WL(T) Long-Term Water Absorption in XPS Explained

9 September 2026

WL(T) is the declared long-term water absorption by total immersion, expressed as a percentage of board volume (Vol.-%), measured and classified under EN 13164:2012+A1:2015. A lower WL(T) class means the board absorbs less water over time, preserving its thermal resistance and mechanical integrity in permanently wet or buried conditions. PNP XPS smooth and waffled-surface boards achieve WL(T)0,7 (≤ 0.7 Vol.-%), while milled-and-grooved surface boards are classified WL(T)3 (≤ 3.0 Vol.-%).

What WL(T) Measures

WL(T) stands for long-term water absorption by total immersion. Under EN 13164:2012+A1:2015, a board is submerged in water for an extended period and the volume of water absorbed — relative to the total board volume — is recorded as a percentage. The result is declared as a class, for example WL(T)0,7 meaning the board absorbs no more than 0.7 Vol.-% of water over the test period.

This is distinct from WD(V), which measures water absorption by diffusion through the board faces under a partial-immersion regime. The two tests reflect different exposure scenarios: WL(T) is most relevant where a board may be fully surrounded by water or saturated soil, such as in inverted roofs, foundation slabs, or below-grade perimeter insulation.

Why Low Water Absorption Matters in Practice

Water is a far better conductor of heat than air or closed-cell polymer foam. Any water that enters an insulation board displaces the insulating gas in the cells and directly raises the effective thermal conductivity of the board. A board with a high WL(T) value — absorbing several percent of its volume as water — will deliver less thermal resistance in service than its declared λD value suggests.

Beyond thermal performance, absorbed water can:

Add significant mass, increasing loads on the structure below.
Reduce the long-term compressive and tensile strength of the board.
Create conditions for freeze–thaw damage in cold climates — which is why EN 13164 also defines the FTCD (freeze–thaw after diffusion absorption) class.

For buried and submerged applications, a specifier should therefore treat WL(T) as a primary durability indicator alongside compressive strength and dimensional stability (DS(70,90)).

WL(T) Classes Across PNP XPS Products

PNP XPS boards fall into two WL(T) groups depending on surface type. Smooth and waffled (W / X-Grip) boards achieve the tighter class; milled-and-grooved (MG) boards, where the surface has been mechanically opened for adhesion, are classified at the higher class.

Surface typeWL(T) classMaximum absorptionExample productsSmoothWL(T)0,7≤ 0.7 Vol.-%XPS 300, XPS 500, XPS 700Waffled (W / X-Grip)WL(T)0,7≤ 0.7 Vol.-%XPS 300 W, XPS 150 W X-GripMilled and grooved (MG)WL(T)3≤ 3.0 Vol.-%XPS 150 MG, XPS 200 MG, XPS 300 MG

The higher WL(T)3 value on MG boards is a direct consequence of the milled surface: the mechanical grooving opens the outer cell structure to provide adhesion for plasters and renders, but in doing so increases the potential uptake path for water. MG boards are therefore typically used in facade and ETICS applications where they are protected by a render coat, not in direct long-term water contact.

WL(T) and Related Hygric Performance Declarations

EN 13164:2012+A1:2015 allows manufacturers to declare additional hygric properties alongside WL(T). Several PNP XPS grades also carry the following declarations:

WD(V) — long-term water absorption by diffusion (partial immersion). Smooth and waffled boards in the 200, 300, 500 and 700 grades are declared at WD(V)3 (≤ 3.0 Vol.-%); XPS 300 MG is declared at WD(V)4 (≤ 4.0 Vol.-%).
FTCD — freeze–thaw resistance after WD(V) conditioning. Grades carrying this declaration (XPS 200, 300, 300 W, 300 MG, 500, 700 and their variants) are rated FTCD1 (≤ 1.0 Vol.-% change after freeze–thaw cycling), confirming dimensional stability is maintained after combined moisture and frost exposure.

Where a project involves frost-exposed buried or exposed edges — for example, perimeter insulation at ground level in a cold climate — specifiers should verify that the chosen grade carries both a WL(T) and an FTCD declaration. Consult the current TDS and DoP for each grade, and involve a qualified designer for the overall hygrothermal assessment.

Selecting the Right WL(T) Class for Your Application

The table below maps common applications to the WL(T) class most frequently specified. This is guidance only — the final decision requires assessment by a qualified designer using project-specific conditions.

