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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{ 2 comments… read them below or add one }

Anthony May 29, 2013 at 12:00 pm

So, apart from removing them, what are techniques to improve them? I have 3 skylights in my home, one in a bathroom, and this year had problems with all of them. The change that I made was to install much better heating and cooling in the house, tightening up the building envelope (although there is some way to go there). That, along with a colder winter than last year, caused a lot of condensation to form on the skylights. Using a digital thermometer, I found that there was a lack of insulation around the skylight, which exacerbated the problem, so I drilled some holes in the drywall and injected foam to try to rectify that. I also applied window film over the skylight in the bathroom which immediately fixed the condensation problem. What else would you recommend?

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Erik North May 29, 2013 at 12:20 pm

Anthony,

Thanks for writing. Like you’ve sussed out, solving the probems with skylights is a matter of controlling temperature and moisture. You’ve done some smart things to help the situation. Improving the insulation around the skylight is important and can help a great deal if the space is accessible.

The biggest condensation problems are (unless there are some odd ventilation/insulation issues going on) usually closest to the exterior. Adding that window film keeps the warm, moist interior air away from cold surfaces where it might condense. During retrofits, I usually suggest a more permanent version of this, using a tempered glass panel, finish wood trim and caulk to create a airtight barrier.

Erik

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