Image Credit: Christian Guthier
More Guest Blogs
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.
Weekly Newsletter
Get building science and energy efficiency advice, plus special offers, in your inbox.
34 Comments
windows in bathroom
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.
That's a lovely space!
"I like a window in my
"I like a window in my bathroom. But it is a problem. "
what are the issues with windows in bathrooms? thanks
In some cases it's a matter of privacy. But it's very easily solved.
As Martin Holladay mentions below, available wall space can often be an issue. When I design homes I try as much as possible to put plumbing against interior walls. This has the added benefit of making it easier to put windows in bathrooms, because by definition I don't have plumbing fixtures on those walls.
Devil's advocate
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. :-)
As far as skylights being like windows, due to their orientation a skylight of a given size will usually let in far more natural light than a window of the same size. So if the purpose is to obtain natural light, they could be a better solution than a window, at least in some situations. Of course there are several other factors involved.
Response to Gideon Brontë
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.
Bath positives with aj
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
Sizzle and drizzle
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.
Marc, if I installed your
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
I didn't think it through
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!
skylights
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.
There are 2 sides to this...
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... ;-)
skylights
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.
Having the shaft near, or even better in line with, an interior wall really gets the full effect of what skylights can offer. They can really add beautiful natural light to a space. But they do it best when that light is casting onto a wall surface. Just popping a hole in the ceiling in the middle of a space is rarely the best option.
But Why?
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.
"There is a reason why skylights are loathed by building scientists"
There are also reasons why skylights are loved by homeowners, and that is more than enough justification to install them in homes where people actually live their lives.
When the house is built the engineers, architects and contractors walk away. But the residents spend the rest of their lives there.
Design for natural light, and install skylights when they are an appropriate solution. Detail well. Control the work of contractors to ensure quality...
And give the residents a place they will love living in.
Skylight discussion
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.]
Response to Velux America
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.
Skylights - What they do
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.
Response to Miles Forsyth
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.
Rants aren't useful
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).
Suggestions are useful
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.
How about an insulated window beneath the skylight in the shaft?
How about a removable and insulated window at the bottom of the shaft, a glass ceiling, to improve the energy loss and moisture damage at the skylight. During the conditioned or heater seasons, the insulated windows works as a glass ceiling. That make the shaft a dead space. Will we need a vent in the shaft? Where will the air go? In spring and fall, people can remove the window.
Interesting article. I don’t disagree but at same time, they are a required “evil” at times.
I am also intrigued by skylight or any window blind heat loss or gain effectiveness. Unless there has been enormous strides in the technology, not sure how really effective they are. Years ago, I ran hourly analysis software modeling large buildings and the energy models showed time shifts in peak heat gain but not much diff in total heat gain, with interior blinds. If reflecting heat back, can you risk overheating the sealed units? What will glass manuf say?
I hope that the author of this blog will accept all my details below as I believe I have some good information to share which was learned the hard way.
I live in Montreal. From 1995 to 2014, I installed 7 skylights in the roof of a small bungalow that was sorely lacking window space. I started out with two skylights that were manufactured in Maine and followed the manufacturer's instructions. What the manufacturer didn't say was that a glacier of ice would form a few feet below my skylights and grow to be 6" thick and 6’ long by the end of the winter.
When I replaced these 2 skylights in 2002 with those from another manufacturer, I added 3' wide by 8’ long seamless copper sheet flashing that created a 6" wide skirt on each side of the skylight fixture and extended 3' below where the glacier typically formed. I also added 2 more skylights. In 2010, I wanted to add natural light to a windowless bathroom, so instead of following instructions, I built a tunnel with thick foam insulation and instead of having a 4" high curb on the roof deck to mount the skylight, I made the curb a foot high, completely covered in soldered copper sheets, and with not one, but 4 double pane thermo glass systems tucked neatly into the top of the tunnel with spacers between each glass system (see attached photo). No condensation in winter while taking a shower when there are a total of 8 sheets of glass.
