You might think that any duct installation is a team effort, and you would be right. There has to be a fair amount of coordination between installers, fabricators, vendors and other contractors for a project to be done right and on time. But when Team Sheetmetal, Inc. of Irving, Texas, chose the new PhenoliDuct system from Spiral Pipe of Texas for the recent Haas Hall renovation at Texas A&M University, they became the key player in a team that had been working for several years to solve a problem that has become more common around the US. And this solution checks off all the boxes.
Let’s start with the first problem. Texas A&M University has been on a program for several years to upgrade old campus buildings with more efficient HVAC systems and energy recovery. This requires a new duct system to capture energy from exhaust air that was formerly thrown away, transport the air to new air handling units, then return that captured energy to the existing fan coil units as conditioned make-up air. When you’re doing this on 40-year-old, four story modular dormitory buildings, the logical place for the new duct and units is on the roof. That’s where problem #1 had to be addressed. Texas A&M, like many other institutional building owners, has dealt with problems of water in their duct system. Water can get into a duct in several ways — through poor seams, holes, condensation at thermal breaks, or even sucked in through the louvers. When ducts are on the roof, they should be insulated, and traditional mineral fiber insulations can wick and retain moisture. That leads to the potential for mold problems.
When engineers started looking for solutions to the first problem, an answer seemed pretty simple — change from a fibrous insulation to a closed-cell insulation. At the time, there was only one closed-cell insulation product with a history of use on ducts — elastomeric foam. And that’s where you ran into problems #2 through #5.
Elastomeric foam is expensive. Duct insulations on a rooftop need to be at least an R=8 value, and most elastomeric foams don’t meet flame spread/smoke developed requirements in a 2” thickness. You can’t weld over elastomeric foam, so duct fabrication options are limited. But the biggest problem for elastomeric foam insulations was that these rooftop ducts needed to be round.
Rain and snow roll off round ducts. Round ducts see only about 40 percent of the wind load rectangular ducts see. Round ducts are lighter, yet stronger and more resistant to damage and impact than rectangular ducts. But when you have branch diameters as small as 6” diameter, the better 2” elastomeric foams are too difficult to work with. Some newer elastomeric foam products are offered for use in round ducts, but the compromise is that they open the cellular structure below the 90% threshold of “closed-cell content” and don’t have a 2” thick R=8 option to meet the ASTM E84 25/50 flame and smoke standard.
About two years ago, I started working with Eduardo Ramirez, Steve Simon and Arturo Serrano of Ramirez-Simon Engineering in Houston, Texas, on a solution to these problems. They are the mechanical engineering design firm for much of the Texas A&M University work and were open to new and better solutions to the university’s requirements. Their specification was based on a 2” elastomeric foam with a 12 mil laminated covering membrane, but after several rounds of rooftop duct projects there were fabrication issues the duct manufacturers were struggling to solve. One alternate solution tried was foam injection of a double-wall duct. This solved the closed-cell issue and prevented moisture wicking, but it was a difficult and costly process and not all foams met the 25/50 flame and smoke standards.
In working with ASHRAE technical committees, I had been introduced to the Kingspan KoolDuct phenolic board duct products. They checked off several of the boxes — closed cell, great flame and smoke performance, lower cost than elastomeric foams, and thermal performance that was actually 50 percent better than either mineral fibers or elastomerics. A couple of companies were already marketing phenolic board ducts as a rooftop system, but when considering them for these projects there were two critical obstacles. First of all, they weren’t round. We were back to the problems of excessive wind loading and possible water pooling. Second, they were not durable. In central Texas, you don’t have to worry just about wind. You also have hailstorms and a broad range of temperature variances. A built-up foam board system with lagging did not compare with the proven durability of sheet metal duct systems fabricated to SMACNA standards. So, the obvious question was, can we make this product round?
