RoofViews

Building Science

Value Engineering Part 2: Retaining Performance

By James R Kirby

June 08, 2020

A wooden scale with balancing coins and a yellow hard hat

In the first Value Engineering post about the attributes of high performing roof designs, value engineering is defined as "a concept that states there are less expensive ways to get equivalent performance," and the post described the performance attributes that make for a long-lasting, high-performing roof system. These performance attributes include: images

  • Energy efficiency (reflectivity, thermal resistance, and air leakage)
  • Impact resistance
  • Wind resistance
  • Condensation prevention
  • High heat (and UV) resistance
  • Positive drainage
  • Wear resistance

These performance attributes manifest themselves in certain aspects of the overall design of the roof system. What type of membrane, membrane thickness, cover board or no cover board, adhered or mechanically attached cover board, insulation layout (including tapered), insulation attachment method(s), vapor retarder / air barrier or not, and attachment to the roof deck are some of the questions that drive the performance of a roof system.

And the roof design is…

After considering all of the above performance attributes, your design strategies for a long-lasting, high performing, and durable roof include:

  • Adhered reflective roof membrane with high heat-resistance
  • Adhered HD polyiso coverboard
  • 2+ layers of adhered insulation, staggered and offset, with tapered insulation and crickets and saddles
  • Air barrier (over a substrate board) at the deck level
  • Third-party-tested edge metal details
  • Walkway pads


Energy Efficiency – reflectivity and thermal resistance

The attachment method is integral to a roof system's energy efficiency. Consider, for example, a roof that is designed to have the first layer of insulation fastened to the deck and the remainder of the components above are to be adhered. There is a cost for fasteners as well as adhesives. One way to "value engineer" this roof is to use fasteners through the entire system thereby removing the cost of the adhesive and labor to install the adhesive. While eliminating the adhesive is a cost reduction strategy, it is possible that longer fasteners will be needed. Sometimes something that is eliminated creates the need for a greater cost elsewhere. This is often not mentioned or discussed, so ask the question!

Value engineering the removal of the adhesive also, unfortunately, can result in a roof system with a 15 to 30% reduction in R-Value relative to the as-designed roof system. If the roof was designed to have an R-30, the actual R-value as installed would be about R-21 to R-25! It's been shown (read this blog about optimizing R-value) that over the life of a roof, roof systems that use adhered membranes and an adhered top layer of insulation can offset the cost of adhesives when the initial cost of 'lost' insulation performance and the additional annual cost for increased heating and cooling are factored into the analysis.

One of the key issues with value engineering of a roof system is that proponents of the reduction of costs are generally only looking at the initial cost to install a roof. However, the operational costs, the energy efficiency, the wind resistance, are commonly compromised. Operational costs, which are tied directly to a roof system's energy efficiency, are often much higher over the life of a roof. (Read this blog about analyzing long-term costs.) Also, roof systems with different wind-resistance ratings may need to be selected after value engineering revises the attachment method.

Also, is the mechanical system design and whole-building energy-use analysis predicated on a roof system R-value of 30? Determining the effect on mechanical unit sizing and cost, including differences in whole building energy use may show that the savings from value engineering the roof will have a negative effect on annualized energy use and the ability of the HVAC system to maintain occupant comfort. Whole building energy simulation modeling, such as EnergyPlus™ (EnergyPlus.net), can be used. EnergyPlus™ is funded by the U.S. Department of Energy's Building Technologies Office, and managed by the National Renewable Energy Laboratory.

Each "dot" is a fastener and plate; fasteners and plates are thermal bridges that reduce the R-value of the insulation layer.


Impact Resistance

Using any type of coverboard will improve the impact-resistance of a roof system because of the toughness of coverboards in general. However, a key issue with impact resistance is the location of the plate and fastener relative to the roof membrane. If a coverboard is mechanically attached (so the plates and fastener head are immediately beneath the membrane), impacts at fastener heads and plates have been shown to result in damage to the membrane. Adhered coverboards remove this concern, but do require the use of some type of adhesive. It's not just the use of a coverboard, but the installation method that matters.

ice.impact

Ice-ball impact above fasteners always punctures the membrane. Left to right: increasing damage from hairline crack to complete puncture.