And in 2014, I added another 2 skylights above a new kitchen island, for a grand total of 7.
In 2016, I redid the roof shingles and put down peel and stick membrane on 100% of the roof deck before installing asphalt shingles. Winter 2018 delivered a lot of snow and I naively believed that my new 100% membrane coverage would allow me to ignore the 5' of snow that was accumulating, whereas in years prior to 2016, I had regularly cleared snow from around the skylights after each snowfall.
In the spring of 2018, I gad a gallon of water that somehow got through one of the skylights and found its way into my ceiling below. So I asked myself why the membrane had not completely sealed the roof and I concluded that there was a detail that no membrane manufacturer ever mentions. When you install 3' wide rolls of membrane in horizontal rows, and you have to use 2 pieces that overlap by 6", there is a microscopic vertical channel that is created at the seam because the membrane material can't perfectly mould itself around the edge that it is stepping over. Capillary action draws water into this grove and water will move upward. With each horizontal row of membrane overlapping the previous one by 6”, with my low sloped roof, the water only had to rise 2” vertically in the groove until the water contacted the bare wood. However, I also cut my 3’ wide membrane into 12" wide pieces for easy installation around the skylight curbs and my patchwork had minimal overlap. So even small ice dams would be high enough for water to penetrate my membrane pieces. Now I no longer trust peel and stick membrane for use around skylights.
So during the winter of 2018 with no snow shovelling having taken place, I most certainly had an ice dam below my number 2 skylight and that is how my roof leaked.
So my advice to anyone wanting to install a skylight:
(1) Make your tunnel go 2'-3' above the roof deck so that the wind will blow off as much snow as possible from the heated glass surface, and there will be less snowmelt to re-freeze into a glacier below.
(2) Build your tunnel with 12" thick wall cavities filled with batting below the roof deck and with 4" thick wall cavities, filled with rigid or sprayed-on foam, above the roof deck.
(3) Install torch-on commercial roofing material (designed for flat roofs) on the roof deck around the skylight tunnel where it protrudes through the roof and extend the torch-on material 2' on each side, and 2’ above, and 6' below the sides of the tunnel.
(4) Extend the torch-on membrane up the sides of the outside of tunnel to the very top where the skylight fixture is located. Every seam of torch-on material has to be fused together perfectly.
Had I known all this back in 1995, I would have avoided all the leaks and avoided the often daily visits to the roof to clear off the latest snowfall.
Two more things to remember. A skylight that is covered with snow yields no benefit of natural light to the room below, so some snow clearing is unavoidable. And if you let snow build up all winter, while a well sealed roof may not leak, the weight of the snow could pose a hazard to the roof structure, so again, some routine snow clearance is necessary even if your home doesn't have skylights.
I live in the Hudson Valley in New York. I live in a converted barn open space with SSW exposures and vaulted ceilings. I have four large skylights facing SSW.
In winter, the heat gain is negligible. The one over my bed is a waterfall of cold air at night. You can actually feel the breeze.
In summer, the heat gain is stiflingly hot. It gets over 110f on an 80f day on the insides at the top of the skylights according to my crappy IR camera.
I made some crappy covers out of 1/2" plywood painted, and glued over and 1.5" solid foam with 1/2" plywood sides. I slide them over my skylights and they don't fly away in the wind. They fell apart after two summers, but while in service they actually helped quite a lot.
They're very pretty, but they're really not worth it. I have lots of SSW facing glass windows that I shade from the outside with mesh screening common in the Southern US. When I get rooftop solar installed I'll have roofers come in before and remove the skylights.
Any designs for insulated skylight coverings until the roofers come?
