain fully editable.

How k affects the whole wall U-value

Changing one layer's conductivity changes that layer's resistance, which changes R_total and therefore changes U = 1/R_total. The effect is strongest when the modified layer accounts for a large share of the total resistance.

The WattCostLab resistance bars make this visible. An insulation layer with a large R contribution causes a larger modeled temperature drop across that layer and a larger reduction in the overall U-value.

Thermal conductivity is not U-value

k has units W/mK and describes a material. U has units W/m²K and describes the complete modeled assembly. A wall can contain many materials with different k-values, and the final U-value depends on all layer thicknesses, all conductivities and the surface resistances.

Using the material table responsibly

The table above is best used to understand scale and to build preliminary scenarios. It should not be cited as a product performance declaration. If two candidate materials differ only slightly in generic k-value, product-specific data and construction details become even more important.

Scientific basis

The layer resistance and multilayer series model used by the WattCostLab calculator follows the methodology documented in Paraschiv et al., Energy Reports 6 (2020), 343–353. DOI: 10.1016/j.egyr.2020.08.055.

How to convert conductivity into resistance per thickness

A convenient way to compare candidate materials is to calculate resistance for the same thickness. For a 100 mm layer, x = 0.10 m. If k = 0.040 W/mK, R = 2.50 m²K/W. If k = 0.13 W/mK, R ≈ 0.769 m²K/W. If k = 0.72 W/mK, R ≈ 0.139 m²K/W. These are mathematical examples using the entered k-values, not declarations about any specific commercial product.

You can also calculate the thickness required for a selected layer resistance: x = R·k. If the desired added R is 2.0 m²K/W, a material entered at k = 0.040 W/mK requires 0.080 m in the idealized model, while a material entered at k = 0.035 W/mK requires 0.070 m.

Why material names alone are not enough

Broad names such as “brick,” “concrete,” “wood” or “mineral wool” describe families, not single invariant thermal properties. Density, porosity, formulation and moisture can differ. Even products with the same generic name may publish different declared thermal conductivities. The calculator therefore lets you overwrite every preset immediately.

Conductivity and temperature profile

For a fixed steady heat flux, the temperature drop across a layer is qR. Since R = x/k, a low-k or thick layer produces a larger temperature drop. This is visible in the calculator's temperature chart and is useful for checking where the thermal gradient is concentrated.

Choosing a value for preliminary versus project-specific analysis

For a classroom example or early sensitivity study, an illustrative preset can be sufficient as long as it is labeled. For a project calculation, use the value required by the applicable method: a product declaration, project specification, standard table or measured value as appropriate. Record the source and basis with the calculation so another person can reproduce it.

That source discipline is more important than adding extra decimal places. A precisely typed generic value is not necessarily more accurate than a rounded product-specific value that actually applies to the construction being modeled.