RoofViews

Building Science

Air Barriers and Vapor Retarders: The Current Conundrum in the Roofing Industry

By James R Kirby

September 10, 2018

woman in rain with brief case over head by office building

The roofing industry continues to discuss vapor retarders but should be discussing air barriers.

One of the primary purposes of a building envelope is to keep moisture out of a building. What makes this difficult is that water comes in many forms and can take many paths into a building. Therefore, building designers need to account for bulk water, capillary water, air-transported moisture, and water vapor, and deal with each of these with different forms of defense.

Water in its many forms

Bulk water (i.e., rain and snow) is kept out of buildings with roof membranes and facades of all types. Capillary water is primarily a ground-based issue involving water moving into and through the building envelope via capillary action. Foundation waterproofing and water barrier layers or components are used to prevent this intrusion. Air-transported water, as the name implies, is carried into a building by air that infiltrates the building envelope. Water vapor enters a building by the process of diffusion of water through the building envelope materials.

air flow around building

Figure 1: 1) Bulk water, 2) Capillary water, 3) Air-transported water, 4) Vapor diffusion


There is an order of priority for the prevention of water intrusion. Bulk water is most critical, capillary water is second, then air-transported water, and finally, of least concern—although still important—is vapor diffusion.

The design, manufacturing, and construction industries are quite good at keeping bulk water out of buildings. They are also quite good at keeping capillary water out of buildings. They have only recently begun to focus on the importance of keeping air out of buildings. That is why the International Energy Conservation Code (IECC), since 2012, requires all new buildings to include an air barrier. The main reason for air barriers is to keep conditioned air from escaping and exterior air from infiltrating, but air-leakage prevention in building envelope keeps the moisture in the air from passing into and out of buildings. It may be a secondary reason, but nonetheless, this reason is important!

You may be wondering why air-transported water is a bigger issue than vapor diffusion. Indeed, the roofing industry has discussed vapor retarders for decades, but only recently focused on air barriers. But "back in the day," multi-ply asphalt-based vapor retarders that were installed above the roof deck and below the insulation were also acting as very effective air barriers.

Diffusion versus air movement

Let's compare vapor diffusion and air-leakage from the perspective of how much water is transported for each process. Lstiburek1, et al, determined that—in a warm climate—approximately 1½ pints of water will diffuse through a 4'x8' gypsum board, and approximately 14 pints of moisture will be transported by air passing through a 1"x1" hole in that same gypsum board. The same research showed that—in a cold climate—approximately 2/3 of a pint of water will diffuse through a 4'x8' gypsum board, and approximately 60 pints of water will be transported by air passing through a 1"x1" hole in the same gypsum board.

Another way of stating this is: In a warm climate, air transports 10x more water than diffusion, and in a cold climate, air transports 100x more water than diffusion. This is why air-transported moisture is much more critical to prevent than water vapor that enters a building by diffusion.

It has been suggested that air infiltration and exfiltration make up 25 to 40 percent of the total heat loss in a building in a cold climate and 10 to 15 percent of total heat gain in a hot climate.2 This is likely why the IECC have air-barrier requirements and do not have any significant vapor retarder requirements for building envelopes.

2nd law of thermodynamics

There is one simple rule that defines how heat, air, and moisture move—the 2nd law of thermodynamics. That sounds like a mouthful, so let's distill it. What it means in terms of building and roofing science is this:

  • Hot moves to cold
  • Moist moves to dry
  • High pressure moves to low pressure

Heat, moisture, and pressure always equalize when possible (i.e. if paths are available to do so). That's why there is a drive for warm, moist air to leave a building during winter when it's cold and dry outside.

2nd law of thermodynamics

For roof systems, the 2nd law of thermodynamics helps explain why—during the cold winter months—warm, moist interior air (e.g., 75F, 50% RH) infiltrates up into a roof system that doesn't have a vapor retarder/air barrier in the system. The warm, moist air equalizes to the exterior where the air is cooler and drier. There can be other reasons why this happens, like the stack effect, wind and associated membrane billowing, and internal pressurization from mechanical systems. So let's discuss those, too.

Air Movement

The stack effect, in laymen's terms, is the fact that warm air rises. In a tall narrow column, such as a skyscraper, this effect can be very pronounced. When warm air rises, it creates a higher pressure in the upper interior portion of a building. That increase in pressure also means the warm, moist air will escape through any pathways that are available. It will escape 'into' the roof or any air passage that goes to the exterior. (That's why air barriers are now an energy code requirement for new construction. More on that in a bit.)

Membrane billowing occurs when wind creates a negative pressure above a roof system and lifts the membrane between the rows of fasteners in the seams. A billowing membrane brings interior air into the roof system regardless of temperature or moisture levels.

