Reference Corner

The Solar Reflectance Index designates the "coolness" rating of a roof on a scale of 0 to 100.  The higher the SRI, the cooler the roof.

Low emissivity (low-E) denotes a material's ability to restrict the transmission of radiant heat. Such materials are designed to help keep heat inside in winter and outside in summer.

The "Hybrid" Roof: Solar Shingles
 
Roofing

Map courtesy of BECP Resource Center

 

Energy Efficiency Overhead

 

Climate Zones 1 through 3

 

So you live in a warm climate, where the summer's sun and heat are oppressive. Air conditioning is unavoidable and expensive, as your energy usage climbs with the temperatures. Here it's all about reflectance.


- A COOL ROOF VIDEO QUICKIE -


Cool the roof! The idea is to lower the amount of heat passing into your house by lowering your roof's surface temperature. If you need a new roof, you'll find many types of "cool" products. They're mostly white, but there are colors - even in darker hues - to choose from, depending on the products. And yes, it is possible to use a field-applied coating to cool an existing roof, as well. Two important factors determine the efficiency of a cool roof:

  • Solar reflectance (R), or the ability to reflect sunlight (between 0 and 1.0% efficiency).
  • Thermal emittance (E), or the ability to release heat that has been absorbed (between 0 and 1.0% efficiency).


The coolness of a roofing product is established using the Solar Reflectance Index (SRI). The SRI assigns a coolness rating of 0 to a black roof on a sunny afternoon. Its R value (solar reflectance) is given as .05, and its E value (thermal emittance) is 0.90. The SRI of a white roof on that same afternoon is 100. Its R is .80, and its E is .90. The higher the SRI, the cooler the roof's surface.


Aging is also a factor in the coolness rating. Exposure to the elements, stress, thermal expansion and biological growth all affect a roof's SRI over time. A new roof typically reaches steady R and E values after about 3 years, after which the roofing material is measured again for any changes in coolness.


Check our "GO DEEPER" column (above right) to find out how much $$$ you could save if you cooled your roof, then find the products that can do it in the "Cool Roofs Ratings List." You'll have some real facts at your fingertips when you start to talk with roofing contractors.


Climate Zones 4 through 7

 

What if you live in a cold climate? So your heating bills far surpass your cooling bills? Don't you want every bit of sunshine warming your roof in the wintertime? Yes, but it's not that simple.


EnergyStar has found "evidence that low emissivity [E] may benefit those buildings located in colder climates by retaining heat and reducing the heating load. Research on the benefits of emissivity is ongoing." In other words, when your furnace is roaring, you want to keep that heat inside the house and not escaping through the roof.


First, insulate the attic! That will prevent the condensation that occurs when warm, moist air from your heated spaces below drifts up to the attic. A dehumidifier may also be advisable, where humidity is especially high.

Second, provide good attic ventilation! Your goal is to replace warm attic air with cold air from the outside. That allows a constant flow of cold air between the insulated attic floor and the rooftop. It prevents indoor heat from melting snow or ice on the roof, which can cause problems like

  • Run-off - water seeps under shingles and into the attic
  • Ice dams - melting snow runs down the roof, hits colder eaves and re-freezes, beginning a thaw/freeze cycle. After several days of this, an ice dam can build up, with puddles of liquid left behind it. That backed up water can leak under the roof and into attic or along exterior walls.


 

 

Ice Dam DON'Ts:

  • Don't try to remove or chip away an ice dam. Shingle damage is likely.
  • Don't install water heaters or other large mechanical equipment in your attic. They increase the attic's temperature.
  • Don't use chemicals to melt snow on your roof. Their corrosive effects can affect metal gutters, flashings and downspouts; their runoff can damage landscaping elements below.

 

- A ROOF-SHIELDING VIDEO QUICKIE -

Illustration courtesy of State Farm

 

The University of Massachusetts School of Building and Construction Technology's Paul Fisette advises that "the most efficient way to vent a roof is to use continuous soffit and continuous ridge vents." The illustration on the right (above) shows the roof vent at the top and the soffit vent at the roof line.

Go Deeper