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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

Posted on Mar 4 2014 by Erik North

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.

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 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.

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.

Erik North, the owner of Free Energy Maine, is an energy auditor and home performance specialist in Westbrook, Maine. He is also the author of the Energy Auditing Blog.


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Image Credits:

  1. Christian Guthier
1.
Tue, 03/04/2014 - 08:58

Edited Tue, 03/04/2014 - 09:01.

windows in bathroom
by Robert Swinburne

Helpful? 0

I like a window in my bathroom. But it is a problem. I need to rebuild that entire wall.
I have a skylight in my barn! A great place for a skylight.
If you must have light perhaps http://www.kalwall.com/aerogel.htm is a better solution with a U of .05. I would still want a sealed panel of glass or plexi at the ceiling plane though.

barninterior1-small.jpg


2.
Tue, 03/04/2014 - 10:27

"I like a window in my
by gideon brontë

Helpful? 0

"I like a window in my bathroom. But it is a problem. "

what are the issues with windows in bathrooms? thanks


3.
Tue, 03/04/2014 - 13:43

Devil's advocate
by Nick Welch

Helpful? -1

It's a window. The R-value is about what other windows are. We like windows because we like natural light. The R-value sacrifice is worth it.

The concerns with cathedral ceilings, I would argue, are just as much the fault of cathedral ceilings as they are the fault of skylights.

Ice dams are a legitimate concern. I'm glad I live in a mild climate. :-)


4.
Tue, 03/04/2014 - 13:44

Edited Tue, 03/04/2014 - 13:46.

Response to Gideon Brontë
by Martin Holladay, GBA Advisor

Helpful? -1

Gideon,
Q. "What are the issues with windows in bathrooms?"

A. Of course, it's perfectly possible to put a window in your bathroom. The main problem is that many bathrooms are so small and crowded that there is no wall space left for a window.

For one thing, you really don't want to put a window in your shower, since that location guarantees water, mold, and rot problems for the window stool.

If the bathroom is small, available wall space is often covered with medicine cabinets, mirrors, and towel racks. So designers end up installing a skylight.


5.
Tue, 03/04/2014 - 19:06

Edited Tue, 03/04/2014 - 19:46.

Bath positives with aj
by aj builder, Upstate NY Zone 6a

Helpful? 1

I and many of my customers love windows in shower stalls. And we have installed many skylights in bathrooms.

All positive.
Very happy customers.
No mold.
Not hard to do. Velux actually copied what carpenters learned on the job as to how to properly flash their skylights by running ice and water shield onto the curbing and the other basic is to use the ice and water shield continuously till the eave. That is also how to install an ice damn safe woodstove chimney just use ice and water shield surrounding the flue and down to the eave. Done. No water issues there after. Nada. Customer joy. Contractor joy.

Interior flashing idea is the same for windows in showers. Kurdi by Schluter is what I have used mostly along with clear silicone made for wet use. I can post pics. I have installs that date back thirty positive years.
positive aj ;)

I have also installed the solar tubes. Customer liked... me not so... I prefer seeing a view of the sky and an tree's upper canopy and more not just a bit of light thank you.

Lastly, skylights are rapidly improving. Someday we all will have super high performance glass and we can go back to putting glass where we want glass. I clipped some U values shown to be in the low .2s. Wasco is the second clip, Velux the first.

I love love love skylights. Love the sky don't hide the sky.
aj

Velux.JPG wasco.JPG


6.
Tue, 03/04/2014 - 20:36

Sizzle and drizzle
by Marc Labrie

Helpful? 0

I've been living with two Velux skylights for 15 years and I agree about the daylight and views benefits. What i'm not a fan of is the summer sizzle due to the solar heat gain and the winter drizzle either due to some internal condensation. Oh and I learned the hard way about the need to always rake the snow off the roof below them to avoid ice dams and water infiltration.


