What’s Cool About This Roof?
I recently worked on a tiny shed project. The smart client finally picked this one. (see image above).
As you can see, the design is simple, almost boring. But why did I say my client is smart?
Because she asked these TWO questions.
1. This roof is designed with an assembling thickness of 9-inches. Why?
a. Snow load vs. Wood Framing
This has to do with the required snow load in your neighborhood. For example, in the project’s jurisdiction area, the required designed load is 40 pounds per square foot (ground snow load of 30psf + dead load of 10psf= 40psf). Once we have this required load in mind, the next step is to look at its wood framing layout. A conventional textbook would list how rafter span and spacing limitation based on the roof’s size and required design load. The table looks like this.
This single plane roof is sized as 16 feet by12 feet, therefore one efficient framing would use 2x8 rafters to assemble the roof frame. With the plywood (minimum of 19/32”) to enclose the top and bottom side of the frame, the total thickness of this roof assembly is about 9–10inches.
b. Climate Zones vs. Added Living Space
There is an 8-inch void inside the roof framing, and I use it to put insulation materials. According to the climate zone map, this project is in the 4A climate zone. Therefore, continuing insulation of R30 is required if you would like to have an added living space. The R30 insulation often comes with this fluffy, pink, batt form, which will fill all the void gap between the rafters. (Very good use of the space!).
In this particular case, the shed structure is only function as a storage. But the client wants exposed rafter at ceiling to complete the aesthetic look that she likes. Therefore, after installing 2–3 inches of R13 insulation, she can still see the exposed rafter.
2. The roof is designed in white color. Why?
The color is white. “So what? What’s cool about this white color?” you might be wondering.
a. Heat Island Effect vs. Material Attributes
Three main effects happen when sun rays hit the roof surface:
Thermal Emittance: The relative ability of the roof surface to radiate the absorbed heat.
Solar Reflectance: The fraction of solar energy that is reflected by the roof.
Heat Absorption: Some heat is absorbed by the roof and transferred to space below.
And “Heat Island Effect” is a phenomenon that often occurs in an urbanized area. When a built environment absorbs and re-emit the sun’s heat more than nature landscape, we experience higher temperature than average circumstances.
b. Cool Roof vs. SR/SRI
One way to defeating unwanted temperature rise in our environment is to use a “Cool Roof”.
Cool roofs reflect more solar energy, emitting more thermal radiation into the atmosphere and absorb less heat into the building. It also slows the formation of ground-level ozone and smog.
To achieve these highly emitting, less absorptive characters, most cool roofs are made of highly reflective material. One way to see if the product is really “cool” is to see their SR and SRI values.
SR means Solar Reflectance. It is a measurement range between 0–1 with 1 representing 100% reflective with no absorption. Many lighter tones material is designed with an “energy star” label which is available throughout the US. To qualify to get the “Energy Star” label, their initial solar reflectance SR must be greater or equal to 0.25.
SRI means Solar Reflectance Index. It is a calculation using the aged solar reflectance and thermal emittance measurement of the roof surface material. It ranges from 0–100, with 100 representing maximum solar reflectance can be achieved. This index measurement can be used as an indicator of how hot the roofing material is likely to become when they are exposed to the sun. The lower the SRI value the hotter the roofing material is likely to be under the sun. Typically, higher SRI also runs into lighter hues, such as white, light grey, light tan. In a LEED standard, the SRI value is required by the following standards to achieve credits.
By the way…
· An added living space is also referring to as a “conditioned space”-where temperature and humidity are controlled. Insulating a place offers the ability to have a comfortable living space that allows you to spend long hours inside.
· You can also find your local climate region by viewing the Guide to Determining Climate Region by County from U.S. Department of Energy.
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