What is a potential problem of mixing algae-resistant shingles with non-algae resistant shingles on the same roof?

Mixing algae-resistant shingles with non-algae resistant shingles can lead to an issue with appearance, which is the reason the answer is correct. Algae-resistant shingles are designed to prevent the growth of algae, which often manifests as dark streaks on roofs, particularly in humid areas. If non-algae resistant shingles are used alongside them, the areas with non-resistant shingles may become discolored due to algae growth, while the algae-resistant sections remain clean and unaffected. This can create an uneven and unattractive look, diminishing the overall aesthetic appeal of the roof.

Regarding structural integrity, cost efficiency, and insulation effectiveness, these factors are generally less impacted by the mixing of different types of shingles. While structural issues could arise from improper installation or material incompatibility over time, this is not inherently caused by the presence of algae-resistant versus non-algae resistant shingles. Cost issues might arise in terms of purchasing different shingle types, but appearance is the more immediate and noticeable problem when both types are installed together. Insulation effectiveness is unrelated to the shingle type's algae resistance, as shingles primarily serve as a weather barrier rather than an insulator.