Why I Hate, Hate, Hate Skylights

by Erik North on May 27, 2013

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Title reference in memory of Roger Ebert and his book, I Hated, Hated, Hated This Movie.

Why the skylight hate? 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 group 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.

Why I Hate, Hate, Hate Skylights

The Problem – 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 roof’s R-value which someone punched a hole through.

Skylights are also very difficult to improve. If installed through an insulated ceiling slope, you have a rectangular hole in the insulation to which there is no access. If installed through an attic flat, you’ll have a very difficult to access rectangular tube rising up through the roof deck. And it had better be well insulated as it is essentially outdoors.

Skylights are often installed in bathrooms…another nightmare scenario. The appeal is understandable. Much of a bathroom’s solid surfaces are already spoken for with cabinets, tiles, toilets, towel racks, etc. Nowhere to add a window and 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.

Problems Inside and Out – Skylights cause problems on the exterior as well. Standard ventilation can be a real pain. 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.

Problem the second: skylights in cold climates substantially warm the surrounding roof deck. Suppose a skylight is installed through an 18” cathedral ceiling with doubled up fiberglass batts. The skylight allows conditioned air right into this hold 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, saves you climbing on the roof and shoveling. Which leads us to problem the third: ice dams.

The skylight allows warm air near the roof deck, melting the snow. With a ventilated roof, the cold vented air is stopped below the skylight. Melted snow? Cold roof deck? That’s the formula for ice dams, some of the worst of which I’ve seen below skylights.

Skylights provide great natural light but at a cost. Ice dams, moisture damage in the surrounding drywall, pleasant sunshine…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.

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