Keeping Up with Changing Roof Requirements

Many of you are trying to keep up with what’s going on in the evolving world of roof systems, but are finding that the subject is becoming increasingly complex.

So this edition will update you on a couple of hot issues.

Proposed Changes to ASHRAE Standard 90.1-2007, Energy Standard for Buildings Except Low-Rise Residential Buildings
At any given time, ASHRAE has many proposals under consideration for changes in their existing documents. These changes may be adopted, rejected, or modified, and those that are adopted will appear in the next edition of 90.1 (expected in Fall 2010).

One of these proposed changes concerns thermal performance of metal roof systems.

ASHRAE has divided the United States into eight climate zones and published minimum R-values (for insulation) and maximum U-values (for the roof assembly) for each of these zones. Further information can be found at Professional Roofing or ASHRAE.

On Jan. 12, 2010, ASHRAE issued an announcement concerning a proposed change for the thermal efficiency of insulated metal building roof systems: “Studies by a metal building task group found that typical installation practices of single- and double-layer assemblies in Appendix A act to compress (batt) insulation and negatively affect the assembly's thermal performance.”

The proposed changes also include a refinement of the definitions of the available metal building roof insulation systems

Single Layer. The rated R-value of insulation is for insulation installed perpendicular to and draped over purlins, and then compressed when the metal roof panels are attached. A minimum R-3 thermal spacer block between the purlins and the metal roof panels is required unless compliance is shown by the overall assembly U-factor.

Double Layer. The first rated R-value of insulation is for insulation installed perpendicular to and draped over purlins. The second rated R-value of insulation is for unfaced insulation installed above the first layer and parallel to the purlins, and then compressed when the metal roof panels are attached. A minimum R-3 thermal spacer block between the purlins and the metal roof panels is required unless compliance is shown by the overall assembly U-factor.

Liner System. A continuous membrane is installed below the purlins and uninterrupted by framing members. Uncompressed, unfaced insulation rests on top of the membrane between the purlins. For multilayer installations, the last rated R-value of insulation is for unfaced insulation draped over purlins and then compressed when the metal roof panels are attached. A minimum R-3 thermal spacer block between the purlins and the metal roof panels is required unless compliance is shown by the overall assembly U-factor.

For non-residential metal building systems (per ASHRAE 90.1-07), the current minimum R-values are:

Climate Zones 1-7               R-value=19.0

Climate Zone 8                    R-value=13.0 + R=19.0

Since a minimum of R-19 thermal insulation is required for zones 1-7 in 90.1-07 and R-13 + R-19 for zone 8, tests conducted at the University of Illinois and Oak Ridge National Laboratory indicated that the typical installation methods for single-layer, over-the-purlin insulation would yield higher U-factors than previously assumed in Standard 90.1 due to additional compression of the glass fiber batt insulation between the purlins. This proposed revision changes the assumed U-factor for those systems as they are “typically” installed.  In the proposal, ASHRAE also developed new equations that can be used to calculate the performance of over-the-purlin installations that have more insulation draped between the purlins, which would yield better thermal performance. 

It should be noted that all forms of construction would undergo significant increases to the required thermal insulation due to the U.S. Department of Energy and ASHRAE’s desire to improve energy efficiency across the board. 

Below is a summary table that shows the new roof insulation requirements for all non-residential forms of construction, which are slated to be approved for the 2010 edition of ASHRAE 90.1:

When retrofitting an existing metal building to bring it up to the forthcoming ASHRAE insulation requirements, options might include:

1. Applying a recover metal roof system, with additional thermal insulation superimposed on top of the existing system followed by a new metal roof.
2. Converting a low slope metal roof system to a sloped roof system with a newly created insulated attic space.
3. Installing a cavity fill or liner system.
4. Paying more attention to the alternative to total reliance on R-value of insulation alone by considering the overall U-factor for the entire roof assembly.
5. Applying sprayed-in-place polyurethane foam and coating directly to the existing metal roof.
   

Grasp the alternative of coating the existing metal roof to increase its albedo and emissivity. A reduction in required thermal insulation may be allowed in climate zones 1-3 (hot zones, for air-conditioned buildings).

The Metal Building Manufacturers Association (MBMA) published Energy Design Guide for Metal Building Systems. It covers metal building systems and applications, energy code fundamentals, cool roofs, PV panels, and much more. It can be purchased directly from www.mbmamanual.com.

PVC and the Environment
In PVC Roofing, I mentioned concerns with PVC and the environment. As a follow-up, on March 3, 2010, the Green Building Council of Australia announced that its new system for rating the PVC plastic should be in full force by April 2010. Green Star Executive Director Robin Melton says the old rating “effectively encouraged using alternatives to PVC-based floorcoverings, pipes, and other products.” The revised credit will “promote the use of PVC products, providing they meet best-practice sustainability criteria.”

Wind Calculations
With assistance from Wanda D. Edwards, Director of Building Code Development, Institute for Business and Home Safety; Jay Crandell, ARES Consulting/ARMA; and Peter J. Vickery, Applied Research Associates, highlights of these changes in ASCE-7-10 follow:

Three-second-gust wind speeds have been changed to ultimate wind speeds. In changing to ultimate wind speed, the return period on the wind map based upon the importance of the structure has also changed. Load factors will no longer be used, so there are no changes to wind loads. However, based upon the modeling, wind speeds in some areas are lower. The windborne debris region is smaller in some areas. The modeling, which is done by ARA, is based on several new factors. The modeling reflects that the degradation of the storm as it hits landfall is greater than once thought to be, the intensity of the storm as you move from the eye decreases more than was once thought to, and many more points were used in the modeling. In brief:

• Complete reorganization of the wind provisions in ASCE-7-2010, Chapter 6.
• New wind speed maps (ultimate wind speed -700 + return period depending on building importance category). The wind speeds shown on the maps are still 3-second-gust speeds. The reason the speeds are much higher is because of the return period of the map and the fact that we’re not using the importance factor anymore.
• New windborne debris region map.
• Reintroduction of Exposure D for water surfaces in hurricane-prone regions.
• Introduction of another new simplified procedure.
• While the wind speeds of these new maps show increased velocity, the wind loads of the revised document may actually go down.
  

Keeping Up
Since there will be an incubation period before these changes reach local building codes, much more information will appear in the next several months in these newsletters.

Consider having one member of your staff focus on roofing. The designated member should attend at least one technical symposium on roofing technology every year.

Sign up for free webinars offered by BUILDINGS and other reliable sources.

Wecome familiar with online software that can automatically determine wind loads based upon building zip code and other basic information. Learn how to use RoofNav from FM Global to find systems that meet your requirements.