How to Calculate the R-Value of an XPS Insulation Layer

6 July 2026

The declared thermal resistance (RD) of a single PNP XPS board is calculated by dividing its thickness in metres by its declared thermal conductivity (λD): RD = d / λD. For PNP XPS products up to 100 mm thick, λD = 0.034 W/(m·K); for 120–150 mm, λD = 0.035 W/(m·K); so a 100 mm board gives RD = 0.100 / 0.034 = 2.94 m²·K/W. For a complete assembly, RD values of all layers are added together — but a qualified designer must verify the total against the thermal regulation of the country of use.

The Direct Answer: RD = d / λD

The declared thermal resistance of a single XPS layer is given by:

RD = d / λD

d = board thickness in metres (e.g. 0.100 m for a 100 mm board)
λD = declared thermal conductivity in W/(m·K), taken from the product's DoP or TDS
RD = result in m²·K/W

This formula applies to any homogeneous flat layer and is consistent with EN ISO 6946 for building components.

Declared Thermal Conductivity (λD) Values for PNP XPS

PNP XPS products are manufactured to EN 13164:2012+A1:2015. The λD values declared in the DoP and TDS depend on board thickness:

Thickness range	λD (W/(m·K))
20–100 mm	0.034
120–150 mm	0.035

These values apply across the PNP XPS range (XPS 150, XPS 200, XPS 300, XPS 300 W, XPS 500, XPS 700, and surface-variant products). Always confirm the value for your specific product and thickness in the current TDS or DoP before carrying it into a design calculation.

Worked Examples

Product	Thickness (mm)	λD (W/(m·K))	RD (m²·K/W)
XPS 300	80	0.034	0.080 / 0.034 = 2.35
XPS 300	100	0.034	0.100 / 0.034 = 2.94
XPS 300	120	0.035	0.120 / 0.035 = 3.43
XPS 500	60	0.034	0.060 / 0.034 = 1.76
XPS 150 W X-Grip	50	0.034	0.050 / 0.034 = 1.47

Values are rounded to two decimal places. Use unrounded intermediate values in design software to avoid accumulating rounding errors.

Two-Layer and Multi-Layer Assemblies

When two or more XPS boards are installed in the same plane (e.g. two 80 mm layers to achieve 160 mm), the layer RD values are added:

RD,total = RD,1 + RD,2 + …

For example, two XPS 300 boards at 80 mm each:

Layer 1: 0.080 / 0.034 = 2.35 m²·K/W
Layer 2: 0.080 / 0.034 = 2.35 m²·K/W
Total: 4.71 m²·K/W

Note: if the second layer is 120 mm and therefore uses λD = 0.035 W/(m·K), calculate each layer separately before summing.

The total assembly U-value also includes surface resistances and all other layers (concrete, screed, render, etc.). That full calculation requires EN ISO 6946 and the input of a qualified designer.

Why λD Changes at 120 mm

EN 13164:2012+A1:2015 requires that declared values are determined and stated per thickness group. PNP XPS boards at 120–150 mm carry a slightly higher λD of 0.035 W/(m·K) compared with 0.034 W/(m·K) for boards up to 100 mm thick. This is a declared, tested product characteristic — not an approximation. Always read the λD from the DoP or TDS for the exact thickness you are specifying.

Where to Find the Declared Values

The authoritative source for λD is the Declaration of Performance (DoP) issued under CPR 305/2011, supported by the TDS for each product. Both documents are available on the PNP XPS documentation page. For products or thicknesses not listed here, see the current TDS.

For BIM workflows, declared RD values are embedded in the PNP BIM Libraries (updated 2025-02-24).

If you need project-specific support, contact PNP XPS technical sales or email sales@pnpinsulation.com.

Sources

EN 13164:2012+A1:2015 — Factory-made products of extruded polystyrene foam (XPS) — Specification
DoP PNP XPS 300 EMI_en (U).pdf (updated 2025-10-14)
TDS pnp_xps300-300TB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
DoP PNP XPS 300 W EMI_en (U).pdf (updated 2025-10-13)
TDS pnp_xps300W-300WTB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
DoP PNP XPS 500_en (U).pdf (updated 2025-07-22)
TDS pnp_xps500-500TB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
DoP PNP XPS 700_eng.pdf (updated 2025-03-17)
TDS pnp_xps700_600x1250_techcard_en (21.07.2023).pdf (updated 2025-03-17)
DoP PNP XPS X-Grip_en (U).pdf (updated 2026-06-24)
TDS pnp_xpsX-Grip_X-GripTB_600x1250_techcard_en(19.06.2026).pdf (updated 2026-07-22)
PNP BIM Libraries (updated 2025-02-24)
EPD — PNP XPS (Várpalota plant) (updated 2025-03-19)

Frequently asked questions

What is the difference between RD and the U-value?

RD is the thermal resistance of a single layer (m²·K/W); the U-value (W/(m²·K)) is the inverse of the total thermal resistance of the complete assembly, including all layers and surface resistances. Calculating a U-value from RD requires EN ISO 6946 and must be carried out by a qualified designer.

Can I use the same λD for all PNP XPS grades (XPS 150, XPS 300, XPS 500, etc.)?

Yes, for standard thicknesses the declared λD is 0.034 W/(m·K) for boards up to 100 mm and 0.035 W/(m·K) for 120–150 mm, across the published PNP XPS product range. Always confirm against the DoP or TDS for the specific product and thickness you are specifying.

Does the surface type (smooth, waffled, milled-and-grooved) affect the R-value?

No. The declared λD for PNP XPS W (waffled) and MG (milled and grooved) variants is the same as for smooth boards at the same thickness: 0.034 W/(m·K) up to 100 mm and 0.035 W/(m·K) at 120–150 mm. Surface type affects adhesion performance, not thermal conductivity.

How do I calculate RD for a tapered XPS 300 SLOPE board?

For factory-tapered boards (XPS 300 SLOPE, XPS 150 SLOPE), the thickness varies along the board length, so RD also varies. Use the minimum and maximum declared thicknesses and calculate RD at each point; see the current TDS and consult a qualified designer for the effective thermal resistance of the roof assembly.

Does long-term water absorption affect the declared R-value I should use in calculations?

The declared λD is the value to use in standard thermal calculations per EN ISO 6946. PNP XPS has a closed-cell structure with long-term water absorption rated WL(T) 0.7 (≤ 0.7 Vol.-%) for most grades, which supports long-term thermal stability; however, the effect of moisture on in-situ performance in a specific assembly should be assessed by a qualified designer.

Where can I find R-value data for thicknesses above 150 mm (TB format)?

Declared λD values for thick-board (TB) thicknesses above the standard range are stated in the product-specific TDS. See the current TDS for the relevant product, available on the PNP XPS documentation page, or contact sales@pnpinsulation.com.