05/30/2007

Roof Reflectivity and Good Design

By Carl De Leon

 

Enhancing Investment in Insulation
From a design perspective, using a reflective material in the roofing system results in a more holistic approach. It’s a decision that works synergistically with other components of the system, particularly the insulation.

In some jurisdictions, local codes allow a reduction in insulation when cool roofing is employed. But, whether the building team chooses to reduce insulation or not, the performance of the insulation used will be optimized – meaning, thermal conductivity kept to the lowest level – with a reflective roof.

Thermal conductivities of roof insulation materials have been measured at different operating mean temperatures using computerized heat-flow meters. The results indicated that higher operating temperatures always lead to higher thermal conductivities in the insulation material. The effective temperature driving this effect in the roof systems is directly linked to the absorbed solar radiation of the surfaceii. So, when insulation temperature rises, such as occurs with dark-colored rooftops, the thermal conductivity of the insulation increases as well. This means that the R-value of the insulation is reduced and along with it, the effective performance of the insulation. According to one study, effective insulation performance below a black membrane can be as much as 30-percent lower than advertisediii.

Cooling-load contributions of the roof system components have been modeled using theoretical nominal insulation thermal conductivities. These nominal values are typically measured at 75 degrees F. Modeled in comparison with these typically reported values were actual field temperature-impacted insulation thermal conductivities. The models indicated significant advantage on roof-induced cooling load through synergistic action of the insulation thermal conductivity, and lower roof surface operating temperature by the use of light-colored membranesiv.

Preserving Reflectivity Over Time
Natural weathering and soiling can affect a roof’s ability to maintain its high-reflectance values. Depending on such variables as geographic location and climate; urban, agricultural, or industrial setting; the amount and type of discharge from the building and adjacent structures; and roof slope, particles and pollutants of all kinds can accumulate and diminish the roof surface’s inherent reflectivity.

Testing conducted by LBNL and the National Research Council (Canada) has shown that even simple cleaning techniques can restore most, if not all, of the original reflectivityv. Washing a weathered cool-roof membrane can result in a practically complete restoration of solar reflectance. Studies and field experience have also found that most reductions in reflectivity occurred during the first year and then leveled off, with further reductions negligible by the sixth year; however, reflectivity levels will continue to be significantly higher than those of traditional dark-colored materials.

Application Methods Offer Choice
But, the vinyl material itself is not the only thing that’s flexible. With vinyl membranes, designers and installers also have flexibility in application methods. Taking into account building code and insurance requirements, vinyl membranes can be installed on the roof deck or substrate in several ways:

  • Mechanically attached with various fastening systems.
  • Loose laid in vegetative roofs or plaza decks, where the weight of the overburden holds the membrane in place.
  • Adhered with either adhesives or self-adhered membranes.

Since vinyl membranes offer a broad spectrum of application methods to accommodate varied building design criteria, the best possible application method for a given building can be employed.

The numerous variables involved make it important to take into account manufacturers’ guidance when designing and specifying a project. A full complement of manufacturer-authorized adhesives, insulation, sealants, plates, fasteners, flashing components, and other accessories are available to enhance the vinyl roofing system’s prospects for a long service life as a cool roof, along with a wide array of product and installation warranties.

REFERENCES:

  1. Freund, S., Dettmers, D. and Reindl, D., "White Roofs in Northern Climates: Simulated Influence of the Roof Reflectance on the Building Energy Balance in Two Northern Cities," HVAC&R Center, University of Wisconsin-Madison, presented at ASHRAE Region VI Chapter Regional Conference, May 2007.
  2. Budaiwi, I., Abdou, A. and Al-Homoud, M., "Variations of Thermal Conductivity of Insulation Materials Under Different Operating Temperatures: Impact on Envelope-Induced Cooling Load," J. Arch. Engrg., Vol. 8, Issue 4, December 2002.
  3. Akbari, H., Berhe, A., Levinson, R., Graveline, S., Foley, K., Delgado, A., and Paroli, R., "Aging and Weathering of Cool Roofing Membranes," Proceedings of the Cool Roofing Symposium, Roofing Consultants Institute Foundation, Atlanta, May 2005.
  4. Budaiwi, I., Abdou, A. and Al-Homoud, M., "Variations of Thermal Conductivity of Insulation Materials Under Different Operating Temperatures: Impact on Envelope-Induced Cooling Load," J. Arch. Engrg., Vol. 8, Issue 4, December 2002.
  5. Akbari, H., Berhe, A., Levinson, R., Graveline, S., Foley, K., Delgado, A., and Paroli, R., "Aging and Weathering of Cool Roofing Membranes," Proceedings of the Cool Roofing Symposium, Roofing Consultants Institute Foundation, Atlanta, May 2005.

 


Pages: 1  2  View All  
 

Need portable cooling?

Rent or buy spot coolers from full-service locations nationwide. On call “24/7”. Primary, supplemental or emergency cooling. Atlas Sales & Rentals, Inc., or call (800) 972-6600.

Click here for more info


Visit our website today to learn about the design flexibility of a Morton building and the endless possibilities of partnering with our designBUILD team.


Wood construction is both cost and energy efficient. Check out Morton Buildings and our designBUILD team online today to discover all the benefits of post-frame construction.


When choosing a metal-clad building for your next construction project, consider Morton Buildings, Inc., and their designBUILD team, we’ll make your dream a reality.

Bluebeam® Revu® simplifies digital facilities document management from design review to leveraging as-builts, maintenance manuals and O&Ms submittals.

Yaskawa drives offer quality performance for air handlers and cooling towers on the roof to secondary chilled water pumps in the basement

We Can Help You Reduce Energy by 30%

Our mission is to help our customers manage their buildings' energy costs, improve reliability, and enhance performance while having a positive impact on the environment.
CLICK HERE to find out how.

Need portable cooling?

Rent or buy spot coolers from full-service locations nationwide. On call “24/7”. Primary, supplemental or emergency cooling. Atlas Sales & Rentals, Inc., or call (800) 972-6600.

Click here for more info


Visit our website today to learn about the design flexibility of a Morton building and the endless possibilities of partnering with our designBUILD team.


Wood construction is both cost and energy efficient. Check out Morton Buildings and our designBUILD team online today to discover all the benefits of post-frame construction.


When choosing a metal-clad building for your next construction project, consider Morton Buildings, Inc., and their designBUILD team, we’ll make your dream a reality.

Bluebeam® Revu® simplifies digital facilities document management from design review to leveraging as-builts, maintenance manuals and O&Ms submittals.

Yaskawa drives offer quality performance for air handlers and cooling towers on the roof to secondary chilled water pumps in the basement

We Can Help You Reduce Energy by 30%

Our mission is to help our customers manage their buildings' energy costs, improve reliability, and enhance performance while having a positive impact on the environment.
CLICK HERE to find out how.


 
comments powered by Disqus

Related Products

Communications
Doors & Hardware
Plumbing & Washrooms
Plumbing & Washrooms

Sponsored Links