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Skylights and sun tunnels: which fits your room

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1The idea

The hallway with no windows has been dark since the house was built, and nobody thinks about fixing that until the day they realize the roof right above it is drenched in sun. A skylight is the honest fix: glass, sized like a real window, sky and weather along with the light. A sun tunnel is a small roof-mounted dome, ranging from about 10 inches across for a home up to 22 inches for a commercial building, feeding a mirrored tube down to a ceiling diffuser that looks like a light fixture. No view, no glass to clean on a ladder, just light.

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2Why this works

Toplighting from the roof is three to ten times more efficient than a window on the wall, because the sun spends more of the day overhead than sideways. A dark hallway, closet, or windowless bathroom is the classic sun-tunnel job: too small to justify a real skylight and its framing, easy to run a 10-inch tube through the attic to. Solar light pipes also hold their heat better than a full skylight, which is part of why they get used in small rooms where a big glass hole in the roof would leak comfort along with the light.

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3What's actually up there

A roof-mounted acrylic or polycarbonate dome, formulated to block UV; a tube lined with reflective material (manufacturers claim up to about 99.5 percent reflectivity); a ceiling-level diffuser that spreads the light evenly into the room; and on a full skylight, a low-emissivity glass unit rated by the same U-factor and Solar Heat Gain Coefficient numbers used to grade windows.

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4Worth knowing

Daylighting design (using the sun instead of a bulb) can cut a building's electric lighting use by up to 80 percent in the right room, which is the whole reason skylights and sun tunnels keep showing up in kitchen and bathroom remodels instead of just attics. It also looks different from a bulb: ordinary daylight-tone fluorescent lamps are built to about 6500 K, a cool bluish-white, chasing the daylight a skylight gets for free.

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