Designing for the overcast sky
Most daylighting advice assumes a clear sky and a sun to track. Portland's winter sky is a uniform grey dome, brightest at the zenith and about a third as bright at the horizon. Under that dome, the quantity of light a room receives depends on how much sky a window can see, not where the sun is. This is why a tall, narrow window set high in the wall delivers more useful daylight here than a wide one at sill height, and why the sky angle from the back of the room is the first number we draw on a section.
We express the result as a daylight factor: interior illuminance as a percentage of the unobstructed exterior under the same overcast sky. Below 2%, a room reads as gloomy and electric light runs all day. Between 2 and 5%, daylight dominates and electric light becomes a supplement. Above 5%, glare and heat gain start to cost more than the light is worth. Our planner on the home page uses the BRE split-flux method to estimate this band for any room.
Reflectance does the heavy lifting
Once light enters, surfaces decide where it goes. A three-coat burnished lime plaster in our warm oatmeal tone measures around 0.82 reflectance, against 0.72 for a typical painted gypsum board and 0.45 for exposed oak. In a 12-by-14-foot room, moving the walls from oak to lime plaster can lift the daylight factor by a full percentage point without touching the glazing. Lime also buffers humidity and holds a soft, slightly uneven surface that scatters light instead of mirroring it.
We keep ceilings lighter than walls, walls lighter than floors. Floors stay in the 0.25 to 0.35 band, which is where oiled white oak and honed basalt naturally fall.
Tunable electric light, 2700 K to 4000 K
Electric light in our houses has one job in daytime, which is to raise the melanopic stimulus at the eye to a level the body reads as morning, and a different job after dusk, which is to get out of the way. We specify tunable-white fixtures with a 2700 to 4000 K range, CRI of 90 or better, and TM-30 fidelity (Rf) of 90 or better so that oak, terracotta and skin tone all read truthfully across the range.
A typical schedule runs 4000 K at 500 lux on the desk from 07:00 to 11:00, drifts to 3500 K through the afternoon, lands at 3000 K at dinner and at 2700 K or lower after 20:00 with vertical illuminance at the eye held below 50 lux. The hardware is ordinary DALI or 0–10 V drivers on an astronomical clock. The part that takes judgement is placing the light so it comes from above and in front, where the sky would be.
Biophilic pattern, applied strictly
- Prospect and refuge. Every principal room has a long view out and a low, enclosed corner to retreat to. In practice this is a window seat with a 2.1 m ceiling under a 3 m room.
- Material connection. Timber that shows its grain, plaster that shows the trowel, stone that shows its bed. Nothing printed to look like something else.
- Dynamic and diffuse light. Daylight from two sides where the plan allows, so the balance of light shifts across the day.
- Non-rhythmic sensory stimuli. Rain on glazing, leaves moving beyond it. We keep at least one window unobstructed to the garden from every workspace.
- Thermal and airflow variability. Operable glazing in every room, a sheltered outdoor room off the kitchen, and radiant floors so the air can stay cooler than the surfaces.
Measuring the result
Before handover we log illuminance and melanopic EDI at eye level in each primary room on an overcast day, and we leave the owner a one-page schedule of what the lights do and why. Projects in Hood River and the Willamette Valley are documented in Selected Works.