Understanding Thermal Performance and How R Values and U Values Are Calculated for Portland Stone

Albion Stone is committed to helping architects, engineers and designers understand the technical performance of the materials they work with. One of the most important measures in modern building design is the U‑value, which describes how easily heat passes through a building element such as a wall, façade or cladding panel. To calculate a U‑value accurately, it is essential to understand how R‑values are derived, and how they relate to the thermal conductivity of Portland Stone.

Where the R‑Value for Stone Comes From

The R‑value of any material layer is a direct expression of its thermal resistance. It is not a fixed number supplied by manufacturers but a calculated value that depends on two things:

1. The thickness of the material (d) — measured in metres
2. The thermal conductivity (k) — measured in W/mK

The formula is simple:

This means the R‑value of Portland Stone changes naturally with the thickness selected in a design.

For Portland Stone specifically, Albion Stone has confirmed a thermal conductivity (k) of 1.53W/mK. Using this value and the thickness of the stone, the R‑value is calculated immediately.

This relationship allows designers to assess the thermal contribution of any thickness of Portland Stone, from thin cladding units to thick ashlar blocks.

What Rsi and Rse Are — and Why They Matter

When calculating a U‑value for a building element, the thermal resistance of the material layers is not enough on its own.

Heat must also cross the thin layers of air sitting on both sides of the wall:

Internal surface resistance (Rsi)
This represents the resistance created by the still air inside a building. Indoors, air movement is relatively gentle, so this layer provides a modest but important amount of thermal resistance.

External surface resistance (Rse)
This represents the resistance created by the moving outdoor air, which is affected by wind and environmental conditions. It is typically lower than the internal value.

These two values do not depend on the stone or any other material.
They come from established international standards, such as BS EN ISO 6946, and are applied universally in U‑value calculations.

The standard values used in the UK are:

• Rsi = 0.13 m²K/W (internal air film)
• Rse = 0.04 m²K/W (external air film)

These values are added to the R‑value of the material layers to calculate the total thermal resistance of the complete building element.

Using the K‑Value of Portland Stone (1.53 W/mK)

With the confirmed thermal conductivity of Portland Stone at 1.53 W/mK, the resistance of any thickness can be calculated directly.

For a thickness d, the resistance is:

Once this is known, the total resistance is:

And the final U‑value becomes:

This approach allows precise modelling of Portland Stone in any wall build‑up, whether used alone or in combination with insulation.

Example: Calculating the U‑Value for 100 mm Portland Stone

Step 1 — Stone resistance

Thickness: 100 mm = 0.10 m
Conductivity: 1.53 W/mK

Step 2 — Add surface resistances

• Internal surface resistance: Rsi = 0.13
• External surface resistance: Rse = 0.04

Step 3 — Calculate U‑value

Final U‑value for 100 mm Portland Stone: 4.25W/m²K

 

The thermal performance of Portland Stone can be assessed accurately using its confirmed conductivity of 1.53 W/mK. By combining this with standard surface resistances and the thickness of the stone, designers can calculate a reliable U‑value for any application.

Albion Stone remains committed to providing clear, transparent and technically robust data to support best‑practice design and specification.

Tags: U Value Thermal Performance