In short: three jobs and a recess for the track
Bedroom curtains do three different jobs: let in daylight, block the view from outside and make the room dark enough to sleep. One fabric rarely manages all three, so the usual answer is two layers: a light sheer and a heavy blackout curtain or roller blind, and in Barcelona external persianas on top of that. The recess for a concealed track and power for a motor are planned before the ceiling is closed.
Curtains seem like a finishing touch, but the first part of the decision is made during first fix. Once the ceiling is painted, a concealed track can only be surface-mounted.
Darkness: fabric, edges and external shutters
A dim-out fabric and true blackout give different results. The first softens the light; the second stops it passing through the cloth. But even blackout fails if light comes round the edges: above the track, at the sides and underneath. So the curtain is made wider than the opening and returns onto the wall, the track is hidden in a ceiling recess, and roller blinds get side channels.
In Barcelona flats the most effective darkening device is often already there: external persianas. During the renovation check the box, the strap or motor and the slats. Working persianas plus a blackout curtain give near-total darkness, and in summer they are also the first line of defence against the sun.
Privacy in the Eixample: test at night
Bedroom windows in the Eixample often face the inner courtyard of the block, with the neighbours' windows twenty metres away. By day a linen sheer hides the room; at night, with the lights on inside, it becomes almost see-through. That is not a fault in the fabric, it is physics: you see into wherever is brighter.
So test the sample by day and at night, with the lights on, looking from outside or at least from the next room. If privacy matters in the evening, you need an opaque second layer or a roller blind in opaque fabric that comes down when the lights go on.
Room for the system: recess, stack and nearby furniture
A concealed ceiling track needs a plasterboard recess of the right width and depth: for two tracks and, if needed, a motor. Its size is agreed before the ceiling goes up; otherwise the recess ends up too narrow for the second layer. If an air-conditioning unit or duct sits above the window, everything is resolved together.
A drawn-back curtain takes up space beside the window. With heavy fabric that stack can be 20 to 30 centimetres each side. Check that it will not hit the built-in wardrobe, the bedside table or a wall corner. In Spain casements usually open inwards, and the balcony door must open freely without catching the fabric.
Heat, cold and condensation
Against summer overheating, external shading beats any curtain: the sun is stopped before it reaches the glass. A lined internal curtain helps, but the heat is already in the room. On south and west elevations the difference is especially clear.
In winter a heavy floor-length curtain pressed against the window cuts the glass off from the room's warm air, and condensation can form on it. Leave a gap, air the room in the morning and do not seal the curtain tight in a room without proper ventilation.
Control: by hand or motorised
A manual curtain on a good track lasts for years and asks for nothing. A motor makes sense for tall windows, heavy fabrics or a morning scene where the curtains open on their own. For a bedroom choose a quiet motor and ask to hear it before ordering.
If a motor is even a future possibility, run power to the recess now. Coordinate automation with whoever installs the smart-home system, so curtain, lights and persianas are controlled from one place rather than three remotes.
Care, samples and sequence
Ask how the fabric comes down and gets cleaned, whether it shrinks when washed and whether it fades in the sun: linen and silk on a south-facing window change colour within a couple of seasons. Look at large samples in the room itself, by day and at night, not a small square in the showroom.
The sequence: decide the system and the recess before the ceiling, choose the fabric during finishing, measure after painting, then make and install at the very end, once the dust has gone. A good choice balances looks, darkness, care and ease of use, not just the colour of the sample.