Question

What is the extra cost of cool roofing materials over standard roofing materials?

Asked by E. Crane
Baton Rouge, LA

What is the average extra cost of reflective roof sheathing materials and reflective roof tiles to reduce the temperature in attics where HVAC ducts are run?

Answer

Cynthia Phakos

Answered by Cynthia Phakos

Los Angeles, CA

Koffka Phakos Design

June 7, 2011

In my files I found a very informative article, “Cool Systems for Hot Cities,” which should give you an in-depth understanding of cool roof systems.

Cool roofs can reduce the heat gain in your attic space up to 10 degrees in some climates.  

From a cost perspective, cool roofing products can range from the coating system described below at $40 per square (10’ x 10’) installed, up to $650+ for  lightweight concrete tile.  As a comparison, two conventional asphalt shingle products recently recommended by Consumer Reports are priced at about $100 per square.

First, let’s briefly discuss these characteristics of a cool roof:

  • solar reflectance;
  • thermal emittance; and
  • Solar Reflectance Index 

Solar Reflectance and Thermal Emittance

A cool roof’s energy performance can be determined by these two characteristics of the surface layer.

When sunlight hits an opaque surface, the solar energy that is reflected is called its solar reflectance, or albedo. Measured on a scale of 0 to 1, the higher the solar reflectance value, the "cooler" the roof.

Energy that is not reflected by the roof is potentially absorbed by it. Thermal emittance is the measure of how quickly the absorbed energy is given off.

Solar Reflectance Index (SRI)

SRI combines the solar reflectance and thermal emittance of a surface.

  • Black roofing material with a high emittance value (90 percent) but a low reflectance value (5 percent) makes for a hot surface and is given an SRI value of 0.
  • On the other end of the scale, a white roofing material is highly reflective (80 percent) but may have the same emittance as the standard black surface (90 percent). Its surface is much cooler, and the white roof is assigned an SRI value of 100.

An exceptionally “cool” roof can have an SRI value greater than 100.

Coating Systems for Existing Roofs

There are liquid spray or paint coating systems that can be applied to an existing built-up roof.

  • The cost for this is minimal; roughly $40 per square installed (a roof square is a roofing measure -- 10’ x 10’ for 100 sq. ft.).
  • A well-known coating is Karnak’s Base Coating and White Energy Efficient Reflective Elastomeric Coating, with an initial reflectance of 86%, and aged, 77%; and with an initial emittance of .91, and aged, .90.

These coatings also work for roof restoration and can extend the life of your roof. For these liquid-applied roofs, it may take more than one coat to meet the required mil thickness specified by the manufacturer, and if it is not applied properly it could degrade up to 20% in the first year.

Style of Roofing

As you live in Baton Rouge, Louisiana, I assume you have a pitched roof (greater than 2:12). The style of roofing is a consideration as well as the cost, and you will want to consider what type works with the vernacular of your area.

Also, you should select a product that meets the Miami-Dade County Product Control Approval System, which is the leading standard set for products that can resist the wind uplift of a high velocity hurricane zone.

Roofing Recommendations by Cost

A cost-effective roof is GAF’s Timberline Cool Series asphalt shingle-style roof, and this roof is also listed under the Miami-Dade product list.

  • The granules have a metallic reflective coating that qualifies the roof as an Energy Star Cool Roof.
  • The Cool Antique Slate and Barkwood have an initial solar reflectance of 27%, which is not a high value, but with a thermal emittance of .92, making for an SRI of 29%. The minimum value for a cool roof’s solar reflectance is 25%.
  • The cost of this roof installed is approximately $275 per square (until June 23, 2011, when there will be a price increase).

The next-level roof -- a standing-seam metal roof -- becomes more costly. A standing-seam metal roof can cost anywhere from $500 to $600 per square furnished and installed.

  • We have had good experiences with Custom-Bilt Metals’ Titan mechanical seam roof, which is an Energy Star Roof and is on the Miami-Dade product list.
  • There are numerous colors to choose from with SRI values varying from 87% (white) to 23% (black), with a solar reflectance between 25 and 71%. There is a distribution center in New Orleans, and so I think it is a popular choice for your area.

The premium roof would be lightweight concrete tile such as Monier LifeTile made by the Boral Roofing Company, furnished and installed at $650 to $750 per square.

  • The material runs roughly $125 per square with the total costs being greater due to the installation costs.
  • The LifeTile is listed on the Miami-Dade County standard product list and qualifies as a Cool Roof with an SRI value between 82% and 27% based on color.
  • The tiles come in a hand-split cedar shake look typical of the vernacular in your area and also in a slate style. This is also the preferred roof for hurricanes, in comparison to the metal roof, since losing a tile is less significant than losing a large metal panel. 

 

For more information:

Read "What type of roof should I choose in a hot climate?" a Q&A answered by Mick Dalrymple.

Also, read "What are the sustainable options for re-roofing our 30-year-old home?" a Q&A answered by Cynthia Phakos.

Tagged In: energy star

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