An oft-overlooked characteristic of building materials is stability when exposed to ultraviolet (UV) light (i.e. sunlight ☀️). Some building materials are specifically intended for long term exposure to UV light, but many are not. Those that are not often have limited periods of time for which they remain stable when exposed to UV, and after that time they may degrade to the point that performance is reduced.
One genre of materials where this characteristic is particularly important is underlayments for cladding, including both roofing and exterior walls. Underlayments, for example house wraps, are often installed well ahead of their overlying cladding. They provide a convenient way to “dry in” the building during construction. Usually this works out fine, but occasionally these materials are left exposed longer than anticipated.
This isn’t just an issue for the construction phase. A lot of finished cladding assemblies, intentionally or unintentionally, leave portions of the underlying materials permanently exposed to sunlight. In these cases a UV-stable underlayment is essential.
Being aware of this potential vulnerability allows us to choose appropriate materials to mitigate the risk when needed. As an example, the black stuff in the image at the top of this post is an air and water barrier membrane for walls that is made to be stable even under long term exposure to ultraviolet light.
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