Types of Rooftop Skylights – PART I

For a flat roof commercial building or residential rowhomes in one of the historic neighborhoods of Washington DC, skylights are a perfect way to bring daylight into an otherwise dark building. City buildings are a bit darker than buildings built almost anywhere else. Unlike buildings that are built completely freestanding with windows on all sides, most buildings built in densely populated historic urban areas have buildings built directly next to them. City streets even, can feel like a valley, like walking through a ravine deep in the shadows where the sun cannot reach you.

There are practical reasons for why cities are built like this, with buildings packed together.   One of the downsides though is the lack of exposure to sunlight and opportunities for fenestration.  Fenestration is the arrangement of doors and windows on the exposed facade of a building.   If a building’s sidewalls are shared with neighboring buildings on both sides, the amount of opportunity for fenestration (and ventilation as well) is very limited.

Skylights are a good alternative to typical windows and fenestration at the facade.  They aren’t perfect,  But skylights allow a building to use its roof to allow sunlight to enter the building. There are downsides to skylights though.

Some of the downsides to using skylights in the construction of a building and rooftop.

  1. Increased maintenance / upkeep
  2. Solar gain can be high in the summer
  3. Additional cost: product itself, light well, finished, insulation, curb, and flashing.
  4. Thermal heat loss in the winter
  5. Ice damming

The issue of solar gain is particularly interesting. Solar gain is thermal ingress that comes from the sun. In other words the heat that comes from sunlight. In the summer the heat that enters a building through skylights can be extreme.   In the Winter though, a time when solar gain would benefit the building in specific ways by reducing heating costs, the solar gain is much less. This is intuitive because the sun is less intense during the Winter.  We perceive that condition and recognize that to be a factor and condition of the seasonal changes. But another aspect which is a bit counterintuitive is that because of the lower angle of the sun in the daytime sky, solar gain is also reduced significantly in the Winter.

In a future blog, we will delve deeper into the concepts listed above to explain the concepts around the cause and effect of each of the items, including solar gain.

Photographs follow up several different types of skylights. These photos are good for reference, because they show examples of each of the different classifications and types. These types of skylights have changed over time and perform different functions.

The photo below shows a historic skylight, built with vents at each of the four sides of the skylight frame. The vents are built as part of an integral passive louver in the skylight housing.   These vents work well in a cooling air draft phenomenon known as the vent stack effect which was one of the best ways to reduce the temperature of a hot house in the summer before the advent of air conditioning.

historic skylight

There are a few common downsides of this type of louvered skylight. It solves one problem of cooling the house but also introduces new problems.

  1. The downward and outward angled louver’s protect against regular vertical rainfall but do not completely resist water entry from directional rain and they do not protect against upward ( including capillary action) effects of water movement in ice damming conditions.   Capillary action is the name for the phenomenon of water to rise or fall within small tubes or interstitial space between building materials when faced with differential pressures. Differential pressures can be caused by air pressure, such as wind, simply blowing over the surface of a building facade or rooftop or building material. In other words, with wind or other types of pressure differentials, water can literally move upwards Because of capillary action in certain contexts and configurations of materials.
  2. Rodents, like squirrels, are always searching for gaps and openings between building materials for themselves to use as places to crawl into and store food or live or keep warm. These louvers tend to invite rodents, not just squirrels, but also other animals such as birds and vermin will often try to get into a building or rooftop through louvered skylights.
  3. While these types of louvers are primarily intended to vent an exhaust hot air during the hot months of the summer, they do the same during the Winter, hot air will passively bleed through these events and introduce drafts of cold air into the house every time a door window is open. Windows and doors are open much less during the Winter, but every time someone comes in or out of the building, the door opens and these louvers allow a greater, increased, level of draft and exchange which reduces the overall temperature of the house. Old boiler radiator systems or forced air heat systems can sometimes struggle to keep up with the increased heating load.  In effect, combined with poor roof insulation, omitted or lacking recovery board, and poorly insulated foundations, these effects combined, can make for very cold living spaces in the Winter.


Smart proactive replacement, construction, upkeep and maintenance of low slope roof systems requires an enthusiastic interest and understanding of waterproofing principles and building science.   Here in Washington DC, historic and modern residential and commercial buildings are extremely expensive and the roof and related systems provide the shield that preserves the building.

We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the value of quality construction and building maintenance, and develop a relationship with our company.  You can learn a lot more on our blog.  Feel free to check it out.  If you have questions about the roof and related systems of your building in Washington DC, fill out the webform below and drop us a line.  We will be in touch if we can help.

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