Why I Hate, Hate, Hate Skylights
Skylights have a low R-value, provide cold surfaces for condensation, interfere with roof venting, and contribute to ice dams
Why do I hate skylights? Because I’ve rarely seen one that isn’t either causing a problem or in the process of causing one. They fall squarely into a category with recessed lights and cathedral ceilings: Homeowners love them and energy pros come to loathe them.
They lead to uncomfortable conversations that can be summarized as: Yes, they’re a problem; no, they can’t be easily fixed.
Think of skylights as windows installed at an angle. And as energy folks are fond of saying, windows make lousy walls. The problems are multiple. First, the R-value of even the best skylight is a fraction of the R-value of the roof — the roof which someone just punched a hole through.
Skylights are also very difficult to improve. When installed through an insulated sloped ceiling, skylights create a rectangular hole in the insulation to which there is no access. If installed above an unconditioned attic, you’ll have a very difficult-to-access rectangular shaft rising up through the attic to the roof deck. And the shaft had better be well insulated, as it is essentially outdoors.
BLOGS BY ERIC NORTH
Let's put one in the bathroom
Skylights are often installed in bathrooms — another nightmare scenario. The appeal is understandable. Much of a bathroom’s wall space is already spoken for with cabinets, tiles, toilets, towel racks, etc. So there is nowhere to add a window to provide some natural light. Besides, who would honestly want a window in your bathroom?
Instead, designers add natural light by cutting a giant hole through the thermal envelope — in an ultra-high humidity environment.
On an even somewhat cold day, the skylight interior will dip below the dew point, condensing all that 100% relative humidity shower steam. Lots of condensation plus lots of cracks equals incipient moisture problems.
They interfere with roof ventilation and contribute to ice dams
Skylights cause problems on the exterior as well. Standard roof ventilation can be a real pain if there is a skylight in the way. A well ventilated roof needs clear air flow from the soffit to the roof peak. A skylight would disrupt that flow, stopping the air flow below the bottom edge.
There's another problem: skylights in cold climates substantially warm the surrounding roof deck. Suppose a skylight is installed through an 18-inch-thick cathedral ceiling insulated with doubled-up fiberglass batts. The skylight allows conditioned air right into this hole punched through the insulation. This warm air would be inches from the roof deck, heating the surrounding shingles and melting snow. Big deal, right? Melting snow is good — it saves you climbing on the roof and shoveling. Which leads us to the next problem: ice dams.
Melted snow flows downhill
The skylight allows warm air near the roof deck, melting the snow. With a ventilated roof, the ventilation air is stopped below the skylight. Melted snow? Cold roof deck? That’s the formula for ice dams. Some of the worst ice dams I’ve seen are below skylights.
Skylights provide great natural light but at a cost: ice dams, moisture damage in the surrounding drywall… They’re a real horror show. While the desire for natural lighting is understandable, a solar tube might be a better alternative. Skip the skylight and avoid the hassle.
The blog's title is a reference to Roger Ebert and his book, I Hated, Hated, Hated This Movie.
- Christian Guthier
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