Design guidelines for ventilated facades

National building codes seldom give any requirements for wind protection. In such cases, follow our recommendations below. If requirements are given in national building codes and they exceed these recommendations, follow the national requirements. The recommendations below are based on scientific research carried out in Finland and Lithuania by external research institutes, and on our extensive experience in the Nordic countries. Ventilated facades can be designed in many different ways, but all systems should prevent deterioration of the inner shell due to moisture. If the thermal insulation has an open structure, it needs to be shielded with a wind barrier so that the thermal performance of the insulation is preserved. The air ventilation openings in the facade layer and the thickness of the gap determine the wind protection needed. The examples shown below create a foundation for a durable and functioning building.

Air infiltration through the structure

An air infiltration barrier on the inside of the building envelope prevents air from flowing through the structure and causing negative effects. National building codes often provide requirements for the air tightness of barriers, but the general trend is towards improved air tightness. This is especially true after the adoption of the Energy Performance Directive in Europe. In practice, massive structures such as concrete or masonry achieve sufficient air tightness, but in the case of (light) frame structures, an air infiltration barrier made of, say, plastic foil is necessary. To measure the air tightness of a building envelope, use the standardized pressure test EN 13829. Subject the building to a 50 Pa overpressure and evaluate the air exchange rate of the building. The rate should not exceed 1 per hour.

Cold air intrusion

In a ventilated exterior wall, there is an air gap behind the facade. The gap removes excess moisture from the structure by the flow of air, and keeps it dry to ensure proper functioning. The air flow in the gap is normally upwards. Openings at the bottom allow the air to enter the gap. In the gap the air warms, picking up moisture, and flows up until released through the openings at the top of the wall.

On the exterior side of the wall, the wind barrier stops wind from blowing through or around the porous thermal insulation and causing forced convection in the insulation layer. Forced convection has a negative impact on the thermal performance of the universal insulation. Wind protection must have suitable water vapour permeability in order to lead possible vapour into the ventilated air gap. Choose the surface material for the wind protection so that complies with the fire safety requirements in your market area. Fire safety requirements are usually only imposed for high-rise buildings. Wind protection can either be a faced or non-faced stone wool board or slab, structural board, or a foil. Corners are often a critical point in ventilated wall structures, so take special care to avoid air intrusion. See examples of solutions in the installation guidelines.

Air flow resistance

Definitions with an example, PAROC WAS 25, 30 mm
Air permeability or l value (m3/Pa m s 10-6) is a material property independent of thickness. The numerical value in the product names for PAROC WAS and WAB products indicates the air permeability.

For example, PAROC WAS 25 has an l value of 25 x 10-6 m3/Pa m s, measured according to the European standard EN 29053.

Air flow resistivity r (Pa s m/m3, or usually given as kPa s/m2) is the inverted l value. This is also a material property independent of product thickness.

The air flow resistivity of PAROC WAS 25 is 1/25 x 10-6 m3/Pa m s = 40 000 Pa m s/m3 = 40 kPa s/m2

Specific air flow resistance Rs (usually given in kPa m s/m2) is the air flow resistance of a slab with a thickness d and is the resistivity times the thickness. Use this value when dimensioning wind protection. The examples describe how it is used.

The specific air flow resistance of PAROC WAS 25 is Rs = r x d = 40 kPa s/m2 x 0,03 m = 1,2 kPa m s/m2)

For wind protection or products with a wind protection facing, the specific air flow resistance can be given directly (see table 3 Tyvek -faced WPS products)

The principles of ventilated wall design

The required specific air flow resistance of the layer against the ventilation depends on how fast the air flows in the ventilation layer, and how high the air permeability of the underlying insulation is. A wall can be designed without ventilation, with poor ventilation or with more or less high ventilation. The ventilation openings in the facade control the degree of ventilation. Table 1 shows different types of wall insulation systems based on the size of the air vents. Av is the ventilation opening area in the bottom of the wall per metre.

Table 1. Examples of walls with different ventilation openings.

Size of ventilation, Av(cm2/m)  Structure
Non or poorly ventilated
 Av ≤ 5 Exterior walls without ventilation or walls with plates; materials with sealed/tightened joints such as rendered cement fibre plates, slabs of concrete or glass facades. Slabs of concrete and cement fibre sheets.
5 ≤ Av ≤ 300 Exterior walls as above with low degree of ventilation. Most walls are placed here. Nordic walls.
300 < Av≤400 Curtain wall with ventilation openings ≤400 cm2/m
Very intensively
 Av > 400 Curtain wall with ventilation openings >400 cm2/m with
multiple openings.

Table 2 shows the minimum values recommended by Paroc. If national building codes give requirements for wind protection, follow these. In other cases, use our recommendations.

Main wall insulation air
resistivity ->
r < 5.2
(kPa ⋅s⋅ m/m3
5.2 ≥ r < 17
(kPa ⋅s⋅m /m3
r ≥ 17 
(kPa ⋅s⋅ m/m3
Wall ventilation
Recommended minimum air resistance (m kPa s m/ m3) of wind protection material and recommended products
Av<300 Rs > 1.2 Rs > 0.85 Stone wool slabs for thermal insulation can be used without a wind-protection layer. Fix these slabs mechanically or glue them to the other partition layers in order to eliminate air gaps between the slabs, as well as between the other layers of the partition.
300 < Av ≤ 400 Rs > 1.2*
400 < Av ≤ 1000 Rs > 28.6*
Note *) Fix these slabs mechanically to the other layers to eliminate air gaps between the slabs as well as between the other layers of the partition.

Table 3. Specific air flow resistance Rof PAROC products

WPS 3n
WAB 5t  WAB 10t  WAS 25  WAS 35  WAS 50 
Air flow resistivity     200  100  40  29  20
Tyvek  100          
13 mm    2.6        
20 mm      2.0      
30 mm        1.2  0.9  
40 mm        1.6  1.2  0.8
50 mm        2.0  1.5  1.0
70 mm        2.8  2.0  1.4
80 mm         3.2  2.3  1.6
100 mm          2.9  2.0
150 mm            3.0

Recommendations and working methods

The methodology below only applies to dimensioning the wind protection layer if you are using PAROC stone wool products as a wind protection layer.
  • Start from the wall structure and find the relevant ventilation level in table 1. If necessary, measure or calculate the ventilation opening Av. Place the structure in the correct row in table 2.
  • Check the U-value requirement and choose a suitable insulation product with a suitable thickness.
  • Decide if you want a double layer system with different air resistances and if the wind barrier can be part of the thermal insulation.
  • Check the air flow resistivity r of the main insulation and locate the structure in the right column in table 2.
  • Check if additional wind protection layer is needed.

Note: If a product has an air flow resistivity lower than 17 kPa s/m2, for example PAROC UNS 37, always protect it with a product that has sufficiently high air flow resistance.

  • Check how thick the wind protection layer can be, or if it can be part of the main insulation.
  • Choose a relevant wind protection product and thickness from table 3. The specific air flow resistance Rs must be equal to, or higher than, the minimum value given in table 2.