Most residential structures are insulated with either fiberglass batts or cellulose because both are cheap sources of added R-value. But there's more to insulation than R-value. For best results, the insulation must be accurately cut to fit the joist or stud cavities, and an effective air barrier is needed to keep unconditioned outdoor air from penetrating the insulation like wind blowing through a sweater. In most climates, a poly or kraft-paper vapor retarder is also needed to limit the flow of moisture-laden air and prevent condensation from forming within the insulation.
None of that is exactly rocket science, but doing the job right does take some care and attention to detail. Unfortunately, because both fiberglass and cellulose installations are typically subbed out to the lowest bidder, vapor retarders, air barriers, and the insulation itself are often thrown into place with little regard to quality.
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