Blog · Insulation and energy efficiency
Ventilated facade or SATE: two ways to insulate from the outside and keep the rain out
The panel held away from the wall, the air cavity and the insulation behind it: what the Spanish Building Code asks of a ventilated facade and why it stops rain differently from SATE.
7 min read
Two buildings insulated from the outside can look nothing alike. On one, the insulation is stuck to the wall and finished with render: from a distance it is a rendered, painted facade like any other. On the other, there is a gap of air in front of the insulation and, hung from a frame, panels or slats with open joints.
The first is SATE, the Spanish name for external wall insulation (ETICS); the second is a ventilated facade. Both insulate on the same side, but they deal with rain in opposite ways, and that is what matters most on a facade that faces the sea.
What a ventilated facade is
In the CTE (the Spanish Building Code, Código Técnico de la Edificación), what you find is its parts, one by one. What you see is a discontinuous external cladding fixed mechanically: small units (slate, fibre cement, timber, fired clay), timber or metal slats, or large panels of fibre cement or metal, on their own or forming a system with their insulation.
Behind it is the ventilated air cavity, which the Code defines as a gap in the build-up that “allows water vapour to diffuse through openings to the outside arranged so as to guarantee cross ventilation”. And behind the cavity, against the wall, the insulation, which has to be non-hydrophilic: it must not soak up water if any reaches it.
SATE puts the insulation in the same place, against the wall, but with no air in front of it: it is covered directly by a base coat reinforced with mesh and a finish. How it works layer by layer and why insulate from the outside: what SATE is.
What the Code asks of the cavity
The Code’s document on damp, DB-HS 1, counts a ventilated air cavity with non-hydrophilic insulation as a barrier with “very high” resistance to water penetration, class B3. But only if it meets some conditions:
- The cavity is on the outer side of the insulation.
- It is between 3 and 10 cm wide.
- It has ventilation openings adding up to at least 120 cm² for every 10 m² of wall between floors, half at the top and half at the bottom. Grilles, joints left without mortar or open joints wider than 5 mm all count.
- At the bottom, and wherever the cavity is interrupted by a floor slab or a lintel, there is a system to collect and drain “filtered or condensed” water: a continuous waterproof sheet or profile sloping outwards, and pipes or open joints no more than 1.5 m apart.
In other words, the Code takes it for granted that water will get in through those joints. It does not try to stop it; it gives it a way out.
Two ways to stop the water
That is the underlying difference from SATE. With SATE, water is stopped at the surface: the reinforced render is what keeps it out. On a ventilated facade the panel holds back the rain, but not all of it: what gets in through the joints runs down behind, through the cavity, without reaching the wall.
Two ways to stop the rain
Both insulate from the outside. In both, the insulation sits against the wall; what changes is what shields it from the rain.
The SATE stops it at the surface: its reinforced render will not let it through. The ventilated facade, on the other hand, lets it in through the open joints…
…and the cavity stops it: the water runs down the back of the panel to the bottom, where it is collected, and the cavity is ventilated from bottom to top.
Both can reach the top of the Code’s scale, each in its own way. In a DIT (Technical Suitability Document) from the Eduardo Torroja Institute that assesses SATE and ventilated facade systems from the same insulation manufacturer, the SATE systems can be considered a continuous cladding with water-penetration resistance R3, the highest for a cladding, if everything the document asks for is met. And the cavity of its ventilated facade can be treated as a “barrier with very high resistance to water penetration” (B3) as long as the cavity, joint and opening dimensions are respected, the insulation is not hydrophilic and water is collected at the bottom.
So the choice is not won with the line “ventilated is better against damp”. Both stop water; what changes is where, and therefore what needs looking after: with SATE, that the render does not crack and the edges are properly finished; with a ventilated facade, that the cavity stays open top and bottom and the water has a way out.
On the seafront, the Code asks for more
The degree of impermeability required of a facade comes from two things: the rainfall of the area and how exposed it is to the wind. And for wind, besides the building’s height and the wind zone, the class of surroundings counts. The Code puts terrain types I, II and III in class E0, and type I is “the edge of the sea or a lake with a clear stretch of water in the direction of the wind at least 5 km long”.