Impacts come from hail as well as rooftop use by occupants and anyone performing work on rooftop mechanical units or to rising walls, for example. Reducing membrane thickness, changing from a fleece-back membrane to a smooth-back membrane, as well as changing to a more commodity-type membrane versus one that is shown to have a long service life are value engineering choices that can reduce cost, yet also reduce the impact resistance.

Wind Resistance

Designers make wind-design decisions that are intended to reduce risk (e.g., choosing Partially Enclosed versus Enclosed, selecting Exposure Category C instead of Exposure Category B). If a designer and owner have determined to reduce the potential risk of high-wind-event damage by increasing the design wind loads and subsequently the capacity of a roof system, that high capacity could be value engineered out. Coordinating wind design assumptions and documenting owner expectations and decisions can be critical pieces of information that alleviate the reduction of wind load capacity based on cost. Owners have their reasons; value engineering should not undermine owner expectations. By including design wind pressures for each roof zone as well as providing design loads for edge metal, and parapets and coping, minimum performance requirements are established in the construction documents.

For more on wind design, read our blog here.

Condensation prevention

Roofs that include air barriers above the deck and below insulation provide the best protection against the development of condensation in a roof system. Your roof design took into consideration the interior and exterior design temperatures and calculations were performed to determine potential dew point locations, which resulted in the correct placement of the air barrier.

This roof design is "above code." While the energy code allows the roof membrane to be the air barrier, air can still move up into the roof system. This is called air intrusion and can bring moisture into the roof system, potentially causing condensation issues.


Placing the air barrier layer on the top surface of a rigid board fastened to the roof deck is one option. However, with "first layer fastened and the upper layers (insulation, cover board, membrane) adhered" systems, there is an opportunity to include a vapor retarder / air barrier between the insulation layers without fasteners penetrating the vapor / air barrier layer. Reducing air intrusion and the potential for condensation is achieved using both designs; this is a real value engineering opportunity! The costs for materials and labor for each design can be determined, and the least expensive design that provides condensation control can be implemented, instead of eliminating the properly located air barrier.

While removing the air barrier and the associated adhesives and components reduces costs, changing to a roof system that uses only mechanical attachment also negatively affects the potential for condensation and thermal performance of the roof system. For example, consider a high-humidity building, such as a natatorium—the use of an air barrier at the deck level reduces air intrusion into the roof system from the interior and the moisture that air carries. This reduces the potential for condensation and damage to the roof system.

High Heat (and UV) Resistance

Heat and UV are two environmental factors that affect roof membrane service life. Understanding this and that storms are increasing in severity and frequency, in addition to recognizing temperatures are rising (ASHRAE's climate zone maps are moving northward), your roof design includes a membrane that has been shown to have a long service life. It's likely the specification includes GAF's TPO Everguard Extreme® membrane due to its ability to withstand high heat conditions.

It's possible that a high-performing roof membrane will be value engineered down to one of reduced initial cost. The rationale will be that it still keeps water out! But to what long-term cost? You designed a roof to meet your client's expectations for a long service life. By considering annualized costs, not simply initial cost, it may be shown that a longer service life roof is actually a better long-term value for the owner. A key strategy is to use annualized costs when pushing back against value engineering.

Positive Drainage

Most roof systems are not guaranteed or warranted against ponding water. The combination of UV and water is one mechanism that advances the deterioration of many roof membrane materials. And because water needs to be removed from a roof quickly and efficiently, your roof design includes properly sized drains and scuppers, a tapered insulation layout to efficiently move water to drains and scuppers, and crickets and saddles to prevent localized ponding.

Example of a tapered layout, courtesy GAF Tapered Design Group


When asked to reduce the cost of the tapered insulation, one possible answer is to install more drains that are closer together. This can reduce the build-up of tapered insulation, and the associated cost. Perhaps the use of lightweight insulating concrete is less costly? Perhaps the use of a TPO membrane, which does not have ponding water limitations in the guarantee/warranty, makes sense?

Wear Resistance

Thicker membranes, granulated surfaces, and walkway pads are helpful in preventing unwanted wear to roofing membranes. You've coordinated expectations with the owner, your client, that rooftop protection against wear and tear is important during the operation phase of the building. Reducing the thickness of a membrane reduces its potential wear resistance, as well as its impact resistance (as previously noted in this blog). As with many things, none of these issues are stand-alone issues.

