Moving Toward Net Zero in Pre-engineered Buildings

by Jonathan McGaha | 7 December 2013 12:00 am

As an industry, pre-engineered metal buildings are facing some stiff challenges over the next several years. I know we have all been hearing of “net-zero” and wondering how that’s possible under our current designs and practices. An executive order signed in 2009 states that by 2030 all federal buildings must be net zero. In other words, they must produce as much power as they consume.

That seems like a long way off and it will only apply to federal buildings. However, the truth is we need all that time to develop new power generation sources and improved building techniques, as I’m sure the intent of the law is that private construction will also pursue this lofty, and worthy goal. This brief paper will look at the law, the way power is consumed by buildings, and some thoughts on how to reach net zero.

 

Executive Order 13514

On Oct. 5, 2009, President Obama signed Executive Order 13514, “ensuring all new federal buildings, entering the design phase in 2020 or later, are designed to achieve zero net energy by 2030.” This sounds like a daunting task, and you may ask why? But if you think about it, even if we can only achieve a 50 percent reduction, wouldn’t that be worth it?

So, how much power do buildings (not houses) in the U.S. consume? How do buildings consume that power? Where does the power come from? And, of course, what can we do to move in that direction?

 

How much power do commercial buildings in the U.S. consume?

According to the U.S. Department of Energy, there are approximately 4.9 million buildings in the U.S., covering approximately 81.1 billion square feet. The power consumed by them is equal to that of the third biggest country in the world, consuming over 6500 trillion BTUs. This power was generated by electricity (77 percent), natural gas (18 percent) and other sources including coal and renewables. (This is coal used for heating purposes not electricity.) Commercial buildings consumed 18.9 percent of the total energy used in the U.S. federal buildings consumed another 2.2 percent of the power generated.

When confronted with the size of these numbers, one can’t help but imagine the potential for savings. Now, the Executive Order only applies to federal buildings, but what if all design and construction could attain a significant reduction, of say 15 percent? This kind of change would result in many positive impacts.

Table 1. Areas of Power Consumption Area of Power Percent of Total Consumption Power Consumption

*Service station equipment, ATMs, medical equipment, etc.

Heating and cooling account for more than 30 percent of the power consumed by buildings, according to data from the U.S. Energy Information Administration. What can be done to improve this? What causes the temperature loss or gain through the building? This paper is only addressing pre-engineered metal buildings, so keep that in mind as you read my comments.

 

Heat transfer in metal buildings

Pre-engineered metal buildings (PEMB) are a very cost-effective building method. These buildings achieve wide spans and rather rapid construction. However, they are not the best when it comes to heating and cooling. You know they are quite hot during the summer and cold during the winter. For years we would use 4-inch insulation with a vinyl backing to achieve an R-13 rating in the ceiling and walls. The backing looks good and provides some brightness to reflect the light, but really does little to insulate the building. It basically works as a condensation blanket that stops the panels from trapping water between the purlin and the panel, which leads to rust.

We did some simple studies in our company building here in Denver, N.C., using a surface thermometer. In the summer, a college intern working for us kept an hourly log for one week. We looked at the relationship of the outside temperature to the component temperature inside the building. Average temperatures for a one-week period are given below in degrees Fahrenheit.

Table 2. One-week average temperatures of structural members of an unconditioned Galvalume R-13 roof.

 

Please keep in mind that this is not a scientific test. We were merely looking for ideas and ways to become better sales people for our product. There is a lot of information available by people who have the knowledge and resources to do this type of testing.

But if you look at the numbers, you can draw some interesting conclusions. Obviously, the Galvalume screw down roof panel gets very hot. But even though a 4-inch layer of insulation is between the panel and the purlin, almost all that heat has transferred into the building. Not only that, the heat has transferred to the rafter as well. These materials, inside the building, are much hotter than the conditions outside. Also, notice the components are warmer than the exterior temperature in the morning, meaning the heat is stored, or at least unable to fully dissipate before the next day. This also tells you that the screws and bolts are conduits for the heat.

Granted, a screw down roof with 4-inch insulation is the worst-case scenario. And of course, none of these findings are new or surprising. But what difference do innovations such as a standing seam roof and R-30+ insulation make?

We recently constructed a school which was a pre-engineered building, approximately 20,000 square feet with a 3-in-12 pitch roof, trapezoidal standing seam in dark blue, and we used the Simple Saver System by Thermal Design, Madison, Neb., with a layer of R-19 and R-13. We conducted the same test but did not take as many readings. The results are summarized in Table 2 below.

The blue roof is considerably hotter than a Galvalume roof. But this is a consistent measurement, and I attribute that to the color absorbing more heat. You also see that the heat is unable to transfer to the rafter, probably because of the standing seam roof and the improved insulation technique. This study was done in an unconditioned space, and the temperature of the building interior is still noticeably cooler than the outside environment.

As an interesting aside, this new building adjoined an existing building with the same features, except a “bag and sag” insulation system was used. This space was conditioned with air conditioners running full blast. We took the same temperature measurements with the results also shown in Table 2.

It is apparent that the mechanical systems are cooling the structural members, but they are still warmer than the liner system.

Table 3.Comparison of different roofing methods with structural member temperatures recorded at 3:00 PM in degrees Fahrenheit.

*Purlin temperature is read through the fabric that covers the structural members.

^† Column temperature measurement could not be obtained.

^YSimple Saver System

 

Conclusions and recommendations

None of these findings are new, or surprising. If you are selling or designing a building that requires a conditioned space or will have people working inside, here are my suggestions:

1. Standing seam roof: In my opinion, a standing seam roof is the best roof currently available. If installed properly, it will never leak and last 20 or more years easily. It also appears that a colored roof will have a tremendous impact on the temperature inside the building. There is a lot of information out there about the heat characteristics of colored roofs.

2. Liner system with R-30: I love the liner system for insulation-from both an installer’s and building owner’s point of view. The building is aesthetically pleasing, and, of course, the extra insulation does its job, even without heat or air conditioning.

3. Improved installation techniques: There are some areas we need to improve on as installers and manufacturers. The eave strut needs to be filled with insulation. I would also like to see the manufacturers change to a channel on the rakes instead of an angle. This should also be filled with insulation. The base angle should be replaced with a Cee channel where possible and also filled with insulation. All the Cee channels used for framed openings should also be filled. This would only leave the corners as suspect in terms of the building envelope.

4. Join your local MBCEA: This organization can do more to help with these new challenges than anyone. This group provides a forum for manufacturers, erectors, suppliers, architects and others to address these issues.

These suggestions are the simple things that need to be done first. These are not new ideas, but as an industry we need to embrace them and, more importantly, sell them.

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Wade Wilson is the owner of jwWilson Co., a general contractor located in Denver, N.C. He currently serves on the board of directors for the Carolinas Chapter of the MBCEA.

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