Putting Up Appearances

Have you ever seen a roof with shingles that appear to bulge at the joints of insulated roofing panels? When sunlight washes across the roof, you can see the outline of every insulated panel. The term “picture framing” was coined to describe this phenomenon. Something appears to be wrong, even though there are no leaks or other failure.

Picture framing may be just a matter of appearance. Properly installed insulated roofing systems with shingles can “picture frame,” most commonly when moisture causes the felt underlayment to expand. But this is no consolation to a school official who has a new roof that looks flawed and a nervous feeling that the joints will leak before the warranty is up. Here is an explanation of how it happens.

Insulation and moisture

Manufacturers of fiberglass or organic mat roofing shingles warn that installing them over standard non-vented polyisocyanurate roofing panels may shorten the warranty life of the shingles. The heat buildup tends to accelerate the aging process. To avoid the problem, the shingle industry recommends using vented systems.

The usual vented panel construction consists of a layer of plywood or OSB (oriented strand board), ¾-inch or 1½-inch clear vertically aligned airspace (depending on the distance between the eave and the ridge) on polyisocyanurate insulation. The manufacturing process usually includes a fiber-reinforced facer (moisture retarding) on both sides of the insulation. This protects the product during shipping and installation, and provides a glue surface to attach the blocking and nail board.

Polyisocyanurate insulation is a generic description of an open-cell expanded rigid foam insulation product that is available in a wide variety of thicknesses. It has the highest initial insulating resistance per inch of standard foam insulating products. However, its resistance can diminish with age and moisture content. The insulation must be protected from water and humidity. It also must be protected from ultraviolet light, which breaks down the foam. Once installed properly under other building products, such as roof shingles, the insulation theoretically is well protected.

Using asphalt-saturated roofing felt is the industry standard for shingle underlayment.

In practice, roofing felts allow the builder to cover a newly installed wood roof deck quickly and protect it from moisture until the shingles can be installed. The felts lie flat on the decking; if they are exposed to moisture, they absorb some of it and buckle in “waves.” Once felt has absorbed moisture, it does not appear to be flat, even after drying out.