4  FM quick reads on Roofs

1. What Metrics to Look at For Exterior Energy Efficiency


Today's tip is about the important measures facility managers should be aware of when trying to quantify the quality and energy efficiency of exterior building elements. For new construction, the ASHRAE 90.1 standard is the baseline for measuring how efficient a building is. Soon, though, ASHRAE 189.1, a new green building construction code, and the ICC's International Green Construction Code, will be the standards. These standards reference several metrics for exterior building elements that exterior components must meet when choosing the prescriptive path to compliance.

For roofs, the two main measures are solar reflectance and infrared emittance. Solar reflectance measures a surface's ability to reflect infrared, visible and UV light. Generally, the higher the solar reflectance, the more energy efficient the roof will be. Infrared emittance measures a surface's ability to re-emit any energy absorbed back into the atmosphere. A so-called "cool roof" generally has an emittance value higher than .90 and a reflectance value of .65 or higher. Finally, solar reflectance index, or SRI, is a combination of the two via a standard calculation. SRI is the new standard used in LEED.

For windows and skylights assemblies, U-factor measures heat loss. The lower the number, the better the performance. Solar Heat Gain Coefficient measures how well a product limits radiant heat gain from sunlight. Visible Transmittance measures how much light, but not heat comes through. The higher the number, the more light is transmitted. Finally, Condensation Resistance indicates a product's ability to resist the formation of condensation on interior surfaces. The higher the rating, the better.

For wall assemblies, the best measure for efficiency is R-value - or the thermal flow resistance. The higher the R-value, the better the insulating effectiveness. When calculating the R-value of a wall assembly, the R-values of the wall, insulation and any other layers are totaled to arrive at the R-value of the total assembly.


2.  Understanding Metal Roofs

Hello. This is Greg Zimmerman, executive editor of Building Operating Management magazine.

Today’s topic is about the types of metals used in commercial metal roofs.

There are five main types of metals facility executives can choose, based on the specific criteria of their buildings.

The two most common are aluminum and steel. The majority of commercial metal roofs are made of steel, especially flat or nearly flat roofs, as well as the architectural standing seam products that are used on steep slope applications. Steel roofs come in various gauges – as the gauge number goes down, the steel is thicker. Common gauges for steel metal roofs range from 22 to 16 gauge.

Aluminum is a good choice for coastal environments because it won’t corrode. It’s lightweight, recyclable, and most manufacturers use a high percentage of recycled content in their roofing products. Because aluminum is soft and easily bendable, it is available in dozens of different commercial profiles and colors – including shakes, shingles and standard standing seam.

The more exotic metals sometimes used in specialty applications for roofs are copper, zinc and terne. Copper is a striking metal that’s been used on roofs for centuries. While copper is the most expensive of these five, it can usually last for hundreds of years. It weathers over time from its shiny salmon color into an attractive bluish/green patina.

Terne metal is steel or stainless steel coated with an alloy of mainly zinc and tin. Terne is very corrosion-resistant, even in marine environments. Zinc roofing is relatively new to the U.S., but is gaining momentum because it is has environmental benefits. It occurs naturally, and water runoff from a zinc roof is 100 percent clear, so it can be captured and reused. Zinc roofs have an expected life span of 60 to 100 years. Many are drawn to the rich gray color which deepens over time.

3.  Garden Roofs Bring the Green

Today’s tip is about green, or garden, roofs.

There are two varieties of green roofs: intensive and extensive.

Intensive green roofs are the fancy kind, most often recognized as roof gardens that allow people to walk around and relax. They often include flowers, bushes, and even small trees. The largest intensive green roof in the world is Chicago’s Millennium park, a 24.5-acre green roof on top of two subterranean parking garages. Intensive green roofs are very heavy, ranging from 80 to 120 pounds per square foot saturated. They will usually only work for new construction, when the extra structure to support the weight can be engineered into the design of the building.  

Extensive green roofs are usually less involved, with a thinner planting medium, and usually include smaller plants with shallower roots, like grass and flowers. They have a saturated weight of 12 to 50 pounds per square foot. The planting media is usually 1 to 5 inches thick. Several modular extensive green roof products have emerged in recent years, meaning that plants can be pre-grown in a factory and installed when needed – essentially an instant green roof.    

4.  How to Measure Exterior Efficiency

Today’s tip is about how to measure the efficiency of certain exterior elements. For new construction, the ASHRAE 90.1 standard is the baseline for measuring how efficient a building is. But there are several metrics for exterior building elements that can show how they contribute to, or are detrimental to, energy efficiency goals.

For windows and skylights assemblies, U-factor measures heat loss. The lower the number, the better the performance. Solar Heat Gain Coefficient measures how well a product limits radiant heat gain from sunlight. Visible Transmittance measures how much light, but not heat comes through. The higher the number, the more light is transmitted. Finally, Condensation Resistance indicates a product’s ability to resist the formation of condensation on interior surfaces. The higher the rating, the better.

For roofs, the two main measures are solar reflectance and infrared emittance. Solar reflectance, also known as albedo, measures a surface’s ability to reflect infrared, visible and UV light from the sun. Generally, the higher the solar reflectance, the more energy efficient the roof will be. Infrared emittance measures a surface’s ability to re-emit any energy absorbed back into the atmosphere. A so-called “cool roof” generally has an emittance value higher than .90 and a reflectance value of .65 or higher.

For wall assemblies, the best measure for efficiency is R-value – or the thermal flow resistance. The higher the R-value, the better the insulating effectiveness. When calculating the R-value of a wall assembly, the R-values of the wall, insulation and any other layers are totaled to arrive at the R-value of the total assembly.


RELATED CONTENT:


Roofs , Walls , Windows , Energy Efficiency , Metrics

3M Window Films
65 Crazy, Outrageous Occupant Complaints. Order your copy today >
QA Graphics


QUICK Sign-up - Membership Includes:

New Content and Magazine Article Updates
Educational Webcast Alerts
Building Products/Technology Notices
Complete Library of Reports, Webcasts, Salary and Exclusive Member Content



click here for more member info.