The Benefits and Drawbacks of Skylights

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skylight-solar-heat-gain-air-conditioning-load.jpg

Everyone loves skylights. Right? They bring so much light into a room they can turn a Seattle kitchen into a bright and sunny Florida room. Especially this time of year (in the Northern Hemisphere), having that extra light brighten even the darkest days of winter. But skylights have a dark side, too. If you're not aware of that when incorporating these roof windows into a home, you can end up with high energy bills, rooms that are unusable at certain times of the year, or expensive repairs due to moisture problems.

The benefits

Of course, the main benefit of having skylights in a home is all the natural light you get from them. This is mainly for winter because that's when we have less overall exposure to the natural light our bodies crave. Nobody likes being holed up in a dark, dim, artificially lit cavern on those overcast winter days when the Sun sets shortly after it rises. (Well, maybe with the exception of video gamers and college students who are trying to sleep during the day.)

A well-placed skylight or two can make the difference between feeling OK about yourself on those dark days or becoming a psycho killer. (Qu'est-ce que c'est?) For those who suffer from Seasonal Affective Disorder (SAD) and can't spend time outdoors, being in well-lit interior spaces can make all the difference.

If you live in a cold climate, you can also get some solar heating from a skylight. To do so, you'll need to make sure you' get one with as high a solar heat gain coefficient (SHGC) as you can. (The SHGC tells you how much solar heat gain you'll get from a window or skylight. It's generally a number less than one, with lower numbers meaning the skylight will block more of solar heat from entering.)

There you have it. The two main advantages of having skylights is the extra natural light you get and the possibility of some solar heating in winter.

The drawbacks

But those two benefits come at a cost. The extra natural light and solar heat you get in winter are welcome in winter, but in the warmer parts of the year they can be a nuisance. Before I get into the details, let me make a list of the main drawbacks of skylights.

  1. Too much light on bright sunny days
  2. Too much heat gain, leading to overheating
  3. Heat loss in winter
  4. Skylight shafts that aren't insulated or air sealed properly
  5. Moisture problems from leaks where the skylight penetrates the roof

Let's take a look at each of them now and discuss how you might overcome the drawbacks.

1. Too much light

Depending on the orientation and size of the skylight, you may end up with a room that's just too bright. When the sun shines directly on the skylight, it can make a room practically unusable at certain times of the day.

What can you do about it? One way would be to keep skylights on north-facing roof sections. That will limit the amount of direct light coming into the skylight, especially if the sunlight has to travel through a shaft to get into the room. If you can't locate it on a north-facing roof, put it higher on the roof to take advantage of a longer skylight shaft.

If you can't do those things or still have a problem, you'll need another way of keeping the excess sunlight out, typically shades or blinds. At this point, you should ask yourself whether it might be better to chuck the whole idea of having a skylight. And we still have four drawbacks to go.

2. Too much heat gain

As I've written in the past, windows, including skylights, are a thermal liability. They aren't required to limit the amount of heat loss or heat gain nearly as much as walls, ceilings, or floors. Here's the table from the 2015 International Energy Conservation Code (IECC):

2015-iecc-fenestration-skylight-u-shgc-value.png

Looking at the U-factor columns, we can calculate that the equivalent R-values range from 1.3 to 3, whereas walls would be required to have insulation of R-13 (CZ 1) to R-23 (CZ 6 and higher).

How does this impact the heat loss in winter or heat gain in summer? We did a little modeling with our Manual J software (RightSuite Universal) and found out how much extra capacity you'd need for each square foot of skylight area. Let's take a look.

Florida, Climate Zone 2

First we looked at what happens in a hot climate, using the following specs:

Windows:  U = 0.33 and SHGC = 0.25

Skylights:  U = 0.53 and SHGC = 0.24

For summer, we got the following results:

Orientation
Window
Skylight
Multiple
north
11.7 BTU/hr/sf
66.7 BTU/hr/sf
5.7
south
13.6 BTU/hr/sf
67.4 BTU/hr/sf
5.0
east or west
30.4 BTU/hr/sf
73.5 BTU/hr/sf
2.4

The data here show that putting skylights on north-facing roof gives you the lowest heat gain of the four choices, but there's not a lot of difference between them. The bigger conclusion to draw from this is that skylights will add a lot more heat to your house than windows.

Of course, we used specs for windows and skylights that just barely meet code. By installing skylights that are much better than code, you can reduce that heat gain.