We got the next members of our team when Kingspan introduced us to Lance Herlong and John Ellington of A-Kool Distribution Company of Jacksonville, Florida. A-Kool is the regional distributor of Kingspan KoolDuct products for the southern US with years of experience promoting phenolic board ducts for indoor applications. At Spiral Pipe of Texas, we had over 25 years of experience in using grooved fiberglass board products for round duct insulation, so we had some ideas on what we needed. But phenolic board is much more rigid than fiberglass, and we knew that simply grooving the board would not be the solution if they did not close when the board was rolled, leading to potential thermal breaks and irregularities in thermal value. Working with A-Kool, we developed tooling and grooving standards unique to each diameter. The result was a round tube in every diameter that had no thermal breaks at the grooves and was “round” enough to be used as the annular insulation in a double-wall round metal duct. And with both outer and inner shells made as traditional round metal ducts, we knew we would meet those performance criteria. Rain and snow would roll off the outer shell and wind would pass around it with 40 percent of the force a rectangular duct would see. And on the inside you would get the same airflow performance of a traditional round metal duct.
We took the prototypes and performance data to Ramirez-Simon and they enthusiastically added it to the allowable construction specification for the next rooftop duct project at Texas A&M.
When Team Sheetmetal got the contract for Texas A&M’s Haas Hall renovation, they had their own checklist of ductwork needs. Besides the obvious need for an approved product, they also needed to avoid penalties in duct cost or labor if they were to have a successful job. To further complicate things, they had a very narrow window to perform a major HVAC renovation on the project—the one-week Spring Break for initial rooftop work and the Summer Break for cutting holes in the roof, installing duct in chases, and completing the renovation. They had worked with Spiral Pipe of Texas before, and after reviewing the new PhenoliDuct product, they decided to try it on the Haas Hall project.
First of all, the cost was better than double-wall elastomeric product. Then, according to Team Sheetmetal’s director, David Mason, the installation was the easy part. “It took minutes to install a duct section and, labor-wise, it was just like installing regular metal duct.” Because the phenolic board is more rigid than mineral fiber, you don’t need spacers to keep the inner metal shell concentric with the outer shell. Inner couplings were not needed. All assembly and sealing was done at the outer shell. On larger diameters, factory-installed SPOT flanges made outer shell assembly even easier.
One huge bonus was the flame and smoke performance of phenolic board. Branches could be cut into the trunk duct on the rooftop — after the holes had been cut for roof penetrations. This allowed substantial completion of the trunk ducts during Spring Break without the cost of offsetting to unknown hole locations two months later. Meanwhile, team members Spiral Pipe of Texas and A-Kool Distribution were performing a complex task of their own — producing a brand new type of duct and solving all production issues for a job with a very tight schedule.
In the end, the team of Team Sheetmetal, Spiral Pipe of Texas and A-Kool Distributing produced a solution for Ramirez-Simon Engineering and Texas A&M University that checked off all the boxes for a rooftop duct system.
Checking off the boxes
- Non-fibrous, closed cell. Doesn’t wick moisture.
- Exceeds ASTM E84 25/50 flame and smoke standard
- Lower cost than elastomeric or foam injection
- It’s round! 40 percent of wind load, rain and snow roll off
- Double-wall sheet metal construction. It’s durable in outdoor conditions
- Round metal inner shell. The same smooth dynamic performance of traditional round metal duct.
- No inner coupling. Rigid phenolic board kept the metal inner shells aligned.
- We could weld the duct, so it was fabricated with traditional methods.
- Very good lead times. Other than the additional step of grooving the phenolic board, fabrication didn’t take much longer than traditional double-wall spiral duct with fiberglass insulation.
- Easy installation. With no inner coupling and insulation an integral part of the duct, assembly is not much different than for single-wall metal duct.
- Field-adjustable. You can easily cut holes in the duct and weld them back up without melting the insulation or starting a fire.
- 50 percent better thermal performance than fiberglass or elastomeric foam — R=12 in a 2” double-wall duct.
This story originally appeared in the October 2019 issue of SNIPS magazine.