@user-1034802, you seem to be able to easily access the top of your roof. Once up there, can you unbolt the skylight from its curb with a screwdriver? If so, can you lift the skylight off its curb and expose the top of the tunnel. My 2’ x 4’ Velux skylight is held onto its curb by 10 screws. You can then insert a panel of rigid foam cut to size into the top of the open tunnel. To keep the foam panel from falling down the tunnel, you would first have to install something at the top of the tunnel to support the foam panel. This could be as simple as a block of wood screwed into each of the 4 corners of the tunnel. After the foam panel is in place, bolt the skylights back onto the curb. You could buy the kind of rigid foam (typically 1/2" thick) that has a alu foil lining and make a sandwich of 2 or 3 layers to get R7.5. The foil will reflect the summer sun towards the outside and the foam will keep the indoor heat from escaping in the winter.
That's a really great idea! Yeah, these skylights are just orange box store 15 year olds. I'll give it a shot! A lot less material to lug around and no chance of flying foam across the yard!
It is unlikely that the top of your skylight tunnel will be a perfect rectangle, or that you can cut the rigid foam to the perfect size. So there will likely be a small gap(s) between the edge of the foam panel and the sides of the tunnel. You can cover over this potential gap with a roll of foam tape (3/16" x 3/8" cross-section) that has a self-adhesive backing. Once the panel is in place, just unroll the foam tape onto the top side of the panel along the edge so that the tape covers any gaps. The foam tape will keep humid air from finding a route to the cold skylight glass and condensing there and ultimately dripping down the tunnel onto your bed.
I have sill seal foam rolls sitting in my shed for a decade. Looks like they just found their purpose!
Thanks so much for the thoughtful replies! I'm going to experiment on the light that's easiest to access and isn't over an in-use living space.
Let's just shove everybody into hyper-insulated boxes with code minimum window openings. That will make the engineers happy (and everybody else miserable).
Sorry but this sort of conversation frustrates me a bit. Yes, performance & energy matters ... a lot!
But so do the pleasures of daily life, which good architecture is supposed to promote. Thoughtfully designed spaces that may include skylights can have very pleasant natural light, something for which there are known physiological benefits.
And yes, in areas where snow accumulates on roofs extra thought should go into the design. Perhaps a dormer or clerestory is a better option. But in the USA the large majority of new homes are being built in zones 1-3. So the ice dam issue is a non-issue for most new homes.
So let's not just throw out skylights.
I hear you, I really do.
The most comfortable place I've ever had the pleasure of spending time had no skylights, an expansive brim of a roof around the house, with several porches and, most importantly *well proportioned* windows around the whole perimeter. I felt in an open safe haven.
What we've created in modern housing is a hodge podge of design ideas that don't actually fit the environments they're in. Deeply and purposefully integrating with the climate and site is more emotionally impactful than "lots of light."
I sat around with a dozen US and EU architect friends and asked them, "what's the hardest thing to design for in building residences." They responded, almost in harmony, "wind driven rain." And one added, "US residences are stupid. Look at a the construction of a Japanese home, for example - LONG EAVES. But our customers reject the look."
And I'm just as guilty - I live in a barn. lol.
My current home is 1 mile from the Atlantic, with plenty of wind driven rain! It's also at a northern latitude, which means long nights in winter and grey days.
I don't have to deal with snow, thank heavens!
I've also installed 4 skylights that radically transformed the interior natural light of my home, making a place that was downright depressing, literally so in the middle of winter if you have to deal with SAD, into a home that I enjoy so much I find it hard to leave just to go to the store. Fortunately I work from home so I rarely have to.
There are a lot of ways to live and I hope we make buildings that promote that rather than push people towards homogeneity. If skylights offer a very good solution to residents, then it's our job as designers and builders to deliver that solution in the best way possible. Of course, if residents want skylights but for reasons they may not be aware of it is NOT a very good solution, then it's also our job to educate them as to the reasons why they should reconsider. But as far as I'm concerned, it's their home. They'll be living with it for many years. So if they want it I should do my best to try to deliver it.
Log in or create an account to post a comment.
Sign up Log in