Michigan, Climate Zone 5

Then we looked at a cold climate, with the following specs:

Windows:  U = 0.33 and SHGC = 0.25

Skylights:  U = 0.53 and SHGC = 0.24

The window U-value in this case doesn't quite meet the 2015 code, but the difference between 0.33 and 0.32 is negligible.

For summer, we got the following results:

Orientation
Window
Skylight
Multiple
north
10.2 BTU/hr/sf
60.1 BTU/hr/sf
5.9
south
16.3 BTU/hr/sf
62.3 BTU/hr/sf
3.8
east or west
28.8 BTU/hr/sf
66.8 BTU/hr/sf
2.3

Again, the data show that you probably ought to go with skylights that are significantly better than required by the energy code if you don't want to overheat in the summer.

Also, the heat gain calculations above don't include any skylight shaft area, as the skylight in question here was like the one shown in the photo at the top of this article. It was in a cathedral ceiling.

3. Heat loss in winter

We did the same calculations for winter heat loss and found this:

Florida

For winter, the windows have a heat loss of 12.1 BTU/hr per square foot. The skylight loses 19.5 BTU/hr per square foot. The skylight in this case is about 1.6 times worse than the window because it's allowed to have a higher U-value. Solar heat gain didn't factor into the calculation because the peak heat loss occurs at night, when it's coldest.

Michigan

For winter, the windows have a heat loss of 20.2 BTU/hr per square foot. The skylight loses 32.4 BTU/hr per square foot. The skylight in this case is again about 1.6 times worse than the window.

Heat loss in winter isn't as bad as heat gain in summer in this case, but it could still create comfort problems if your skylight doesn't have good specs.

4. Poorly insulated or air-sealed skylight shaft

Another problem that occurs with skylights is that they sometimes go through unconditioned attics, as you see in the photo below. The framers will build a shaft that has 2x4s on the flat (look at the one on the left with the wire stapled to it), and that makes it difficult to insulate the shaft with traditional insulation materials. The fiberglass batts you see here, even if they weren't peeling back, would not do a good job of insulating the shaft because of poor contact. These shafts often are air leakage sites as well. 

skylight-shaft-unconditioned-attic.jpg

What can you do? One method would be to move the building enclosure to the roofline. Just make it a conditioned attic, and now you don't have to worry about insulating the skylight shaft. Another method would be to use spray foam insulation on the shaft.

However you decide to handle this problem, you need to be aware that that skylight shaft is part of the building enclosure. If it's running through a unconditioned attic and is poorly insulated or air sealed, it's going to create a thermal liability, adding heat to the house in summer, stealing it in winter.

5. Moisture problems from leaks

You should always consider carefully when thinking about adding any penetrations to the roof. Yes, we have materials and methods that can minimize the risk of water leaks, but over time, things change. If you can avoid the risk by figuring out other ways to get that natural light into a home, that's probably a wiser way to go.

The bottom line

Skylights can definitely bring some necessary cheer to a home by allowing more natural light to enter in winter. But while you might get just the right amount in winter, you can end up paying the price later by overheating your house and making rooms unusable or uncomfortable. Here's my summary of how best to use skylights:

  • Install them in climates that don't get too hot in summer. (Think Seattle.)
  • Install them on the north-facing roofs.
  • Install them where they'll get shaded in summer if overheating is an issue.
  • Get skylights with the best specifications you can find. That means double or triple pane with U-values and solar heat gain coefficients well below what the code requires.
  • Eliminate skylight shafts or make sure they're insulated and air-sealed.
  • Use the best water management materials and methods you can to reduce the risk of water leaks at the roof.

Sometimes, though, the best thing to do is just eliminate skylights altogether. If you're in the design phase, this is a lot less expensive to do than if you've already got the in your house, of course.

 

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Comments

David Butler

And then there's the cleaning issue... skylights quickly become mucky but can be difficult and/or dangerous to clean, especially on a high pitched roof, or a high roof. When I see them on plans, I try to steer my clients away from them. That said, my wife insists our next home have a couple of solar tubes. Guess who's gonna win that one. Sigh. At least those are marginally better than a full blown skylight.

Allison Bailes

And if the skylight is under a tree, it gets really, really dirty. Good point, David.

Dale Sherman

Nice coverage, Allison. Your comment about batts on light shafts didn't quite make sense. Also, batts perform best when fully enclosed.

