PASSIVE SOLAR LIGHTING

	Earth sheltered homes can’t have windows in their sheltered walls, so they need some way to bring in additional light. One could always use electric lights, but there’s plenty of free light available if you provide a way for it to enter. There’s nothing really new about using skylights, but the way we installed them improves on the standard.
	This is the inside of one of our 4 smaller skylights. Since the skylights are already located to put light where it’s needed, we included recessed electric lights for evening illumination. With the gloss white interior, when you turn them on the ceiling panel glows just like in the daytime. We added two sets of screened vent holes at the bottom and top on opposite sides of the light well. Convection moves air from the attic through these light wells, reducing heat buildup. Lacking that ventilation, heat would accumulate in the lightwell and radiate into the house, increasing our cooling load.
	In the large (3 ft x 6 ft) skylight above the kitchen island we used 4 T8 48″ fluorescent tubes for illumination. That’s plenty of light for cooking. The larger volume of this lightwell made convective cooling impractical, so we installed a solar powered electric vent fan inside. It circulates attic air through the lightwell anytime the Sun is shining.

Skylight Physics:

That largest skylight above the kitchen island presented a real challenge. This is a custom-sized 3×6 foot skylight. It spans multiple roof trusses (as you can see in the image). We considered framing the lightwell around that space, to avoid having shadows of roof trusses cast on the frosted white acrylic diffuser. Doing that without compromising roof integrity would have been complex and expensive. So here’s what we did.

The entire interior is painted with high-gloss white paint to help scatter incoming light. Covering the trusses with glass mirror tile would have been better, but again complex and expensive. There are some diffuse shadows visible, as you can see in the image below (trusses marked by yellow lines). But they’re tolerable and barely reduce the overall light.

In 2015 we replaced the 4 T8 fluorescents with new technology: LED lighting in the same form factor as a T8. After researching this rapidly changing market, we decided on Keystone T8 LEDs. This technology:

• uses only 50% the energy needed by fluorescents with the same light output
  
• requires no ballast (avoiding energy loss through a phantom load)
• contains no mercury
• fits into existing fluorescent fixtures
• is available in a range of color temperatures (warm — cool)
• comes with a 5-year warranty, and
• has an estimated lifetime of 50-80,000 hours
• can pay for itself in as little as 2 years (depending how often it’s used)

We installed 2 of these same units in the shop, replacing other fluorescents that get a lot of use. We still have some standard fluorescents, but they’re not used often enough to warrant replacement.