Practical factors for skylights
These performance ratings seem to be the standard guide. (NOTE: These are applicable to doors and windows in addition to skylights.)
- U-factor – This describes the rate at which skylights transfer non-solar heat flow, or heat that isn’t generated by sunlight, such as the warm air in your home during the winter. The lower the U-factor, the better, regardless of the climate in your area.
- Solar heat gain coefficient – In straightforward English, SHGC describes the amount of solar heat that a skylight conducts into a home.
- Air leakage – This simply refers to the tightness of the seal around the skylight. Lower air leakage means less cool air gets out during the summer, and less warm air gets out during the winter, so obviously, the lower the rating, the better.
For the purposes of daylighting, there are two main metrics to examine:
- Visible transmittance: The amount of visible light transmitted through the glass or acrylic. The lower the VT, the less visible light admitted. This can help control glare in a particular room, but it can also reduce the level of daylighting available in the room, so you may need to try to strike a balance here.
- Light-to-solar gain – This is the ratio of SHGC (the amount of solar heat allowed through the window) to VT (the amount of visible light allowed through the window). This is likely the broadest indicator of a skylight’s overall energy efficiency. The higher the LSG rating, the more light can be let into a home without allowing excessive amounts of solar heat.
So what does all this mean? Here in Houston, we would want a unit with a very low SHGC and U-factor, plus a very high LSG rating. All told, this will almost certainly drive up the cost, but I’ll pay a premium for a high-quality unit if that means I can keep the heat to a minimum.
Letting the light into a home can illuminate your space in more than one way. The obvious one is more natural light, which has proven health benefits, not to mention making your space look nicer just by introducing light as a factor that accentuates things like mood and texture in your space. But, another is the effect it can have on your energy usage.
To talk more about the ins-and-outs of skylights is guest writer Ryan Boots, a Houston Texas-based home improvement writer and enthusiast.
When it comes to home improvements, homeowners like me generally want a twofer: something that looks good and works well.
Skylights tend to fit in that intersection rather nicely: they can help offset electricity costs while also enhancing a home’s visual appeal. However, in my research of skylights, it seems like selecting the proper kind of skylight and working out its placement can be tricky. I thought I’d share what I learned about selecting and installing skylights.
Skylights and daylighting
The principle behind daylighting is fairly straightforward: by using natural light for indoor illumination, you can cut back on use of artificial lighting during the day to reduce electricity consumption.
Strictly for the purposes of daylighting , skylights are certainly superior to windows: since they face the sun the entire day, a skylight admits around eight times as much light as a window of comparable size. They’re also highly useful in case you keep plants indoors.
But I think it goes a bit beyond just trying to save on energy bills. Frank Lloyd Wright’s big deal was trying to integrate the indoors with the outdoors, and skylights are a great way to bring this off.
Yes, these kinds of skylights make rooms look beautifully lit and airy; a wonderful place to sit, read, relax, visit. However, I shudder to think of what it would take to keep these spaces comfortable in Texas or Arizona in September. And even if it is possible to keep it livable at triple-digit temperatures, can you imagine the monthly electric bill that would be involved?
Feasible daylighting with skylights
On the other hand, consider this variety of skylight. Now this seems a lot more within the realm of the feasible. You get the outside lighting, just not at full blast. And because it’s paired with the glass doors at one end, you can use the blinds to moderate the amount of lighting. It should be a good deal easier to cool in the summer.
Don’t get me wrong – if you can handle the expense involved, sure, go all in and fill your ceiling with glass or acrylic. But for the rest of us who try to keep our monthly electric bills at least within the range of a normal car payment, it’s much more realistic to go minimalist with centrally-placed skylights for optimal outdoor lighting. It can brighten up a room, making it more pleasant for visiting with friends or family or just plain hanging out.
Different types of skylights
The main three classes of skylights are as follows:
- Ventilating skylight – This can be manually or automatically opened or closed to allow for ventilation to outside air. (This is really for functional purposes, such as in a kitchen to ventilate excess heat or fumes, rather than for aesthetic or lighting purposes.)
- Tubular skylight – Think of it a small skylight for hallways or small rooms.
- Fixed skylight – Intended solely to allow exterior light into a house, this is the main type of skylight with which people are most familiar.
Among fixed skylights, there are a number of shapes and styles available. However, once you start looking beyond flat rectangular skylights, some shapes, such as domed or hip ridge, can drive up the cost rather quickly. Skylight panes may also be glazed or coated, and flat skylights may be double-paned for additional insulation. I’m sure storm-resistant materials are available (this is hurricane country, after all), but I’m getting sticker shock just thinking about the price.
Skylight placement: choose wisely
Discussion of materials and energy transfer is one thing; skylight location is another matter entirely, and placement is crucial.
Realize that skylight placement should be based on your home’s internal needs, not external aesthetics. Put another way: while I’m definitely a believer in curbside appeal, for this exercise I’m interested in how it will light up my home, not how my home will look from the curb.
In addition, make sure the room you decide to bless with a skylight will always be used for a given purpose. For instance, I’d be interested in installing it in the dining room or kitchen, since those locations are highly unlikely to change.
But I imagine that installing a skylight in a given room will pretty much render it useless for a home entertainment room. In addition, I’m pretty picky about lighting conditions in any area where I use a computer regularly, so I’m very much disinclined to consider my home office. (Side note: With the arrival of mobile computing, I wonder how much impact the iPad and Kindle will have on the selection of interior lighting?).
For an area I currently use as a sitting area or study, then yes, a skylight would probably do a lot to open that room up and make it seem more spacious, or make it a very nice reading room or study.
There are other factors to take into consideration, not the least of which is the climate in your particular area.
It’s all about balance
It seems to me that to get it right with skylights, one must use the right materials, and select the location carefully. A couple of rectangular skylights with quality fixtures over a study or living room could help it feel brighter, but not measurably hotter.
Conversely, a well-placed skylight built from poor materials might provide lots of light in August, but will make the room feel like a convection oven. And excessive concern over temperature changes might result in poor daylighting, which seems like it defeats the purpose.
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Skylights – worth it?
Overall, I’m kind of on the fence as far as skylights are concerned. They can certainly let in lots of sunlight, and they could help make our home’s interior feel more airy. And I can see how using skylights, plus switching to a different Houston electric company, might help to lower electricity bills. But given the price involved, and the number of variables involved to avoid sacrificing comfort in the process, it seems like this project would primarily be for design/remodeling, not energy efficiency.
I don’t know – what do you think?