My experience with skylights in cathedral ceilings is they can cause moisture condensation issues if not air-sealed properly or bypass channels installed for blocked rafter bays. Oftentimes homeowners think they have a roof leak at the skylight when it is actually moisture condensation due to air leaks and blocked roof ventilation.

Allison Bailes

I didn't know why you said my comment didn't make sense but when I read it again, I saw that I'd left out a word. One little well placed "not" makes a huge difference. Thanks, Dale.

I thought about mentioning condensation and should have included that, too. Thanks for bringing it up, Dale.

Nate Adams

Thanks for the great review! I sometimes tell clients I'm schizophrenic about skylights - the aesthete in me loves them, the auditor in me hates them. I love the North side recommendation, I hadn't considered that. (Facepalm.) The shaded part is also a nice tip, since you'll only get lots of heat gain in winter when you want it.

One product you might want to mention is a tubular skylight like SolaTube. I think they solve or reduce a lot of these issues. The roof penetration is much easier to seal, much like a plumbing stack. Insulating the walls is still problematic, a foam kit is probably best. They transfer a nice amount of light but not nearly as much as a big one, so overheating is a smaller issue. Since they are typically in the 1-2 square foot range, heat loss is a smaller problem, too.

Thanks again!

Allison Bailes

You're welcome, Nate. I was going to include the Solatube in this article, but then it got too long and I decided to save that for later and just write one article about it.

Steve Lefler

Since 2009, we have installed the Sola Tube as the skylight replacement to resolve your listed negative items. The LED light upgrade reduces can lights in kitchens and brighten dark hallways day and night. We find it as a solution.

Allison Bailes

I like the Solatube, Steve. It's not a perfect solution, but it's almost always better than a full-blown skylight.

David Eakin

"Bandaids" on bad designs. Most designs for double-pitched roofs go with architectural designs that go back several centuries (or try to resemble them). None have good "day lighting"; none are particularly easy to integrate into a high-performance design, but are most often used "because we've always done it that way".
A flat roof canted to the angle of the Winter Solstice in your area combined with clearstory windows on the South side will provide illumination to the Northern-most interior recesses, provide maximum solar gain (when needed), help shade the solar gain when not desired, is easy to construct and integrate into a high-performance design. But it is not "normal" looking in a culture where Western European design rules.

Allison Bailes

David, I assume you mean that skylights are bandaids to make up for bad design. I agree. I like clerestory windows for bringing in light, but they seem to have disappeared from residential architecture in the US after the 1980s.

Another option I read about way back in the '90s is to use fiber optic cables to bring natural light to the interior of a building. I read in a book titled Renewables Are Ready about a building in the Netherlands doing that and thought it was a clever idea. Haven't heard a word of it since.

Debbie Coleman

Excellent article! I will now send clients this article link instead of trying to describe the drawbacks of skylights myself less eloquently.

On my own remodel, I had an interior bathroom on the second floor with no exterior walls and the roof just 2' above - wow, perfect for a sky tube! But then it arrived and I just stared at it. I just could not bring myself (well, the builder) to cut the hole in the roof! That roof was high above the ground and with a slope that I would not want to be on to inspect it. I instead ordered a $500 Custom door with frosted glass to get some light form the part of the bath room that did have a window. I can now sleep at night...But then I donated the skylight to an affordable housing project I was working on that had a second floor interior bathroom. Gain the sky tube sat in the box for awhile and then on site the builder and I just stared at it again...once again could not authorize the hole in the roof. I am not sure what happened to that sky tube. I hope it ends up on a one story home with a roof that is easy to access.

Allison Bailes

Interesting story about your sky tube, Debbie. Who knows? It may still be making the rounds in search of that perfect project. Another idea: Install it in a wall instead of a roof. There's a photo on solatube.com showing that use.

Armando Cobo

I disagree that skylights and solar tubes are bad design. There are different qualities of both of those products to avoid too much heat gain/loss, lighting, condensation, and of course, there is always the inevitable bad craftsmanship. Unfortunately, most builders and homeowners buy the cheapest quality products. Dormers can be a good alternative to the issues above, but they cost considerably more than even the high quality skylights or tubes, and they are not always a good solution to some architectural styles. Let’s be careful of having an architectural design police or mandated architectural styles to fit energy efficiency.
For many of my clients, designing bonus rooms or rooms in attic space is a big cost reduction on the house and a good alternative to using space that otherwise would be empty. I’ve been designing high-performing houses exclusively for over 20 years, and I’ve yet to have one house with the problems mentioned in the article. The bottom line is to buy quality products and install them correctly.

