air barriers Archives

3 Energy-Saving Technologies to Consider with Metal Roofs

Posted on December 16, 2015 by Amy Crenan

A roof’s primary function is to keep a building weatherproof. A roof’s secondary function—and approaching nearly equal importance—is to be an energy-efficient element of the building envelope. From an energy efficiency standpoint, we’re accustomed to the inclusion of insulation. Are we as accustomed to the ideas that roof color and air leakage matter for energy efficiency? The building industry is embracing all of these technologies in an effort to save energy. So how does an installer make it all work?

Insulation

Photo Courtesy of NAIMA

Insulation requirements for roofs on metal buildings (according to the 2015 IECC) range from R-19+R-11 LS up to R-30+R-11 LS, depending on climate zone. The first layer is draped over the purlins and requires a thermal spacer block with an R-3.5. A second layer is installed at perpendicular and is required to include a liner system (LS), which is a continuous vapor barrier installed below the purlins and is uninterrupted by framing members. The crisscrossed layers help reduce convective air movement within the insulation layer, making the insulation layer more effective. And, good news!—the vapor barrier can also be an air barrier. So, on to air barriers.

Air Barriers

Even small air leaks in buildings can account for a 30 to 40% heat loss during heating season (winter), regardless of the amount of insulation. It can’t be overstated—air barriers are critical to an energy-efficient roof and overall building envelope. The LS, or vapor barrier, can be an air barrier only if the seams of the LS are sealed to prevent air passage. The junction between the air barrier in the roof and walls is critical; it must be joined to be continuous. Often, a separate material (adhered membranes or spray-applied foams) is used as the transition from wall to roof. Or, the roof and wall air barriers might end on opposite sides of a perimeter beam or purlin, connecting the two air barriers. Also, any penetrations through the roof need to be sealed to the air barrier. Being continuous/having continuity is key to constructing a properly functioning air barrier!

Roof Color

We’ve heard a lot about roof color. Where air conditioning is prevalent (e.g., the Southwest), highly reflective roofs make sense, especially if there is minimal insulation. Where heating is prevalent, roof color becomes less effective for energy efficiency for a couple reasons. One, buildings require significant amounts of insulation, and two, there is much less direct heat gain from the sun over the course of a year. Where heating and cooling are both used regularly (e.g., Nashville, Chicago), it’s not a matter of “black or white.” There are many metal roof colors that are moderately reflective, so they balance reflectivity and heat gain as the seasons change.

Contemplate the interaction of insulation, roof color and air barriers on each metal roofing project.

Better Barriers: Meeting Thermal Performance and Controlling Air & Moisture

Posted on December 10, 2015 by Robert Baker

Panelized metal exteriors have joints. It’s just a rule of best-practice design. Yet these joints are seen by some as interruptions in the façade or roof, when in fact they are connections — the opposite, one can argue, of the word “interruption” that suggests a discontinuity.

In fact, engineered metal panel systems offer arguably the best possible continuous exterior system. Not only are they properly applied exterior to the building structure—outboard of columns, joists and girts—but they are also designed to ensure an unbroken chain of thermal control and barrier protection. Combined with controlled penetration assemblies as well as windows, doors and skylights that are engineered as part of the façade and roof system, the insulated metal panel (IMP) products provide unequaled performance.

That’s the main reason that specialized facilities designed for maximum environmental barrier control are made of IMPs: refrigerated warehouses, R&D laboratories, air traffic control towers and MRI clinics, to name a few.

But any facility should benefit from the best performance possible with metal roofing and wall panels. Consider insulation shorthand for the code-mandated thermal barrier required for opaque wall areas in ASHRAE 90.1 and the International Energy Conservation Code (IECC). For a given climate zone, says Robert A. Zabcik, P.E., director of R&D with NCI Group, the project team can calculate the functional amount of insulation needed by using either the “Minimum Rated R-values” method or the “Maximum U-factor Assembly” calculation. For IMPs, teams use the Maximum U-factor Assembly, which can be tested using ASTM C1363.

With IMPs, the test shows thermal performance values up to R-8.515 and better per inch of panel thickness, meaning that a 2.5-inch-deep panel would easily meet the IECC and ASHRAE minimums.

With metal roofing panels and wall panels, a building team can achieve needed energy performance levels with this single-source enclosure, providing a continuous blanket of protection.

The same is true for air and moisture control. In a July 2015 paper by Building Science Corp., principal John Straube wrote, “Insulated metal panels can provide an exceptionally rigid, strong and air impermeable component of an air barrier system.” He noted that, “Air leakage condensation cannot occur within the body of the insulated metal panel, even if one of the metal skins is breached, because all materials are completely air impermeable and there are no voids to allow air flow.”

In terms of water control, Straube writes that IMPs have a continuous steel face that is a “high-performance, durable water control layer: water simply will not leak through steel, and cracks and holes will not form over time. The exterior location of the water barrier,” he adds, “offers some real advantages.”