ApplicationTypical WL(T) class neededNotesFoundation slabs, below-grade walls, perimeter insulationWL(T)0,7Board in permanent contact with saturated soil or groundwaterInverted (upside-down) flat roofsWL(T)0,7Board sits above waterproofing membrane, exposed to rainwater pondingFloor insulation under screed (dry interior)WL(T)0,7Good practice; vapour barrier still required per designETICS facades (render applied directly to board)WL(T)3 acceptableBoard protected by render; MG or W surface required for adhesionRoad and civil infrastructureWL(T)0,7Long service life, difficult to replace; smooth boards specified

Sources

EN 13164:2012+A1:2015 — Factory made products of extruded polystyrene foam (XPS)
TDS pnp_xps300-300TB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
TDS pnp_xps300W-300WTB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
TDS pnp_xpsX-Grip_X-GripTB_600x1250_techcard_en(19.06.2026).pdf (updated 2026-07-22)
TDS pnp_xps150W-150WTB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
TDS pnp_xps500-500TB_600x1250_techcard_en(20.07.2026).pdf (updated 2026-07-22)
TDS pnp_xps700_600x1250_techcard_en (21.07.2023).pdf (updated 2025-03-17)
TDS pnp_xps300MG_600x1250_techcard_en (21.07.2023).pdf (updated 2025-03-17)
TDS pnp_xps150MG_600x1250_techcard_en (21.07.2023).pdf (updated 2025-03-17)
TDS pnp_xps200MG_600x1250_techcard_en (21.07.2023).pdf (updated 2025-03-17)
DoP PNP XPS 300 EMI_en (U).pdf (updated 2025-10-14)
DoP PNP XPS 300 W EMI_en (U).pdf (updated 2025-10-13)
DoP PNP XPS X-Grip_en (U).pdf (updated 2026-06-24)
DoP PNP XPS 500_en (U).pdf (updated 2025-07-22)
DoP PNP XPS 700_eng.pdf (updated 2025-03-17)
DoP PNP XPS 300 MG_eng.pdf (updated 2025-03-17)
EPD — PNP XPS (Várpalota plant, updated 2025-03-19)

Frequently asked questions

Is WL(T)0,7 the same as 'zero water absorption'?

No. WL(T)0,7 means the board absorbs no more than 0.7% of its own volume as water during the full-immersion test — not literally zero. In practice this is extremely low, and the closed-cell structure of XPS limits water entry almost entirely to cut edges and surface damage rather than through intact cell walls.

Why do waffled (W) boards achieve the same WL(T)0,7 class as smooth boards when their surface is embossed?

The waffled embossing creates a mechanical pattern of bumps on the outer skin but does not cut through the closed cells in the way that milling and grooving does. The cell structure beneath the waffled skin remains intact, so long-term immersion absorption stays within the WL(T)0,7 limit, as confirmed in the declarations for XPS 300 W and XPS 150 W X-Grip.

Can I use an MG board in a below-grade perimeter wall application?

MG boards carry WL(T)3 (≤ 3.0 Vol.-%), which is significantly higher than WL(T)0,7. For applications in direct long-term contact with soil moisture or water, smooth or waffled boards with WL(T)0,7 are the appropriate choice. Involve a qualified designer to confirm the correct product for your specific detail.

How does WL(T) relate to the declared thermal conductivity (λD)?

λD is declared and tested on dry boards under standard laboratory conditions. In a wet application, absorbed water increases the effective thermal conductivity in service. A board with WL(T)0,7 loses negligibly little of its declared thermal performance to moisture uptake, whereas a board with higher WL(T) in a permanently wet location may underperform its λD value. See the current TDS for each product for declared values.

Is WL(T) declared in the DoP, and where can I find it?

Yes. WL(T) is a harmonised essential characteristic under EN 13164:2012+A1:2015 and must be declared in the Declaration of Performance (DoP) for each product. PNP XPS DoPs are available from the <a href="/documentation/">documentation page</a>; the exact declared value is also listed in the product TDS.

Does a low WL(T) value affect the environmental profile of the board?

The EPD for PNP XPS boards (EN 15804, Várpalota plant) addresses the environmental product declaration. Whether WL(T) influences environmental scoring depends on the assessment methodology and system boundary used. See the EPD — PNP XPS (Várpalota plant, updated 2025-03-19) for full environmental data.