Why insulate metal buildings?

With skyrocketing energy costs, builders look for ways to distinguish themselves from the competition. The best way is to offer high efficiency insulation packages for metal buildings. Fiber glass insulation systems are the most sustainable and cost-effective for saving energy.

Insulating metal buildings is especially important because the metal skin and structure are highly conductive. In winter, heat moves from the warm interior to the colder outside. In summer, heat outside moves into air conditioned interiors.

Fiber glass metal building insulation acts as a barrier, slowing movement of heat, keeping it inside the building in winter and outside in summer. By controlling the rate of heat transfer through building walls and roofs, insulation reduces energy consumption, resulting in lower fuel bills and a cleaner environment.

Insulating a metal building is a good idea not only because it improves energy efficiency and reduces condensation problems, but it is probably required by local building codes. Most states require heated or cooled metal buildings to meet roof and wall insulation levels specified in the ASHRAE 90.1 Energy Standard.  Check with local building departments for energy code requirements.

A variety of fiber glass metal building insulation systems are available. Some are designed for use in both new and retrofit construction, while others are designed for new construction only.

The insulation systems discussed in this article are commonly available and offered by a number of manufacturers.  In addition, there are patented fiber glass insulation systems sold by licensed companies. While these can be excellent alternatives, they are not covered in this article due to their proprietary nature.

Regardless of which system is chosen, it is important to consult the building manufacturer before specifying an insulation system to assure the system is compatible with the building.

Roof insulation systems

Over-the-purlin insulation systems are the most common for use with metal building roofs. Prior to installing the roof sheeting, blankets of insulation are rolled out on top of the steel structure with the vapor retarder facing down and toward the inside of the building.

The insulation blankets extend from the eave to the roof peak — perpendicular to the roof purlins.  Seams where blankets meet are joined by folding and stapling the facing tabs or taping the joint to prevent water vapor from getting through the insulation and condensing on the roof.

When installing this type of system, it is important to be sure insulation materials are allowed to expand to their full thickness at the mid point between the purlins because insulation effectiveness is reduced with compression.

After insulation blankets are in place, thermal spacer blocks made from strips of foam are placed directly above the purlins before attaching the corrugated roof panels. The foam strips add insulation value where the insulation is compressed between purlin and roof sheet.

(See Figure 1)

There are three types of two-layer insulation systems that can provide a significant increase in energy savings over the single layer system. The first system is similar to the over-the-purlin system.  The difference is the first layer of faced insulation is draped more between each purlin to allow the installation of a second layer of unfaced insulation parallel to and between each purlin. This system is commonly referred to as the “sag and bag” system.

(See Figure 2)

The second two-layer system has support bands installed below and attached to the bottom of the purlins. A layer of faced insulation is then installed between and parallel to the purlins. Facing on this insulation has wide tabs that extend to the tops of the purlins on both sides, allowing the insulation to more fully expand in the roof cavity. A second layer of unfaced insulation is laid perpendicular to the purlins on top of the first piece of faced insulation.

This system includes foam thermal spacer blocks on top of the purlins and is called the “long-tab” system.

(See Figure 3)

The third two-layer system has a continuous vapor retarder and support bands installed below the purlins. This system has the vapor retarder facing totally below the purlins which adds a finished look to the roof system.

Similar to the previous system, the space or cavity between the purlins is filled with unfaced insulation, usually two layers of fiber glass insulation blankets. This system also has thermal spacer blocks on top of the purlins and is called the “filled cavity” system.

(See Figure 4)

Side and end walls

The most common method of insulating side and end walls of metal buildings is to attach the insulation blanket to the eave strut or rake angle and drape the insulation to the bottom. Seams between the blankets are joined by folding and stapling or taped to form an effective vapor retarder. Blankets are trimmed to fit. Finally, the corrugated metal wall panels are attached to the wall girts.

Fiber glass insulation for metal buildings always includes a vapor retarder facing. This facing limits passage of water vapor into the insulation and prevents it from condensing within insulation or on interior surfaces of the building.

The vapor retarder used on metal building insulation should be strong enough to withstand handling during installation as well as to function as an aesthetically pleasing interior building finish. Therefore, the facing must have good tensile strength, good rip-stop characteristics and puncture resistance. It also must be fire retardant, provide good light reflectivity, have a good appearance and low water vapor permeance. Rips or tears in the facing can be patched with facing tape to ensure a tight seal. 

Metal building insulation greatly improves building acoustics by reducing transmission of exterior sounds to the interior and absorbing reverberating sounds within the building. Laminated facings on insulation provide a bright, attractive wall and ceiling treatment that acts as a reflector to increase lighting efficiency.

In addition to saving energy, fiber glass insulation provides other benefits, including preventing problems due to condensation and making buildings quieter and brighter inside.

Charles Cottrell is vice president, technical services, North American Insulation Manufacturers Association. For information on fiber glass metal building insulation, visit www.NAIMA.org

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