Air conditioning and heating equipment force air through ductwork and into the interior of a building. By forcing conditioned air into a space, the space can become somewhat pressurized. Not to a great extent, but enough to create an imbalance between the interior and the exterior, forcing interior air into the roof system.

air movement

Figure 3: Processes that create air-flow across the building envelope. SOURCE: Building Science Digests, BSD-014: Air Flow Control in Buildings, John Straube, October 15, 2007


A less desirable scenario for air and moisture infiltration into a properly installed roof assembly is to use a mechanically attached (MA) system with a single layer of insulation without a vapor retarder/air barrier (VR/AB). The MA system billows; the lack of VR/AB allows warm moist air to enter the roof system; and the board joints allow a direct path for air flow from the deck to the membrane.

membrane billows roof

Figure 4: A less desirable roof-design scenario for air and moisture infiltration into a roof assembly.


A more desirable scenario is an adhered roof system with multiple layers of insulation (with board joints offset and staggered) over a VR/AB. This system helps lower the risk of these detrimental processes from occurring. The end result can be a roof system with better longevity and thermal performance, and a building with improved energy efficiency. (Of course, actual energy savings may vary based on a number of factors, like climate zone, utility rates, etc.)

adhred roof wind

Figure 5: A roof design that improves longevity and thermal performance.

Vapor Retarders and Air Barriers

Vapor retarders do just as they are named—they reduce vapor diffusion, but not all vapor retarders are equal. There are 3 classes of vapor retarder materials, as shown in the figure. The lower the perm rating, the less diffusion occurs through a material. Most roof membranes are Class I vapor retarders. A single layer self-adhered, bituminous vapor retarder has a perm rating of 0.03 perms. Plywood (1/4" thick, Douglas fir, exterior glue) is a Class II vapor retarder with a perm rating of 0.7 perms. The same plywood with interior glue is a Class III vapor retarder with a perm rating of 1.9 perms. Perm ratings for additional roofing materials are shown in the figure. Remember, these are material ratings; the full system needs to be designed and installed correctly for proper functionality.

three classes of vapor retarders

Figure 6: Three classes of vapor retarders

Perm ratings of common roofing materials

Figure 7: Perm ratings of common roofing materials.


From a designer's perspective, if a vapor retarder is needed, which class should be used? If a Class I vapor retarder is used, the concern is that any moisture (e.g., construction moisture due to installation methods, weather, etc.) that enters a roof system won't be able to dry out. It's often a good idea to select a vapor retarder that will allow some amount of drying from diffusion. Exceptions to this idea include roofs over indoor swimming pools and other high-humidity producing activities or processes. Another exception is a Class I vapor retarder should be installed over a new concrete deck to prevent the moisture in the concrete from drying into the roof system.

Here's a key takeaway—all vapor retarders block air, but not all air barriers block vapor diffusion. That means that when we use a vapor retarder in a roof system, it's also acting as an air barrier. The caveat is that the vapor retarder needs to be sealed at all perimeters and penetrations, and tied to the wall air barrier so air does not bypass the vapor retarder layer. So, practically speaking, all vapor retarders are air barriers if they are installed to block the passage of air.

Moving Forward

The traditional way of designing roofs with vapor retarders is to install an asphaltic vapor retarder (a single layer modified sheet or a double mopping of asphalt) either directly to the deck or over a fastened hardboard. What if that hardboard or plywood deck was recognized to be the air barrier layer, but had a moderate or high perm rating? Since air movement brings 10-100 times more moisture as compared to diffusion, perhaps we should be considering the use of a Class II or III vapor retarder (e.g., hardboard or plywood deck) installed to be an effective air barrier that also allows some drying potential? The wall industry has been doing this for quite some time. Should our roof systems be designed similarly? A gypsum-fiber board has a perm rating of approximately 24 to 30 perms, depending on thickness. If this board is fastened to a steel deck and the joints and transitions are taped, it could be an effective air barrier that allows some drying. Something for roof designers to consider!

Roof system design is always the responsibility of the designer, but perhaps the designers in the roofing industry can find some takeaways from the wall industry. There is always more to learn and understand about the building science of our roof systems.


1Building Science Corporation, Build Boston—2005, Thermal and Air Leakage Control, ppt from Betsy Pettit, AIA

2"The Hidden Science of High-Performance Building Assemblies," Environmental Building News, November 2012

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. A range of codes and standards are beginning to address this problem, though it's important to note that there is often a time lag between development of codes and their widespread adoption.What Is the Industry Doing About It?Long in the business of supporting high-performance building enclosures, Phius (Passive House Institute US) provides a Fastener Correction Calculator along with a way to calculate the effect of linear thermal bridges (think shelf angles, lintels, and so on). By contrast, the 2021 International Energy Conservation Code also addresses thermal bridging, but only considers framing materials to be thermal bridges, and actually pointedly ignores the effects of point loads like fasteners in its definition of continuous insulation: "insulation material that is continuous across all structural members without thermal bridges other than fasteners and service openings" (Section C202). 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. 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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. 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