Now, however, manufacturers of fiberglass batts, blown insulation and spray foam are clamoring for your attention and your hard-earned dollars. Each product has unique strengths and drawbacks. It's up to you to make sure your builder is using each insulator where it works best, and that the job is done right the first time. But adequate performance isn't a sure bet. First, the batts must completely fill each stud cavity.
Empty spaces allow air leakage, quickly reducing the effective R-value.
Proper installation is key. An air barrier and a vapor barrier must be installed in order for fiberglass to be effective. In addition, you should be able to stand across the room and see no voids at the top or bottom of the batts, or around electrical boxes and pipes. Fiberglass is not the best material for insulating floors. The protection from air movement that fiberglass needs is tough to achieve.