As the song goes, let the sun shine in. "Go back 2,000, 3,000 years, and all homes probably had skylights and vents," says Hawaii "daylight engineer" John Callaway. "They just make sense, letting light in and heat out."

Callaway Cooling Skylights is a kamaaina company that manufactures its own skylights, and business is looking bright. Callaway believes sunny Hawaii might have more residential skylights than anywhere. "We’re the skylight capital of the world!" he says.

The concept is pretty simple: Put a hole in your roof, allowing natural daylight to come in and illuminate dank, dark corners. Thanks to convection, hot air rises and drifts out the vent, taking with it excess humidity. A good deal all around, except when it rains or when mynahs decide your house is actually their birdhouse.

Skylights once were carefully constructed, leaded and sealed glass windows. They leaked. They allowed sunlight to fade carpets and artwork. They trapped hot air. The invention of fiberglass and blown-Plexiglas domes after World War II, however, led to prefab units almost anyone could install.

They’re available in big-box DIY stores for $100 to $200. Custom-made units with higher-quality materials can be $400 to $800, typically. As with everything, you get what you pay for. Just remember that a fragile, leaking skylight can lead to bigger repair costs down the line.

Jeffrey Tong at Skylights of Hawaii cautions the cost of the skylight itself might be the cheapest part of the project. "The hassle is in the construction of the ‘light well’ between the skylight and the ceiling," says Tong. "It takes a fair amount of carpentry and finishing skills, and that adds up."

A new type of skylight, consisting of a tube that channels light via highly reflective inner surfaces, has become one of the most popular skylight designs because it creates its own light well, including a neatly finished ceiling fixture.

"Many people just want light channeled deep into their homes, like in inner hallways and bathrooms," says Tong. There are several manufacturers, and Tong points out the ubiquitous SolaTube brand can use reflective film instead of polished metal and can degrade over time. When in doubt, ask.

Tube sizes range from 10 inches to 22 inches.

If you’re not afraid to use a sabre saw on your roof, installing a skylight is relatively simple. Fit it between the joists by drilling a center hole from the inside and using it as a guide to cut out the space from the outside. Frame the hole with two-by-fours as header joists; make it stout. This supports the "curb" that elevates the skylight above the roof.

The curb will come with flashing that must be interlaced with the existing shingles. "The biggest problem is making sure the size of the curb is followed exactly," says Callaway. "If there’s a leak, it’s almost always because the flashing wasn’t installed properly."

Follow the instructions, agrees Tong. Caulk and roofing cement are inappropriate sealants; the sealing should be an industrial-strength material and roofing paper. It helps if you understand that water flows downhill when interlacing the flashing with the shingles. The shingle can be opened slightly with a pry bar to slip in roofing paper and sealant.

Once the skylight is in place, you can proceed with the light well, which is simple if it’s an open ceiling and more complicated if additional structure and drywall are involved. Ventilating skylights can have rain blow in sideways during gusty weather, so protect the drywall and plaster with several coats of paint.

Over the years it was discovered that fiberglass skylights deteriorate under ultraviolet rays, so few are made that way these days. The inexpensive plastic skylight domes shatter easily. "Polycarbonite and glass skylights are almost the same price," says Tong. "They’re more expensive but they’re tougher and last longer." And some glass domes are laminated safety glass, just in case you live near a golf course.

Callaway’s skylights, which are manufactured in Hawaii, are shaped Plexiglas. The models include "Hawaiian Skyvents" that overlap the curb with leakproof air vents.

Glass skylights can be treated with low-emissivity coatings that reduce heat radiation. Tinted or white skylights can soften the light broadcast into the room below.

The biggest problem Callaway sees in skylight repairs is when homeowners attempt to stop leaks with lots of caulk. "The caulk then keeps the skylight from moving, and it has to move, because the dome and the aluminum frame expand and contract at different rates," he says. "It will eventually crack. Your skydome will commit suicide over time."

Well, that’s not very sunny, is it?