The building envelope is a key element of an energy-efficient design. However, compromises are not uncommon and are caused by a variety of factors that can easily be avoided.

Improper placement of insulation, improper sealing or lack of sealing around air barriers, incorrect or poorly performing glazing and fenestration systems, incorrect placement of shading devices, misplacement of day lighting shelves, and misinterpretation of assembly details can compromise the energy performance of the building, according to ASHRAE’s “Advanced Energy Design Guide for K-12 Buildings.”

Insulation entirely above deck should be continuous insulation (CI) rigid boards. Continuous insulation is important because no framing members are present that would introduce thermal bridges or short circuits to bypass the insulation.

When two layers of CI are used in this construction, the board edges should be staggered to reduce the potential for convection losses or thermal bridging. If an inverted or protected membrane roof system is used, at least one layer of insulation is placed above the membrane and a maximum of one layer is placed beneath the membrane.

Attics, and Other Roofs

Attics and other roofs include roofs with insulation that is entirely below (inside) the roof structure (attics and cathedral ceilings) and roofs with insulation both above and below the roof structure. Ventilated attic spaces need to have the insulation installed at the ceiling line.

Unventilated attic spaces may have the insulation installed at the roof line. When suspended ceilings with removable ceiling tiles are used, the insulation needs to be installed at the roof line. For buildings with attic spaces, ventilation should be provided equal to one square foot of open area per 100 square feet of attic space. This will provide adequate ventilation as long as the openings are split between the bottom and top of the attic space. Additional ventilation can further improve the performance of the building.

In metal roof building construction, purlins are typically z-shaped, cold-formed steel members, although steel bar joists are sometimes used for longer spans.

The thermal performance of metal building roofs with fiberglass blankets is improved by addressing the thermal bridging associated with compression at the purlins. The two types of metal building roofs are standing seam roofs and through-fastened roofs.

Standing seam roofs have very few exposed fasteners and utilize a concealed clip for the structural attachment of the metal roof panel to the purlins. The larger gap between the purlin and the roof sheets, along with the thermal spacer block, provides a thermal break that results in improved performance compared to the standard through-fastened metal roofs.

It is recommended that the thermal resistance between the purlin and the metal deck be at least R-8. One way to accomplish this is by using a 3/4 x 3 in. foam block (R-5) over 3/4 in. of compressed fiberglass blanket (R-3).

Alternatively, a 2-inch space filled with compressed fiberglass insulation will provide roughly R-8.

Through-fastened metal roofs are screwed directly to the purlins and have fasteners that are exposed to the elements. The fasteners have integrated neoprene washers under the heads to provide a weather tight seal.

Thermal spacer blocks are not used with through-fastened roofs because they may diminish the structural load carrying capacity by “softening” the connection and restraint provided to the purlin by the metal roof panels.

To meet performance recommendations, through-fastened roofs will generally require insulation over the purlins in the conventional manner, with a second lay of insulation added to the system. The second layer of insulation can be placed either parallel to the purlins (on top of the first layer) or suspended below the purlins.

In tropical climates, the recommended construction is standing-seam roofs with R- 19 insulation blankets draped over the purlins.

In other climates, the recommended construction is standing-seam roofs with two layers of blanket insulation. The first layer is draped perpendicularly over the purlins with enough looseness to allow the second insulation layer to be laid above it, parallel to the purlins.

Single Rafter Roofs

Single-rafter roofs have the roof above and ceiling below both attached to the same wood rafter, and the cavity insulation is located between the wood rafters. Continuous insulation, when used, is installed on the bottom of the rafters and above the ceiling material.