Perform a cost comparison of a granule-surface modified bitumen membrane and a smooth surface system with a high-solids coating system to see if there are less expensive systems that still meet wear-resistance requirements. Don't just look at initial costs; consider maintenance costs over the life of the roof that affect annualized costs. A relatively small number of walkway pads, strategically placed around rooftop units and systems that will require regular maintenance (e.g., HVAC units and solar arrays), can be an effective and economic solution.

Product substitutions

Not all products with the same intended function (and described with the same terminology) are created equal. A vapor retarder is a good example. A self-adhered vapor retarder with a perm rating of 0.03 has a desired performance and was included for a specific reason – to effectively block the diffusion of moisture into the roof assembly. Substituting a less expensive vapor retarder with different properties (e.g., a higher perm rating) and physical characteristics (30 mil asphaltic vs. a 6 mil poly), won't be installed in the same fashion—even though they are both vapor retarders. The least expensive material may be the most costly to install properly.

Details

Details matter. Without a doubt, the ability for a roof to keep water out is most challenging at the details—penetrations, perimeters, and locations where there is an interruption, end, or change of direction. You designed a two-part counterflashing for ease of maintenance and future reroofing; a single-piece counterflashing is less expensive initially. When it's time to reroof, will the facade material (e.g., masonry, stucco, EIFS) need to be removed and repaired in order to install new counterflashing? Again, you've established with the owner that future efforts for maintenance and reroofing are important.

There are ways to value engineer roofing details, especially when it comes to air barrier and vapor retarder constructability. A good example is an overhang. It seems logical to design an air barrier to enclose the overhang (the blue lines in the graphic), but there are many potential dis-continuities (the red lines in the graphic). The air barrier can be inboard of the eave; the wall air barrier could be tied to the underside of the roof deck, and the above deck air barrier (whether at deck level or the roof membrane) is tied to the top of the roof deck making a continuous air barrier system. There likely are labor savings associated with this value-engineered design revision.

Screen shot 2013-01-20 at 11.33.52 AM.png

Weather

A change of seasons is often the impetus for changing from an adhered system to one that strictly uses mechanical attachment. While this may not be value engineering, a change of seasons can mean daily temperatures are below manufacturer recommended minimums for adhesive materials. However, manufacturers have (and are developing) lower temperature adhesives. Regardless of why a roof system design eliminates adhesives in favor of fasteners, there are energy efficiency and an impact-resistance trade offs.

Owner expectations

Much of the prevention of value engineering is having a clear understanding of the owner's performance requirements and expectations. Getting 'buy-in' from an owner for high-performance roof systems is key. During the design phase, explaining the importance of design decisions and aligning them with the owner's performance requirements should be documented. Documented design decisions that are made in conjunction with the owner provide a defense when "it costs too much" is used as the basis for value engineering.

The Whole Building Design Guide, from the National Institute of Building Sciences, includes detailed information about value engineering during the design and construction phases of a project. You can find this resource here.

About the Author

James R. Kirby, AIA, is a GAF building and roofing science architect. Jim has a Masters of Architectural Structures and is a licensed architect. He has over 25 years of experience in the roofing industry covering low-slope roof systems, steep-slope roof systems, metal panel roof systems, spray polyurethane foam roof systems, vegetative roof coverings, and rooftop photovoltaics. He understands the effects of heat, air, and moisture movement through a roof system. Jim presents building and roofing science information to architects, consultants and building owners, and writes articles and blogs for building owners and facility managers, and the roofing industry. Kirby is a member of AIA, ASTM, ICC, MRCA, NRCA, RCI, and the USGBC.

