- Report
- FR-012
- Published
- Written by
- Inês Carvalho
- Reading time
- 02 min / 375 words
Daylight autonomy versus occupant comfort
The west-facing homes at Casa Mares scored best for daylight and worst for comfort. Sensors on the blinds showed us why.
Section 01
Two numbers that disagree
At design stage the west-facing homes at Casa Mares had the best daylight scores in the block. Spatial daylight autonomy, the share of floor area that reaches 300 lux for at least half of the occupied hours, came out at 78 percent, comfortably above the target. A year after move-in, the same homes reported the lowest comfort in the resident survey. Half of the households said their living rooms were too bright in the afternoon, and several had covered the glass with film. On paper the light was a success. In the homes it was a problem.
Section 02
What the blinds told us
Each home has a position sensor on its external blind, installed to check the shading strategy rather than to study behaviour. Its data turned out to be the most useful in the building. On the west facade the blinds were closed for most of every summer afternoon, from around two in the afternoon until sunset. With the blinds down, measured daylight in those rooms fell well below that of the east-facing homes. The design had delivered plenty of daylight. The residents could not live with it, so in practice they never received it.
Fig. 012.1/ Blinds closed, west and east homes
Unit / %
| Period | West | East |
|---|---|---|
| 08:00 | 6% | 41% |
| 10:00 | 9% | 33% |
| 12:00 | 18% | 15% |
| 14:00 | 61% | 10% |
| 16:00 | 84% | 9% |
| 18:00 | 77% | 8% |
| 20:00 | 38% | 6% |
Section 03
Designing for the hand on the blind
The lesson changed how we model daylight. Every new housing project now runs the daylight model with an occupant blind model in the loop, calibrated against two years of Casa Mares data, so that glare is counted as a loss of daylight rather than a footnote. For the west homes themselves, a fixed vertical fin was added to each balcony in the autumn of 2025. Afternoon blind closures have since fallen by more than half, and the light the design promised is finally reaching the rooms.
Section 04
Measuring comfort, not light
We now report two daylight numbers for every home: the autonomy the model predicts with the blinds open, and the autonomy residents actually receive once the blinds are counted. At Casa Mares the gap on the west facade closed from 41 points to 12 after the fins went up. The resident survey moved with it. Comfort on the west side rose from 2.9 to 4.1 out of five, now level with the east-facing homes. Light that people shut out is not daylight, and a design metric that ignores the hand on the blind will keep rewarding the wrong facade.
Fig. 012.2/ Resident comfort before and after the fins
Unit / of 5
- West, before2.9 of 5
- West, after4.1 of 5
- East, before3.8 of 5
- East, after3.9 of 5
