External wall insulation, done with the details right

What is SATE?

SATE stands for Sistema de Aislamiento Térmico por el Exterior. It is a system rather than a single product: insulation boards bonded and, where needed, mechanically fixed to the outside face of the wall; a base coat with an embedded reinforcing mesh; and a finishing render or coating. Together these form a continuous insulated skin. The components are designed to work together, so they should come from one compatible system rather than be mixed.

Because the insulation is outside, the wall itself stays on the warm side, rooms keep their size, and the junctions between floors and walls — typical cold spots in coastal houses — are covered. Our guide what external wall insulation is explains the layers in more detail.

Which houses are suitable?

A wall is a good candidate when it is structurally sound, dry, flat enough to receive boards, and able to hold fixings. Detached villas with simple façades and reasonable eaves are the easiest. Things that make it harder or unsuitable:

• Active damp in the wall, especially from the ground, which must be resolved first.
• Decorative façades, stone cladding or features you want to keep visible.
• Very shallow eaves or roof edges that cannot cover the extra thickness.
• Townhouses where the neighbours will not insulate and the junctions become awkward.
• Protected buildings or areas with strict rules on appearance.

We look at these at the visit. Our article on when external insulation is not the answer goes through the cases where we would advise against it.

How is it installed?

1. Survey and preparation: hollow render removed, cracks treated, surface cleaned and levelled.
2. Base profile fixed at the bottom of the insulated area, kept clear of ground moisture and splash zones.
3. Insulation boards bonded in staggered rows, with mechanical fixings where the system and wall require them.
4. Reinforcement: base coat with embedded mesh, with extra pieces at the corners of openings.
5. Details: window reveals, new sills with drips, junctions with eaves and roofs, pipes and fittings passing through.
6. Finish: primer and finishing render in the colour and texture agreed.

Most failures of external insulation happen at details — a sill without a drip, a gap at the roof edge, a pipe sealed badly — not in the middle of the wall.

What does the building code say?

Spain’s Código Técnico de la Edificación (CTE) sets energy requirements in its basic document DB-HE, last substantially updated by Real Decreto 732/2019. Its section HE1 applies to interventions on existing buildings as well as to new ones. When envelope elements are replaced, added or substantially modified in a renovation, their thermal transmittance must not exceed the limits set in the document’s tables, and stricter overall criteria apply when a renovation renews more than a quarter of the building’s total thermal envelope. HE1 also requires that any interstitial condensation in the envelope does not cause degradation or significantly reduce its thermal performance. Officially protected buildings are excluded where compliance would unacceptably alter their character.

In practice, the system’s thickness and specification should be chosen with these requirements in mind; where the scope calls for a technical justification, that is a matter for a qualified professional (application to your specific house: to be verified with a technician).

Does it need permission and the community’s agreement?

External insulation changes the thickness and often the appearance of the façade, so it involves the town hall in some form, and in communities it needs the owners’ agreement. Requirements differ by municipality (to be verified case by case). Our guide on permits for façade, roof and window works explains the general framework. Public grant programmes for energy renovation open and close over time; we do not manage applications, and any current call should be checked with the official body that runs it.

How much will I save?

We do not give savings figures. They depend on the house’s orientation, the existing walls, windows and roof, the system and thickness chosen, and how the house is heated, cooled and occupied. What we can say is how the system works and what it changes physically: the walls are protected from temperature swings, cold spots at slab edges are covered, and the inside surface of external walls stays closer to room temperature. If you need numbers, an energy assessor can model the house. Our article is external insulation worth it looks at the decision more broadly.

How is SATE quoted?

We quote by wall area, with separate lines for preparation and repairs, base profiles, insulation and fixings, reinforcement and finish, plus each type of detail: sills, reveals, eaves junctions, penetrations. Access and protection appear separately. A first estimate is possible from photos of each elevation and the approximate built area; the final itemised quote follows a visit, because the substrate condition drives much of the work.

What happens at windows, eaves and the base of the wall?

These three areas decide whether external insulation lasts. At windows, the insulation returns into the reveal where space allows, and new sills with a proper drip replace the old ones because the wall is now thicker. At the eaves, the top of the insulation must be covered by the roof overhang or a capping so that rain cannot run behind it. At the base, the system starts above ground level on a profile, and the lowest band is usually built with a more impact- and moisture-resistant material, because that is where splashing water, garden equipment and footfall hit the wall. We draw these details into the quote rather than leaving them to be decided on site.

Official sources

• Real Decreto 732/2019 – modificación del CTE, DB-HE (BOE-A-2019-18528)