Designing With Daylight

It’s that time of year again for the northern hemisphere – days are getting shorter, and temperatures are dropping. As we approach winter and daylight savings time, our bodies need to physiologically adapt to the environmental changes that are occurring. Our homes should also adapt to these changes and allow us to adjust our spatial organization and functional use of different zones for optimizing wintertime daylight and thermal qualities. There are a handful of strategies for light modulation and adaptation, including operable windows and doors, moveable exterior shutters, adjustable interior blinds, and more advanced glazing designs with spectrally selective coatings and responsive mechanisms (i.e. thermotropic or electrochromic). For now, we’ll focus on the specific concepts related to the qualities of light and different climates.

Daylight Source

First of all, daylight is an abundant resource that is available at any location around the globe. It consists of the visible spectrum of light that emanates from the sun, which includes a range of electromagnetic radiation of 380nm – 740nm.  The low end of the spectrum consists of violet light and the high end of the spectrum consists of red light. In between we have the gradation sequence that you might recall from primary school science class – ROYGBIV, or red, orange, yellow, green, blue, indigo, and violet, providing the colors that you see in a rainbow! These colors correspond to specific wavelengths of light, and a wavelength is essentially a photon-particle energy band – but we’ll get back to this a bit later. For now, let’s consider the qualities of daylight that occur in different regions of the world.

Sky Conditions

When we design with daylight for specific climates, we analyze information about the annual sky conditions. A sky condition describes whether the sky is clear or cloudy. Of course this varies all the time, with clouds moving across the sky, or lingering, or disappearing all together throughout a given day. The amazing climate scientists have long utilized tools and sensors to measure the amount of clear vs. cloud conditions for various locations (typically from airports in different cities for primary measurements). This data is collected at an hourly rate and compiled into an annual dataset, and over many years the data is analyzed for statistical averages that are used to inform our design conditions for a given climate location. 

 This data also provides the specific values of daylight that are available in a quantity of lumens, a measure of light distribution flux (rate per second) that emanates from a given source. Every location has its own nuances of sky conditions, and the elevation of a location also plays a role since clouds stratify and settle at different heights based on broader regional geographies. From my own experience, I can compare Portland, Oregon (primarily overcast skies) with Tucson, Arizona (primarily clear skies) and New York, New York (mixed sky conditions). This is a simplified generalization of the sky conditions for each of these locations, but it begins to guide the possible design strategies that could be helpful for the type of daylight quality available.

Daylight Interactions and Glazing

In order to capture natural daylight to the interior spaces of our home, we incorporate glazing into the building envelope (exterior walls and roof). This can be in the form of windows, skylights, doors, and also glass walls. The glazing provides a transparent or translucent material for light to travel through. Of course, glass can be designed with many different optical properties allowing for spectrally selective functions that modulate the specific wavelengths of light that will be allowed to transmit through to the building interior. There are four functions for light interactions with materials or glazing. The first is absorption, which is when light (or any wavelength of radiation) is received by a material and stored within that material’s microstructure.

The second is transmission, which is when light passes through a material structure such as glass. Transmission occurs because of the particular material microstructure, which in the case of glass is a relatively consistent amorphous solid comprised of silicon dioxide. This microstructure results in zones that have an absence of electron energy states that would absorb visible light photons, resulting in the transmission of those photons to effectively pass through the material. Because of the difference in molecular structure between glass and air, the speed of light also slows down when it hits the glass, resulting in a shift in the trajectory angle known as refraction. Refraction is the offset or bending of the light’s path from its point of entry to its point of exit. Refraction through crystal structures is what allows us to visually see the colors of light in the form of the rainbow spectrum as each wavelength of light slows at slightly different speeds and combine with some scattering interactions that spread the light’s wavelengths apart into a visible array.

The third interface between daylight and glass is reflection, which is the redirect or bounce of light off of a glazing surface. There are two basic types of reflection – specular and diffuse. Specular reflection occurs with very smooth and shiny appearing surfaces and results in the light bouncing off the material surface at an angle that is equal to the angle of incident light. Most glass will have specular reflections, which is what causes us to see faint images on the glass like what we experience with mirrors. Diffuse reflection occurs with rough surface microstructures, which interfere with the incident light to result in a scattering effect of the light’s rays in all directions.

Finally, there is the function of light emission. Emission occurs when the photon energy stored in a molecule’s electrons (i.e. from absorption) are released back out into the surrounding environment. When emission occurs from basic building materials, including glass, it is almost always in the form of long-wave radiation or heat but not visible light. Glazing can be designed to absorb short-wave and long-wave radiation as well as to emit these wavelengths in certain directions, such as back to the outside or towards the inside of the building. In summary, there are four basic interactions of light with glass (absorption, transmission, reflection, emission) and glass can be designed to control light and heat relative to the climate and functions needed for the home.

Daylight Design Strategies

Let’s consider the examples of the three locations cited earlier and determine what type of glazing design strategy will be most beneficial for each particular sky condition.

With the primarily overcast sky conditions the lumens available is decreased due to the absorption of large quantities of light by the clouds and atmospheric interference. So, the amount of light that eventually reaches the site of a house is more limited, but it is evenly and diffusely distributed. In these conditions, it is helpful to try and capture as much natural light as possible to bring into the home’s interior spaces. This can be accomplished with bay windows, also known as oriel windows, which push out from the exterior wall and incorporate glazing on at least three sides. This can also be accomplished with skylights and clerestory windows since the daylight is diffused and will spread evenly into the spaces below.

In contrast, sky conditions that are primarily clear have a very high lumens value and a large amount of light will reach the site of a house in these locations. However, the quality of light is extremely bright and creates high contrast conditions. High contrast results in heavy shadows directly adjacent to super bright zones. These situations create more challenge for achieving even quality light distribution and can also cause vision difficulties as the pupils need to dilate to adjust to the extremes. In bright sky conditions, it is helpful to use windows that are more deeply recessed within thicker walls if possible. A deeply recessed window will allow for some of the light to be controlled and dampened by the surrounding surfaces. Using matte surface materials, such as plaster or stucco, and curving the edges of the window surround will further enhance the softening of the light before it transmits through the window glass.

For the mixed sky condition locations, sometimes there will be clear skies with brighter light that creates more contrast and other times there will be cloudy skies with diffuse light and low levels of lumens. Windows that are placed more or less in the middle of the wall system will do well for these climates, providing adequate access to diffuse light while also allowing for some diffusion of brighter sunlight conditions. In climates where there is generally a mix of sky conditions, it is also helpful to incorporate adaptive controls such as exterior and interior blinds to modulate the light on brighter days or to be fully open to allow more daylight to the interior spaces on cloudy days. You can explore more details about windows and glazing for your climate in the Windows for Your Climate blog post.   

Daylight and Your Health

Designing with daylight is an important aspect of both architecture and interior design as these two systems need to be synchronized for optimizing the daylight qualities in your home’s spaces. Daylight has significant benefits for human health and well-being, in particular providing our bodies to produce serotonin and vitamin D, while darkness will be a queue for melatonin production aiding in a restful sleep. You can read up on more content about the benefits of daylight in the Natural Daylighting in Design blog post. Another factor to consider when designing with natural light is the interaction of light and color, which can play a role in your psychology and overall mood. See the blog post on Color Theory for Your Home to read up on the details of light’s interactions with color!

If you are interested in optimizing the daylight quality of the spaces in your home or workspace, reach out to AIDA, LLC today for a consultation. You can always find more information and healthy home resources at Aletheia Ida Design and Architecture, LLC (AIDA, LLC) at www.aletheiaida.com.