7.
Tue, 03/04/2014 - 23:01

Marc, if I installed your
by aj builder, Upstate NY Zone 6a

Helpful? 0

Marc, if I installed your skylights it might have cost a little more and there would be no roof shoveling. Good contracting is worthy but is often not paid for or found in the first place. As to summer heat there are summer shades. As to internal water the newest units have interior gutters that catch it. The last problem that cannot be controlled is homeowners not controlling internal moisture which in my experience many homes are adding too much moisture thinking they need it because winter air is dryer than summer. We get what we get... sow what we sow....

As am example my own home has decades old Velux units in a cathedral ceiling. No problems. Just installed right and used right.
aj


8.
Wed, 03/05/2014 - 08:38

I didn't think it through
by Roy Brohaugh

Helpful? 0

I put 2 Velux roof windows in my heated shop in northern Wisconsin. I love the light that streams in although right now not a lot - they're covered with snow. I'm glad I did it because it caused me to not do it in the nearby house we're building. My shop ceiling is well insulated, including the shaft walls. But what I didn't think about is where the roof window meets the roof - a pretty substantial thermal bridge!


9.
Wed, 03/05/2014 - 20:07

skylights
by Howard Partch

Helpful? -1

I agree with the original comments about skylights. They really only work in a hot desert and even then they cause you to "lose your cool". When we wanted to bring light into the interior of a building with a cathedral ceiling, we always designed a clerestory/dormer into the roof. IE the insulated glass is vertical and we always heated/ventilated the glass to control condensation/drips. Also, we reminded the client when you bring in natural light, you lose heat and build in higher maintenance costs.

We almost never used a clerestory with a cold attic. That is so difficult to do.


10.
Thu, 03/06/2014 - 09:54

There are 2 sides to this...
by David Potter

Helpful? 0

As a Professional Realtor since the mid 1980's I recognize that value and quality of the investment for one side is measured in terms used daily by engineers and that the other side (while appreciating the value and importance of these considerations) enter a home and set about throwing open the window coverings to experience the natural light (artists, architects, perhaps poets... ;-).

I have slightly more in common with the artists, architects and poets... ;-)


11.
Thu, 03/06/2014 - 18:18

skylights
by daniel hagan

Helpful? 0

As a proponent of thermal efficiency, I always hated skylights. At the insistance of my main designer (my wife), I installed an 18" by 18" skylight in a corner of a room where a woodstove penetration had existed. What great light bouncing off of the white walls! I now find very small skylights to be acceptable. I believe the main problem is when designers lose discipline and go grandiose in size. By the way, I have always made any shaft required to be outside the thermal envelope, with glazing at ceiling insulation level, and the double bubble acting as a weather shield only, with venting.


12.
Thu, 03/06/2014 - 22:45

But Why?
by Peter L

Helpful? 0

What is the point of installing a skylight when it adds complexity, energy loss, roof leaks, moisture problems, and a host of other issues? 95% of all contractors are going to mess-up the install details and a roof leak is all but inevitable. Having a R-50 ceiling and then cutting out a 3x3 hole in it is beyond dumb. There is a reason why skylights are loathed by building scientists and loved by GC's. It's a money maker and a great call-back component that will keep making money for contractors for years to come.


13.
Fri, 03/07/2014 - 16:45

Skylight discussion
by VELUX America

Helpful? 1

Eric – “Hate, hate, hate” is a bit strong, don’t you think? Skylights used in moderation and in optimal locations on a home (south or north facing) can contribute to home energy efficiency.

[This comment has been edited. Velux America, a manufacturer of skylights, posted 16 paragraphs of text that amounted to an unpaid ad. The paragraph above gives a flavor of Velux's argument. Needless to say, the company wants people to buy their products. I'm sure that interested GBA readers can track down the Velux website if they are interested in this type of promotional material.]


14.
Fri, 03/07/2014 - 17:00

Response to Velux America
by Martin Holladay, GBA Advisor

Helpful? 0

Dear Velux,
GBA provides a forum where designers, builders, and homeowners interested in green construction can exchange ideas and learn from each other.

We're not really a good place for manufacturers of building materials to try to post unpaid ads.