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What is going on here?No, this roof does not have measles, it has a problem with thermal bridging through the roof fasteners holding its components in place, and this problem is not one to be ignored.As building construction evolves, you'd think these tiny breaches through the insulating layers of the assembly, known as point thermal bridges, would matter less and less. But, as it happens, the reverse is true! The tighter and better-insulated a building, the bigger the difference all of the weak points, in its thermal enclosure, make. 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Likewise, The National Energy Code of Canada for Buildings: 2020 addresses thermal bridging of a number of building components, but also explicitly excludes fasteners: "in calculating the overall thermal transmittance of assemblies…fasteners need not be taken into account" (Section 3.1.1.7.3). Admittedly, point thermal bridges are often excluded because it is challenging to assess them with simple simulation tools.Despite this, researchers have had a hunch for decades that thermal bridging through the multitude of fasteners often used in roofs is in fact significant enough to warrant study. Investigators at the National Bureau of Standards, Oak Ridge National Laboratory, the National Research Council Canada, and consulting firms Morrison Hershfield and Simpson Gumpertz & Heger (SGH), have conducted laboratory and computer simulation studies to analyze the effects of point thermal bridges.Why Pay Attention Now?The problem has been made worse in recent years because changes in wind speeds, design wind pressures, and roof zones as dictated by ASCE 7-16 and 7-22 (see blogs by Jim Kirby and Kristin Westover for more insight), mean that fastener patterns are becoming denser in many cases. This means that there is more metal on average, per square foot of roof, than ever before. More metal means that more heat escapes the building in winter and enters the building in summer. By making our buildings more robust against wind uplift to meet updated standards, we are in effect making them less robust against the negative effects of hot and cold weather conditions.So, how bad is this problem, and what's a roof designer to do about it? A team of researchers at SGH, Virginia Tech, and GAF set out to determine the answer, first by simplifying the problem. Our plan was to develop computer simulations to accurately anticipate the thermal bridging effects of fasteners based on their characteristics and the characteristics of the roof assemblies in which they are used. In other words, we broke the problem down into parts, so we could know how each part affects the problem as a whole. We also wanted to carefully check the assumptions underlying our computer simulation and ensure that our results matched up with what we were finding in the lab. The full paper describing our work was delivered at the 2023 IIBEC Convention and Trade Show, but here are the high points, starting with how we set up the study.First, we began with a simple 4" polyisocyanurate board (ISO), and called it Case A-I.Next, we added a high-density polyisocyanurate cover board (HD ISO), and called that Case A-II.Third, we added galvanized steel deck to the 4" polyiso, and called that Case A-III.Finally, we created the whole sandwich: HD ISO and ISO over steel deck, which was Case A-IV.Note that we did not include a roof membrane, substrate board, air barrier, or vapor retarder in these assemblies, partly to keep it simple, and partly because these components don't typically add much insulation value to a roof assembly.The cases can be considered base cases, as they do not yet contain a fastener. We needed to simulate and physically test these, so we could understand the effect that fasteners have when added to them.We also ran a set of samples, B-I through B-IV, that corresponded with cases A-I through A-IV above, but had one #12 fastener, 6" long, in the center of the 2' x 2' assembly, with a 3" diameter insulation plate. These are depicted below. The fastener penetrated the ISO and steel deck, but not the HD ISO.One visualization of the computer simulation is shown here, for Case B-IV. The stripes of color, or isotherms, show the vulnerability of the assembly at the location of the fastener.What did we find? The results might surprise you.First, it's no surprise that the fastener reduced the R-value of the 2' x 2' sample of ISO alone by 4.2% in the physical sample, and 3.4% in the computer simulation (Case B-I compared to Case A-I).When the HD ISO was added (Cases II), R-value fell by 2.2% and 2.7% for the physical experiment and computer simulation, respectively, when the fastener was added. In other words, adding the fastener still caused a drop in R-value, but that drop was considerably less than when no cover board was used. This proved what we suspected, that the HD ISO had an important protective effect against the thermal bridging caused by the fastener.Next, we found that the steel deck made a big difference as well. In the physical experiment, the air contained in the flutes of the steel deck added to the R-value of the assembly, while the computer simulation did not account for this effect. That's an item that needs to be addressed in the next phase of research. Despite this anomaly, both approaches showed the same thing: steel deck acts like a radiator, exacerbating the effect of the fastener. In the assemblies with just ISO and steel deck (Cases III), adding a fastener resulted in an R-value drop of 11.0% for the physical experiment and 4.6% for the computer simulation compared to the assembly with no fastener.Finally, the assemblies with all the components (HD ISO, ISO and