(This is Part 2 of a two-part series. See Part 1: Skylights done right.)

Last time, we looked at how to avoid some skylight-planning pitfalls. Now, armed with a couple of prospective locations, we’ll learn how to test a skylight’s performance before it’s installed.

Of course, there are lots of complicated ways to do this – with computers, sun angle calculators, and what have you – but there’s also a method that’s simple, accurate and pretty good fun to boot. All you have to do is build a very simple model of the room out of white cardboard or foamcore board. A scale of one inch to the foot (the same one used for dollhouses) will work fine. We’re not talking about fine art here – your model can be stuck together with sewing pins, mailing tape, or what have you. All that matters is that the proportions are fairly accurate, including the size and location of any existing windows or other openings that let in light.

When you’re done, just set the model in direct sun and orient it exactly like the actual room. Because light is scaleable, you’ll get a perfect miniature representation of the skylight’s effect. You’ll see exactly how sunlight will move through the room during the course of the day, as well as how bright it will be at any given time.

If you make the ceiling of your model removable, you can play around with the skylight’s size, shape, and location until you find an arrangement you really like. You can also try out different shapes for the skylight well (the void extending down from the skylight to the ceiling).

It’s important to model the well correctly, because it plays a big role in the quality of light you’ll get. If your house has a pitched roof over an attic, the depth of the well will depend on the roof pitch and the skylight’s location. If the roof is flat or if the ceiling is vaulted, the well will be the same depth regardless of where the skylight is located. In general, a shallow skylight well lets in more direct light, but it can also create a glare problem. A deep well, on the other hand, will diffuse the light, but may not allow enough direct light into the room.

Most people treat the skylight well as an afterthought, so it usually ends up looking very clumsy. Don’t fall into this trap – figure out how the well will look ahead of time. Although the depth will be determined by your roof structure, there are a couple of options on how to treat the well’s sides. Flaring out the sides from the skylight down to the ceiling does a good job diffusing the light but may look busy, since the flared sides can’t meet a sloping roof without a lot of awkward transitional angles. A well with vertical sides usually makes for a tidier transition, as well as less labor, but won’t admit as much direct light. The model will let you see exactly how each kind of skylight well will affect the quality of light.

Aside from demonstrating one of nature’s neat phenomena – the scalability of light – a skylight model serves a very practical purpose: By cutting holes in cardboard instead of in your roof, you’ll know exactly what you’re getting, before it’s too late to change it.


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