Which decisions define a house's insulation
The insulation of a new house is defined by five decisions: how much insulation goes into the roof, which system is used on the facade, how the ground slab is insulated, how thermal bridges are broken, and what airtightness and ventilation the home will have. Windows and solar shading complete the picture, and each has its own article.
Spain's building code (CTE, the basic energy-saving document) sets the legal minimum, and the project demonstrates compliance. That minimum is a starting point, not a goal: above it you decide the real comfort of the house and what it will cost to keep it at temperature for decades.
Roof: the biggest summer heat gain
In a one- or two-storey house in Castelldefels, Garraf or Maresme, the roof takes direct sun all day in summer. It is where each centimetre of insulation pays off most, and the cheapest place to add more during construction: extra thickness in a flat or pitched roof at design stage costs little compared with retrofitting later.
If the roof will be walkable or carry solar panels, the insulation must bear loads and coexist with fixings and cable penetrations without losing continuity. This is coordinated with the waterproofing, covered in another article, because wet insulation loses much of its effectiveness.
Facade: outside, in the cavity or inside
There are three common ways to insulate a facade. External systems (ETICS/SATE or a ventilated facade) wrap the house without interruptions and keep the wall's thermal mass inside the home, which in a Mediterranean climate helps keep it cool in summer. Cavity insulation between two masonry leaves is more traditional and requires care at junctions with slabs and columns. Internal insulation takes away usable floor area and leaves more thermal bridges.
The choice affects the facade finish, total wall thickness and budget, so it is made with the architect before the structure is fixed. Changing system once walls are up means redrawing details and often revising the permit.
Ground slab and thermal bridges: where the invisible losses are
A ground-floor slab in contact with the soil needs insulation below or above it, and if the house has underfloor heating, that insulation is essential so heat rises into the home instead of sinking into the ground. Its thickness is decided before the concrete is poured, because it sets the levels of the whole floor.
Thermal bridges are the points where insulation is interrupted: slab edges, columns in the facade, roller-shutter boxes, balcony projections and window junctions. That is where heat escapes and where condensation and mould appear in corners. Ask the designer for construction details of these junctions and check on site that they are built that way, with photos before they are closed up.
Airtightness and heat-recovery ventilation
A well-insulated house with air leaks loses much of its advantage. Airtightness comes from a continuous layer on the inside of the envelope and from sealing service penetrations, shutter boxes and window junctions. It can be measured with a pressure test (blower door) before finishes, while leaks can still be fixed.
An airtight house needs mechanical ventilation. With heat recovery, outgoing air warms or cools incoming air, and the house is ventilated without opening windows in the middle of August or January. In the 250 m² children's psychology centre we delivered in the Eixample in 2026, heat-recovery climate control was designed together with the false ceilings from the start; a house is no different: the ducts need space that has to be reserved in the design.
When each decision is made and what it costs
The order is strict: facade system and thicknesses before the permit; slab insulation before pouring; thermal-bridge details before the structure; airtightness and ducts before partitions and ceilings are closed. Whatever is left to the end can no longer be improved without redoing finished work.
Insulating above the minimum costs more up front. How much more, and how many years it takes to pay back, depends on the house, its orientation and the heating and cooling systems, so we do not quote a generic figure. In the project we propose specific upgrades, explain what each one changes, and the client decides with the price in front of them. In the house with a pool we built between 2022 and 2024, solar panels were part of the design from the start; an efficient envelope reduces the demand those panels have to cover.