steel deck, a.k.a. Cases IV) showed again that the HD ISO insulated the fastener and reduced its negative impact on the R-value of the overall assembly. The physical experiment had a 6.1% drop (down from 11% with no cover board!) and the computer simulation a 4.2% drop (down from 4.6% with no cover board) in R-value when the fastener was added.What Does This Study Tell Us?The morals of the study just described are these: Roof fasteners have a measurable impact on the R-value of roof insulation. High-density polyisocyanurate cover boards go a long way toward minimizing the thermal impacts of roof fasteners. Steel deck, due to its high conductivity, acts as a radiator, amplifying the thermal bridging effect of fasteners.What Should We Do About It?As for figuring out what to do about it, this study and others first need to be extended to the real world, and that means making assumptions about parameters like the siting of the building, the roof fastener densities required, and the roof assembly type.Several groups have made this leap from looking at point thermal bridges to what they mean for a roof's overall performance. The following example was explored in a paper by Taylor, Willits, Hartwig and Kirby, presented at the RCI, Inc. Building Envelope Technology Symposium in 2018. In that paper, the authors extended computer simulation results from a 2015 paper by Olson, Saldanha, and Hsu to a set of actual roofing scenarios. They found that the installation method has a big impact on the in-service R-value of the roof.They assumed a 15,000-square-foot roof, fastener patterns and densities based on a wind uplift requirement of 120 pounds per square foot, and a design R-value of R-30. In this example, a traditional mechanically attached roof had an in-service R-value of only R-25, which is a 17% loss compared to the design R-value.An induction-welded roof was a slight improvement over the mechanically attached assembly, with an in-service value of only R-26.5 (a 12% loss compared to the design R-value).Adhering instead of fastening the top layer of polyiso resulted in an in-service R-value of R-28.7 (a 4% loss compared to the design R-value).Finally, in their study, an HD polyiso board was used as a mechanically fastened substrate board on top of the steel deck, allowing both layers of continuous polyiso insulation and the roof membrane to be adhered. Doing so resulted in an in-service R-value of R-29.5, representing only a 1.5% loss compared to the design R-value.To operationalize these findings in your own roofing design projects, consider the following approaches: Consider eliminating roof fasteners altogether, or burying them beneath one or more layers of insulation. Multiple studies have shown that placing fastener heads and plates beneath a cover board, or, better yet, beneath one or two layers of staggered insulation, such as GAF's EnergyGuard™ Polyiso Insulation, can dampen the thermal bridging effects of fasteners. Adhering all or some of the layers of a roof assembly minimizes unwanted thermal outcomes. Consider using an insulating cover board, such as GAF's EnergyGuard™ HD or EnergyGuard™ HD Plus Polyiso cover board. Installing an adhered cover board in general is good roofing practice for a host of reasons: they provide enhanced longevity and system performance by protecting roof membranes and insulation from hail damage; they allow for enhanced wind uplift and improved aesthetics; and they offer additional R-value and mitigate thermal bridging as shown in our recent study. Consider using an induction-welded system that minimizes the number of total roof fasteners by dictating an even spacing of insulation fasteners. The special plates of these fasteners are then welded to the underside of the roof membrane using an induction heat tool. This process eliminates the need for additional membrane fasteners. Consider beefing up the R-value of the roof insulation. If fasteners diminish the actual thermal performance of roof insulation, building owners are not getting the benefit of the design R-value. Extra insulation beyond the code minimum can be specified to make up the difference.Where Do We Go From Here?Some work remains to be done before we have a computer simulation that more closely aligns with physical experiments on identical assemblies. But, the two methods in our recent study aligned within a range of 0.8 to 6.7%, which indicates that we are making progress. With ever-better modeling methods, designers should soon be able to predict the impact of fasteners rather than ignoring it and hoping for the best.Once we, as a roofing industry, have these detailed computer simulation tools in place, we can include the findings from these tools in codes and standards. These can be used by those who don't have the time or resources to model roof assemblies using a lab or sophisticated modeling software. With easy-to-use resources quantifying thermal bridging through roof fasteners, roof designers will no longer be putting building owners at risk of wasting energy, or, even worse, of experiencing condensation problems due to under-insulated roof assemblies. Designers will have a much better picture of exactly what the building owner is getting when they specify a roof that includes fasteners, and which of the measures detailed above they might take into consideration to avoid any negative consequences.This research discussed in this blog was conducted with a grant from the RCI-IIBEC Foundation and was presented at IIBEC's 2023 Annual Trade Show and Convention in Houston on March 6. Contact IIBEC at https://iibec.org/ or GAF at BuildingScience@GAF.com for more information.

By Authors Elizabeth Grant

November 17, 2023

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