Multi-layer cladding

On a ventilated facade the outer skin — the one that takes the wind — is the least loaded of the two. EN 1991-1-4 §7.2.10.

Layer configuration

The ordinary ventilated facade: rainscreen outside with uniformly distributed openings, sealed membrane behind. The outer skin takes 2/3 c_pe in overpressure and only 1/3 c_pe in suction.

Pressure coefficients

c_pe

Positive for overpressure, negative for suction. The fraction applied to a permeable outer skin depends on the sign.

c_pi q_p [N/m²]

Air cavity

NOTE 2 applies only when the air flow between the layers is closed at the ends AND the clear distance is below 100 mm.

Clear distance between layers [mm] air cavity closed at the ends (Fig. 7.14a)

▸ Fill in the data on the left and press Calculate

Tool information

What this page computes

The split of wind pressure between the skins of a multi-layer wall or roof, to EN 1991-1-4 §7.2.10, NOTE 2 — the four configurations, plus the conditions of application from Figure 7.14.

The layer that "takes the wind" is the least loaded

This is the case that surprises on site, and it is the most common configuration: a ventilated facade — permeable rainscreen outside, sealed membrane behind.

With c_pe = 0.8, c_pi = −0.3 and q_p = 1000 N/m²:

	c_p,net	pressure
outer layer (permeable)	2/3 · c_pe = 0.533	533 N/m²
inner layer (sealed)	c_pe − c_pi = 1.100	1,100 N/m²

The outer cladding, the one everybody thinks of as "exposed", carries less than half of what the invisible membrane behind it carries. Its fixings are designed for 533 N/m², not 1,100.

The reason is simple: air passes through the openings in the cladding, so the pressure partly equalises in the cavity. The sealed layer is the one that stops the flow, so it takes the full difference.

And the fraction depends on the sign

In overpressure the permeable layer takes 2/3·c_pe. In suction, only 1/3·c_pe — half as much. This is not a symmetry: in suction the cavity drains more effectively through the openings.

The principle behind the four cases

NOTE 1 states it in one sentence: as a first approximation, the wind pressure on the stiffest layer is the difference between the external and internal pressures.

Everything else follows. The four cases of NOTE 2 are just that principle applied to the possible combinations of sealed/permeable and stiff/flexible:

configuration	outer layer	inner layer
sealed inside + permeable outside	2/3 or 1/3 · c_pe	c_pe − c_pi
sealed inside + stiffer sealed outside	c_pe − c_pi	—
permeable inside + sealed outside	c_pe − c_pi	1/3 · c_pi
sealed outside + stiffer sealed inside	c_pe	c_pe − c_pi

The last row is the easiest to miss: both layers are sealed, yet the outer one sees only c_pe. The internal pressure is absorbed by the stiff inner layer, not transmitted through.

The conditions of application, which are not optional

NOTE 2 applies only when:

the air flow between the layers is closed at the ends (Figure 7.14a), and
the clear distance between layers is below 100 mm.

The second condition assumes the thermal insulation fills one of the layers, so the air does not circulate through it.

And if the air inlets connect the air layer to other sides of the building (Figure 7.14b), these rules do not apply at all — the cavity pressure is then no longer the local one, but an average across faces at different pressures. The page flags explicitly when you leave the domain, rather than returning a false number.

One qualification that matters

All the values here are the first approximation recommended by the code. §7.2.10(1) lists five parameters the real split depends on:

the relative stiffness of the layers;
the external and internal pressures;
the spacing between layers;
the permeability of the layers;
the openings at the ends of the cladding between layers.

The National Annex may give different values. For an atypical facade — wide cavity, non-uniform permeability, layers of similar stiffness — the approximation here is a starting point, not an answer.

What it does not cover

§7.2.9 — the internal pressure c_pi, which is entered here as data.
§7.3(6) — two-skin canopies, where the permeable layer takes 1/3·c_p,net. The rule is implemented in the service but has no interface.
The calculation of c_pe itself — that comes from the wind page.
The vertical distribution of cavity pressure on tall facades.

The text of §7.2.10 is transcribed from EN 1991-1-4:2006, pages 49–50.

Wind action — this is where c_pe, c_pi and q_p come from.
Multi-span roofs — §7.2.7, the other distribution rule in Section 7.