Understanding Today’s Vapor Barriers

One of the most misunderstood aspects of a metal roofing system is the proper use of a vapor barrier. There are many sources of information about this topic – some of which are based on science, some based on anecdotal field experience, and some based on journalism. Here, we will try to break it down to the basic principles that can be used to understand the latest options for a metal building roof system today.

What is Vapor?

The observed science tells us that water can take three forms, depending on temperature and its ability to interact with other things around it. Water can be a liquid that we drink, solid ice that we can skate on, or a gaseous vapor that is part of the makeup of the air we breathe. We can’t see water vapor in the air but we can feel it – we call that humidity. High humidity means a lot of water vapor is in the air, typically coupled with higher air temperature – and both can make us feel uncomfortable and “sticky.” Low humidity means the air is dryer – more typical in lower-temperature air – but this may also be uncomfortable for our breathing, skin dryness, etc.

Why is Vapor a Concern?

As long as the gaseous water vapor stays in the air at a moderate or comfortable level, there is no real concern. However, since water vapor responds quickly to temperature, it can turn back into water as soon as it encounters a surface that is cold enough for it to make the transformation. We know this phenomenon as condensation, and anyone who has seen a cold drink collect water on the outside of a glass on a humid summer’s day has experienced it. It is the same phenomenon that shows up on the surface of windows in a building when there is a big difference between inside and outside temperatures. We know that the amount of water vapor (i.e., humidity) present and the air temperature can both be variable at any given time, but there is always a predictable point at which water vapor will condense and form water drops – this is called the dew point. When vapor in the air encounters a temperature at or below the dew point, condensation occurs.

What Does This Have to do With Metal Roofing?

Metal roofing systems and condensed moisture are not a good combination. If airborne moisture seeps into a metal roof assembly, finds a cool surface, and condenses on any surface there, it likely won’t be visible from inside or outside of the building. That trapped water can then cause rust and corrosion of metal parts, resulting in real damage. It can also collect and saturate building insulation, rendering it ineffective. If enough water condenses, it can cause visible staining or grow mold, causing concerns for people inside the building.

Vapor
Vapor barriers are used in metal buildings to reduce the rate at which vapor can move through a material.

Do Building Codes Address This?

Absolutely – they require that the building be protected from the possibility of damage caused by water vapor. Since the concern is to restrict the flow of airborne moisture in relatively warm air from reaching a cooler surface to condense on, they call for something to be installed on the “warm” side to prevent that flow. For most buildings across the United States, the warm side is the interior face of the roof and walls. However, if the building is kept cold as in a refrigerated warehouse or storage building, then the warm side is likely on the exterior. The same is true in southern climates where warmer, humid air is the exterior condition and cooler interiors are common.

What is the Best Solution?

Manufacturers of insulating products have been involved in addressing the best ways to provide not only insulation to keep building temperatures warmer, but also vapor barriers to restrict the flow of airborne moisture. After literally decades of trying different types of vinyl and polyethylene facings over fiberglass insulation, most have realized that those membranes simply don’t provide enough protection to be effective. Instead, most are now offering a choice of laminated facings over the insulation that can be installed so they are exposed to the appropriate warm side of the roofing system. These fairly sophisticated laminations include:

  • Polypropylene-scrim-kraft consisting of layers of white or metalized polypropylene, fiberglass reinforcing, and white kraft paper on the order of 11-30 lb. weight
  • Polyprolene-scrim-kraft consisting of aluminum foil, fiberglass reinforcing, and 30 lb. kraft paper
  • Vinyl-reinforced polyester

All of these latest advancements in vapor barriers can provide comparable, high levels of protection, but their selection can depend on a variety of other building factors. Therefore, it is always best to engage an architect or engineer in the design to review the needs of the entire building to select the most appropriate, specific solution for an given project. It will also be important that all seams, connections, and penetrations of the vapor barrier are addressed in the design and construction, which are similarly best addressed by an architect or engineer to assure there are no breaches in the protection provided by the barrier.

To find out more about vapor barrier and insulation products for metal roofing systems, contact your local MBCI representative.

Understanding Wind Uplift Testing for Standing Seam Roof Systems

When properly designed and installed, a standing seam metal roof system provides the building owner with long-term dependability and value. Standing seam roof systems fulfill requirements for durability and protection against just about any type of weather situation, and they excel in high winds. Overall, they have outstanding performance records.

In the most severe weather conditions, wind pressure can force panels to deflect. This causes the seams to open and the panels to shift in failure mode. Typical failure occurs at the corners and edge zones. As a result, standing seam metal roofs must meet certain standards and testing criteria.

While there are many performance tests out there, the ASTM E 1592 is considered most reliable for the design of standing seam panels.  It is the standard test method for structural performance of sheet metal roof and siding systems by uniform static air pressure differences.

The most common wind testing standards include:

  • Underwriters Laboratories (UL) 580
  • Underwriters Laboratories (UL) 1897
  • Factory Mutual (FM) Global Standard 4471
  • American Society for Testing and Materials (ASTM) E 1592
UL 580 Wind Test
Uplift resistance testing with UL 580 test platform.

UL 580

The UL 580 rating determines the uplift resistance of roof assemblies. The test evaluates the roof panel, panel clips, fasteners and the substrate.

Test Method

  • A 10-foot by 10-foot sample of roofing material is installed onto a test platform. The edges are then sealed with closely spaced fasteners and two purlins in the interior.
  • Next, the sample is subjected to a static uplift pressure for a 5-minute period and an oscillating pressure in 10 second intervals over a 60-minute period.