Allison Bailes

Armando, if you can design and spec bonus rooms with skylights and they have no problems, more power to you. That's a really difficult task, especially in Texas. What U-values and SHGCs are you using for skylights? What roof orientations do you install them on? Do you use any kind of shading?

Armando Cobo

I don’t want to sound like a Velux dealer, but they have great Skylights and Sun Tunnels. They are not cheap, but they are way above the competition in performance. Check them out.
For the Skylights, U=.42-.27 & SHGC=.23-.12 depending on model and shades. For Sun Tunnels, U=.51-.16, again depending on model. The house orientation is more important than the roof orientation, therefore if I have a way to block some of the direct sun, fine, if not, we do have a small penalty. Do have in mind that the Skylight/Sun Tunnel area is typically around 1% or less.

Tyler Kime

What about these new solar tubes? is there any r-value to the flexible tube running through an unconditioned attic?

I have found that most skylight shafts are not sealed at the top and can cause mold build around the shaft in the summer here in Alabama. I remember a few years back when I found one with a large gap in an attic, I used my belt to slide through the opening into the house to show the customer the problem

Allison Bailes

The Solatube (solatube.com) is a good alternative. Their specifications for U-value and SHGC are mostly close to what's allowed by code, but they do have one model with an SHGC = 0.16. As far as I can tell, they don't provide insulation for the tube. Probably the best way to insulate it would be with spray foam.

Avery Colter

Maybe it's different and better now, one would hope. I bought a fixer in 2001 and thought a couple of skytubes would be a good choice, one in the bedroom and one in the living room. They were simple ones, with an inline socket for a light bulb. I eventually put in LED PAR38 so the light would be downward and not have half the light going upward out of the roof. The seals seemed good enough at the roof, as there was never any issue with water. There was absolutely no insulation however on the tube itself. The tube came in two pieces which had gaps that I had to try to tape together, but that wasn't a perfect fix. As a result, bugs would get in the tube, and I had to take off the diffuser periodically to clean it. The diffuser was one of the most annoying things, very difficult to twist on the right way. I had to screw wall mirror brackets into the ceiling to keep one of them on in the bedroom, and in the living room one, having actually lost the diffuser, I ended up taping Saran Wrap over it. The funny part is the taped-on Saran Wrap turned out to be a better air barrier than the diffuser, and I have the blower door readings from the county weatherization program to prove it! Have not put any sky tubes in the new house. My problem is the opposite of one of the posters: my wife would kill me if I made mention of them after the past experience. And I myself, if I did any such thing again, it would need to be a high-end low-e model with a day dimmer.

Allison Bailes

Thanks for sharing your experience, Avery. That's good to know.

Ryan Shanahan

If you can get a good thermal seal at the bottom lense of the solatube (on the flat ceiling plane) in theory it negates the need for insulation on the tube. I've always been curious if the NFRC ratings on solatube are for the top lense or the bottom. Does anyone on here know? Curious...

Allison Bailes

I'm going to be giving them a call soon before I write an article about their products. Stay tuned.

w d

Allison,

Nice summary on the +/- of skylights. I especially appreciate the heat gain data.

Our house has two skylights. I'd prefer zero. We've found a way to live with our skylights. The idea is to accept the solar heat gain in winter while blocking it in summer. First, we created another glass pane by custom fitting an acrylic sheet on the interior in the skylight well. Next, we installed solar grates on the exterior during summers (a/c season). The grates are in storage during winter (heating season) and thus the solar gain is maximized. As far as aesthetics, the lighting benefit from skylights is available year around. When the grates are in place, indirect light is passed while direct light is blocked. Hence, no "too bright" effects.

We also use solar grates on our windows for similar benefits. The reduction in electricity for air conditioning is substantial.

Steve Vollstedt

Allison - For your btu/sqft/hr calculations, how did you come up with the outdoor temperature in the summer & winter, and what did you use for indoor temperatures? I am assuming that for the summer you used the average high temperature for the 2 or 3 hottest summer months and similar for the winter season, right?

Allison Bailes

Steve, we used the 99% and 1% outdoor design temperatures for the locations we chose, which were Pensacola and Detroit. We used the standard indoor design temperatures of 70° F and 75° F, as recommended in Manual J.

For more about design temperatures, see:

We Are the 99% — Design Temperatures & Oversized HVAC Systems

www.energyvanguard.com/blog-building-science-HERS-BPI/bid/53306/We-Are-t...