Connecting the panels at transitions, penetrations and panel joints is the key, of course. Straube notes that sealant, sheet metal, and sheet membranes are effective and commonly used to protect joints.

In my experience, these joint details are incredibly effective. They often outlast most other components of the building. Even more important, they help make IMPs better barriers that meet thermal, air and moisture performance needs. They help make metal panels one of the best choices of all.

Codes: More than the IBC and IRC

Posted on October 13, 2015 by Brad Johnson

We all know to look to IBC Chapter 15 and IRC Chapter 9 for information about roof systems. These two “Roof Assemblies and Rooftop Structures” chapters include the requirements for fire, wind, impact, materials, and reroofing. But did you know the scope of the building code (IBC Section 101.4) references additional model codes that are considered to be part of the requirements of the IBC? From a roofing perspective, this scoping reference brings into play the International Energy Conservation Code (IECC) and the International Existing Building Code (IEBC).

The creators of the model codes are attempting to ensure that buildings (and roofs, in our case) are designed and built according to the most recent model codes even if they haven’t been specifically adopted by a state or local jurisdiction. If a jurisdiction adopts and enforces the 2015 IBC, by reference the 2015 IECC and 2015 IEBC are in effect.

How do 2015 IECC and 2015 IEBC affect roofs?
The IECC Commercial Provisions include energy efficiency requirements for the same buildings for which IBC Chapter 15 roofing requirements are required. The IECC includes minimum insulation, air barrier, and reflectivity requirements for building envelopes. Prescriptive R-values and U-values are provided for roofs, and they are based on climate zone, metal buildings, and attics. Minimum levels of solar reflectance and thermal emittance are required for low-slope roofs on buildings with air-conditioning in climate zones 1, 2 and 3.

Air barriers—used to reduce or eliminate air leakage—are required for new construction. These are based on materials, systems, or the whole building. Sheet steel and aluminum are listed as materials that meet the air barrier requirements. Of course, the joints and seams are critical to the effectiveness of metal roofing panels when considered to be air barriers. When reroofing, air barrier requirements are not triggered, which is significant. But the insulation requirements are triggered.

Roofing and structural considerations
The 2015 IEBC includes sections about reroofing (Section 706, which is new in the 2015 IEBC) and structural considerations (Section 707). The IEBC divides “Alterations” of buildings into three types: Levels I, II and III. A level I alteration includes the removal and replacement of existing materials. Reroofing is a level I alteration, which triggers the requirements of Chapter 7. The Structural section includes a requirement to upgrade a wind-resisting roof diaphragm when more than 50 percent of the roof is removed where the design wind speed is greater than 115 mph, and in special wind zones. While these are small portions of the United States, it’s important to understand this requirement.

Build roofs with the full scope in mind
Look beyond the roofing chapters to ensure that you design and build buildings according to the most recent building codes.

Air Barriers and Vapor Retarders

Posted on May 13, 2015 by Brad Johnson

Building design and code requirements are readily becoming rooted in building science, which is the study of heat, air, and moisture movement across the building envelope.

Reducing the heat energy transfer (which is bi-directional based on geography and climate) is why insulation is used. And arguably more important is the need to reduce airflow (aka, air leakage) across and through building envelopes. This airflow often includes a lot of heat and moisture; therefore, buildings’ HVAC systems work hard (and use energy…and cost money) to make up for the heat and moisture gains and losses in order to maintain proper interior temperature and humidity levels. Environmental Building News, in an article titled The Hidden Science of High-Performance Building Assemblies (Nov. 2012) , stated “Air infiltration and exfiltration make up 25%-40% of total heat loss in a building in a cold climate and 10%-15% of total heat gain in a hot climate.” This is why the model codes are now mandating air barriers.

The 2012 International Energy Conservation Code (IECC), Section C402.4, Air leakage (Mandatory) provides the requirements for air barriers in new construction. Prior to 2012, building codes did not include air barrier requirements. The first step taken in the IECC was to mandate air barriers in Climate zones 4, 5, 6, 7, and 8 (locations north of the Mason-Dixon Line, in a broad sense). Climate zones 4 through 8 are heating climates, where the largest potential for heat loss occurs. The IECC provides three ways to comply; air barriers requirements can be met through material, assembly, or whole building testing. A blower door test, used to test a whole building, seems to be the most common way used to show code compliance currently. The IECC included a list of materials that prescriptively meet the code requirements for air barrier materials; sheet steel and aluminum are on that list.

Three years later the 2015 IECC went a step further. Section C402.5, Air leakage—thermal envelope (Mandatory) extended the requirement for air barriers by mandating their use in all climate zones in the United States except zone 2B, which is a hot/dry zone. Climate zone 2-dry includes only southwest Arizona, southwest Texas, and a small part of Southern California. Essentially all new buildings in the United States are required to have air barriers, and sheet steel and aluminum remain prescriptive air barriers. It’s important to know that when reroofing, the air barrier requirements do not apply.

The IECC is available for purchase on ICC’s website: www.iccsafe.org.