Considerations

  • UL 580 is a pass/fail test and does not specifically determine wind resistance of the panel assembly.
  • It only tests over a specific substrate at a certain clip/fastener spacing.
  • The test standard will not indicate how strong the panel assembly is under load.
  • Most importantly, the test does not simulate real conditions.

UL 1897

The UL 1897 wind test is a continuation of UL 580, and is the standard for uplift tests for roof covering systems. The purpose of this test is to gain uplift resistance data for the panel assembly, and evaluate the attachment of the roof covering systems to the roof decks.

Test Method

  • Utilizing a test chamber, this test is conducted by either pulling a vacuum above the assembly or by pressurizing an air bag placed loosely between the deck and the roof covering.
  • The test is run to failure, and the results are reported as the highest uplift pressure achieved prior to failure (in psf).

Considerations

  • UL 1897 does not consider the strength of the roof deck.
  • The method does not necessarily simulate the actual dynamic uplift pressures encountered by roofing systems.

FM Global Standard 4471

FM 4471, Approval Standard for Class 1 Panel Roofs, states the requirements for meeting the criteria for fire, wind, foot traffic and hail damage resistance.

The test sets performance requirements for panel roofs, which includes all components necessary for installation of the panel roof assembly as a whole. This includes the potential for fire spread on the underside and exterior of the roof panel. It also measures ability to resist simulated wind uplift resistance while maintaining adequate strength and durability.

Test Method

  • FM 4471 utilizes a 12-foot by 24-foot section, including the connecting fasteners and clips used in the field. The panels are subjected to increased wind pressures until the assembly fails.
  • The ratings are stated as 1-60, 1-90, 1-120, and so on, referring to wind pressure in pounds per square foot (psf).
  • This rating is used to apply a classification to roof panels. Class 1 roof panels are rated at 1-90. A safety factor of 2 means the maximum allowable design load is 45 psf.

Considerations

  • FM Global is a non-profit scientific research and testing organization that deals with commercial and industrial property insurance.
  • For roofing projects where FM insurance is required, project designers should work closely with the roofing manufacturer to ensure the roofing system complies with FM requirements.

ASTM E 1592

This test method provides a standard for structural performance under uniform static air pressure differences, and is run to failure to find the ultimate uplift load capacity. This test measures both panels and anchors. ASTM E 1592 is not a pass/fail test; it merely shows how a roof performs under uniform static load.

Test Method

  • A 5-panel-wide sample (10 feet) by 25-foot length is subjected to pressure from underneath to imitate wind load. The sample has intermediate purlin support at varied intervals and covers several spans.
  • The pressure is applied to identify slowly developing failures such as seam separations, and to determine the ultimate failure load of the standing seam roof system.
ASTM 1592 Wind Uplift Testing
MBCI research and development team performs ASTM 1592 wind uplift tests. The wind pressure forces the panels to deflect, pushing the center of the panel above the seams.

Testing for Reliable Design

ASTM E 1592 was developed to account for the many complexities of evaluating uplift properties of metal roofing. The test method “provides a standard procedure to evaluate or confirm structural performance under uniform static air pressure difference. This procedure is intended to represent the effects of uniform loads on exterior building surface elements.” (https://www.astm.org/Standards/E1592.htm)

In conclusion, while all of the standardized test protocols mentioned above were established to determine uplift capacities of roof assemblies, only the ASTM E 1592 uplift test is considered reliable enough for the design of standing seam roof panels. Among its key differentiators, the test takes into consideration the roof’s flexibility, changes in shape occurring under air pressure, and it measures both metal panels and their anchors.

Design to Your Client’s Mindset

Spring Fire Department Station 78

As an architect, when did you last hear your client say, “Money is no object?”  This happens … almost never!  More likely what you hear is “I want high quality for low cost.”  The challenge of the architect is to provide your client with high quality at a reasonable and appropriate price.  A large part of finding that balance is determining the values, goals and long-term perspective of your client.

If a building owner wants a metal roof, it’s likely they already have a reason why.  Perhaps their existing roof didn’t provide the service life they expected it to, or it was damaged disproportionately.  Or the building owner understands that a metal roof can last a really long time.  Or they like the look of a metal panel or metal shingle roof, with all the colors and shapes available.  As an architect, it is important to determine your client’s mindset.  In the end, the question comes down to, “How long will you own this building (or home)?”  And, although less common, a building owner may just want to build a high-end, long-lasting building no matter their desired length of ownership.

The large part of the cost of a metal roof, similar to other roof types, is the labor to remove the existing roof and install the new one.   Upgrading from a 24-gauge metal to 22-gauge metal is a minimal increase in material costs that is easily justifiable for the long term.  Metal thickness, coating type and thickness, and penetration and edge details are the areas where upgrades and enhancements occur.

Argue against value engineering.  Roofs certainly can be out of sight, out of mind to most owners, but building owners who are considering metal roof systems understand the concept of life-cycle analysis, whether they know it or not.  Overtly reinforce their long-term outlook to help ensure that high-end penetration details and edge details are designed and installed.  Look to the industry standards—SMACNA, NRCA—for details that will last the life of the metal panels.  Realize that metal panels don’t leak; joinery and flashings are the potential leak locations.  Upgrade the details to be of the highest quality.

Understanding the mindset of your client is critical to determining the level of design.  This is definitely a cost issue.  The “university” client thinks long term; the “developer” client thinks short term.  However, there is much middle ground that requires inquisitive discussion with an owner to determine his/her goals.  Ask the questions, and design a metal roof based on your client’s mindset.

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