A building in that class comes out more exposed in the table than the same building in a city, and that raises the degree of impermeability required of its facade. On this coast, with the levante and poniente winds driving rain against the facade, that is the position of a building on the front line of the promenade. For the highest degree, 5, the table of solutions accepts, among others, a cladding with very high resistance (R3) or a barrier with very high resistance (B3) on a main leaf of medium thickness: the door stays open for both systems.
Fire and the chimney effect
The cavity that lets vapour out also lets smoke rise. The same technical document warns: “As with all ventilated facade systems, in the event of fire, spread by chimney effect may occur”.
The Code responds with rules of its own for cavities, in its fire safety document, DB-SI. For facade systems in general it asks for a reaction-to-fire class by height: D-s3,d0 up to 10 m, C-s3,d0 up to 18 m and B-s3,d0 above that. For insulation inside a ventilated cavity it asks for more: D-s3,d0 up to 10 m, B-s3,d0 up to 28 m and A2-s3,d0 above 28 m.
On top of that, the vertical run of the cavity has to be limited where it meets the floor slabs separating fire compartments; E 30 barriers are considered a valid way to do it. On facades up to 18 m whose base is within public reach, both the facade system and whatever is inside the cavity have to be at least B-s3,d0 for the first 3.5 m.
What changes for whoever lives behind it
What you see is different: a continuous render on SATE, units with their joints on a ventilated facade. And the upkeep follows what you see. A SATE ends up being repainted, with a product compatible with its finish; a ventilated facade is maintained as its cladding requires, with the cavity kept clear. Units can also be bonded onto SATE, with no cavity, and that is a different solution: SATE finishes.
What they share is the main thing: both put the insulation outside the wall. If the question is whether to insulate inside or outside: insulating walls inside or outside.
Frequently asked questions
Why does a ventilated facade have open joints?
Why does a ventilated facade have open joints?
Because the cavity has to be ventilated, and the Spanish Building Code accepts open joints wider than 5 mm in discontinuous claddings as ventilation openings, just like grilles or joints left without mortar. Some water getting in through them is expected: that is why it has to be collected and drained at the bottom.
Is the air cavity what does the insulating?
Is the air cavity what does the insulating?
That is not its job. In the Code’s solution the insulation sits against the wall and the cavity outside it. What defines the cavity is that it lets water vapour escape through openings to the outside, placed so that air flows right through.
How wide is the cavity?
How wide is the cavity?
For it to count as a barrier with very high resistance to water, DB-HS 1 asks for between 3 and 10 cm, with openings adding up to at least 120 cm² for every 10 m² of wall between floors, half at the top and half at the bottom.
On the seafront, is a ventilated facade better than SATE?
On the seafront, is a ventilated facade better than SATE?
Not as a rule. Next to the sea the Code asks more of a facade, but for the highest degree of impermeability it accepts, among other solutions, both a cladding with very high resistance and a barrier with very high resistance. What decides it is the specific system and how it is built, especially at the edges.
Is a ventilated facade more dangerous in a fire?
Is a ventilated facade more dangerous in a fire?
Its cavity can act as a chimney, which is why the Code sets rules of its own for it: the insulation inside has to meet a stricter reaction-to-fire class depending on height, and the vertical run of the cavity has to be limited at the floor slabs, for example with E 30 barriers.
Sources
- Ministerio de Vivienda y Agenda Urbana, Código Técnico de la Edificación, Documento Básico HS Salubridad (HS 1, Protección frente a la humedad).
- Ministerio de Vivienda y Agenda Urbana, Código Técnico de la Edificación, Documento Básico SI Seguridad en caso de incendio (SI 2, Propagación exterior).
- Instituto de Ciencias de la Construcción Eduardo Torroja, Documento de Idoneidad Técnica n.º 608R/20, «Sistemas ISOVER para SATE y Fachada ventilada» (Saint-Gobain Isover Ibérica; valid from October 2020 to October 2025).
Quotations from Spanish-language documents are our own translations.