15.
Mon, 03/10/2014 - 06:15

Skylights - What they do
by Miles Forsyth

Helpful? 1

I'm told that the 'sky vault' delivers three times as much daylight vertically as it does horizontally. In theory an equivalent window should deliver the same amount of daylight as, say, a rooflight of about 40% of the windows' area.

In a 'model' room, where there are two external walls and a roof above and a window area is equivalent to 25% of the floor area, lets say the heat units lost have a value of 10.

A comparable room with the same dimensions and insulation values, but with a rooflight that is say 10% of the floor area in size and no window the comparable lost heat units would be around 8. Potentially a 20% saving in energy lost.

My maths and theory may be a little wonky but, in the UK's moderate climate I'm often advocating rooflights (where appropriate) because they deliver a great 'bang for the buck' in certain situations. The ice dam issue certainly might dissuade me if it got that cold here !

I guess the alternative is to construct a 'dormer' window (where the room is in the roof) and again these aren't always appropriate in terms of the overall building.

Oftentimes its the view (or not) that determines whether to rooflight or window. I've lived and worked in places with both and agree that direct sun and the cold surface/condensation with rooflights isn't great as an occupant, but then an artificially lit and ventilated room isn't so good either. It's about balance then and rooflights do tend to deliver good daylighting on an overcast day. Overall, in the UK climate, my vote would be FOR them, where they suit the overall and balance of requirements.


16.
Mon, 03/10/2014 - 06:58

Response to Miles Forsyth
by Martin Holladay, GBA Advisor

Helpful? 0

Miles,
Most energy analyses show that skylights are a net loser of energy. In most cases, undesired heat gain during the summer, and undesired heat loss during the winter, increase energy bills so much that the losses overwhelm any savings in lighting energy.

There are exceptions; in commercial spaces, where lighting energy use during daylight hours can be high, skylights can sometimes save energy. However, skylights make less sense in homes. Because many homes are unoccupied during daylight hours, most lighting energy use in homes occurs at night.


17.
Mon, 03/10/2014 - 13:45

Rants aren't useful
by Keith H

Helpful? 0

Articles berating common building materials and techniques aren't that useful IMO. There are a lot situations where things like cathedral ceilings, can lights, or skylights can't be avoided, are required by the customer, or already exist. A discussion which genuinely evaluates two alternative choices (vertical window vs skylight: wufi for vapor, energy model for heat loss) or presents ways to minimize the impact of the disapproved choice are much more useful. For example, if I'm a homeowner reading this, will a new velux skylight help address exists problems with an older skylight? Or is humidity sensing bathroom ventilation a better choice? Has any one modeled the moisture spikes from skylight condensation? Does WUFI or Bio say that you'll grow mold? Are their climates where you can get away with skylights and climate where you cannot? What detail elements are important for minimizing structural damage around the skylight? I personally have what I believe is a 1987 skylight in a bathroom in a low slope unvented roof with no roof deck insulation and dysfunctional ventilation. To boot, the drywall returns in the skylight shaft (~18" shaft) did not have a vb. Yet, the drywall grew no mold during the 5 years I've had it and I didn't find mold on the roof deck near the skylight. On the other hand, I've found evidence of condensation in the cold side of that roof space (4' canti that is not insulated well and perceptibly colder).


18.
Tue, 03/11/2014 - 12:56

Edited Tue, 03/11/2014 - 13:00.

Suggestions are useful
by rjp :

Helpful? 0

As a homeowner in a 30 year home that started with five 4x4 plastic skylights and a huge number of non-IC recessed lights, action to improve this home was the order of the day. New Velux insulated/tempered glass skylights from Lowes easily installed with 4 screws into existing curbs. The ventilated attic with duct work was sealed in two days with fire rated foam. Attic stays 65F or higher all winter, duct temperatures are improved, the skylight shaft is in a conditioned space, the new skylights have a superior rubber gasket, it's been a win/win. All at less then 3 or 4 mortgage payments for all of it.

With energy costs as they are, even the least qualified contractor knows the issues for new construction. So I have to agree with Keith H that rants may not be especially useful for contractors and for homeowners wishing to improve. Otherwise we are just preaching to the choir.


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