Blistering in Shingles
If you have ever had a blister from doing yard work or from too many swings on a golf course you probably have a pretty good idea about what blistering looks like on shingles.
Blistering appears as bubbles raised on the surface of the shingles or small relatively circular cavities that penetrate into the asphalt (featured photo and Figures 1 and 2).
Blisters are due to the vaporization and expansion of the volatiles, such as, kerosene or water, in the asphalt. The vaporization and expansion occur when the shingles are heated by ambient temperatures, the radiant heat of sunshine, and/or excessive heat from a poorly vented attic. When the blisters of the bubble break or are worn away by normal weathering or foot traffic, a cavity is exposed in the asphalt.
Blistering is generally the result of a shingle manufacturing anomaly, but is commonly increased by exposure of the shingles to excessive heat from a poorly ventilated attic.
Theoretically blistering reduces the life of a shingle by exposing a small area of asphalt mat to the degrading powers of sunshine and by providing a depression where water can collect to accelerate the process, but practically speaking by the time the asphalt in a blister degrades and washes away other aging issues have worn the shingles enough to require replacement. Shingles that exhibit blistering require more regular inspection in the spring and fall to evaluate their condition for the next harsh season.