External insulation keeps room sizes, covers cold bridges at slab edges and lets you stay in the house during the work, but it changes the façade, needs scaffolding and often community and town hall consent. Internal insulation leaves the façade untouched and can be done room by room, but it takes floor space, disturbs fittings inside and leaves slab-edge cold bridges partly uncovered. Moisture must be considered carefully in both cases.
How do the two approaches work?
External insulation (SATE) fixes insulation boards to the outside of the walls and covers them with a reinforced render. The whole house is wrapped from the outside. Internal insulation adds an insulating layer on the room side of external walls, usually as boards bonded to the wall or as a framed lining with insulation and plasterboard. The external walls themselves are not touched from outside. Our guide what external wall insulation is explains the external system in more detail.
Space, disruption and appearance
- Room space: external keeps it; internal takes several centimetres from each insulated wall.
- Disruption inside: external work happens mostly outside; internal means emptying rooms and removing and refitting sockets, radiators, skirtings and built-in furniture.
- Appearance: external changes the façade’s thickness and finish; internal leaves the outside unchanged.
- Phasing: internal can be done room by room; external is normally done elevation by elevation with scaffolding.
Cold bridges and condensation
Where floor slabs, pillars and beams meet the external wall, heat escapes more easily. These cold bridges are where condensation and mould often appear. External insulation covers them continuously. Internal insulation stops at each floor and ceiling, so the slab edges remain partly exposed unless extra detailing is added. Internal insulation also changes the temperature of the wall behind it, which becomes colder in winter; if moist room air reaches it, condensation can form inside the construction. That is why vapour control and careful detailing matter with internal systems. Spain’s energy code asks that any interstitial condensation in the envelope does not degrade it or significantly reduce its performance.
Rules, community and paperwork
External insulation changes the outside of the building, so it usually needs some form of town hall notification and, in communities, the owners’ agreement. Internal insulation is interior work and generally simpler on both counts, although town hall rules for interior works still apply. On a single townhouse in a terrace, internal insulation is often the only realistic option. See our townhouse renovation page for the community side.
Which suits which house?
External tends to suit detached villas whose façades need repair, houses with cold-bridge condensation at slab edges, and owners who want to keep room sizes and stay in the house. Internal tends to suit townhouses in terraces, houses with façades worth keeping, individual cold rooms, and situations where external changes are not allowed. For old masonry walls that need to breathe, both approaches need particular care, and breathable solutions are preferred. Our village house renovation page covers that case.
Can the two be combined?
Yes. A villa might have external insulation on its plain rendered elevations and internal linings behind a stone feature wall that you want to keep visible. The junctions between the two need careful detailing, but the combination can make sense where one approach alone does not fit every wall. Roof insulation and better windows complete the picture; insulating walls while the roof leaks or the windows are poor rarely gives the result owners hope for. Our page on external insulation explains how we assess a house.
What should you ask before deciding?
- Are my walls dry, and if not, what is the cause?
- How much room space can I give up, if any?
- Do I need to stay in the house during the work?
- Will my community and town hall allow changes to the façade?
- Are there features, inside or out, that I want to keep visible?
Your answers usually point clearly to one approach, or to a combination.
What about cost and timing?
We do not publish prices, and the comparison depends on the house. In general terms, external insulation carries the cost of access equipment and façade detailing, but it is often combined with façade repairs that would be needed anyway. Internal insulation avoids scaffolding but involves emptying rooms, removing and refitting fittings, and redecorating each room. The more rooms are already being renovated inside, the less extra disruption internal insulation adds; the more the façade needs repair, the more sense external insulation makes. Pricing both as separate lines is the fairest way to compare.
Bauman Raymond Attila · Reforma Villas Costa del Sol · Avenida Antonio Belón 3, Marbella. Last reviewed: 05/11/2026.
