Fiberglass Batt Insulation
Fiberglass insulation is widely used, but it is often installed incorrectly. Small installation errors can reduce the effective R-value by as much as thirty percent.
Fiberglass batts installed in walls must have the insulation surface in contact on all six sides—front and back, top and bottom, and both sides. Any void in the cavity allows convection currents to quickly transport warm air to colder areas and move moisture where it can condense to create a breeding ground for molds. Batts installed in attics can have the horizontal top surface open to the attic.
Batts are intended to fit snugly into cavities, but not be compressed. It requires effort to achieve this where wiring, plumbing or there are other obstructions in the cavity. Care must be taken to slit the batt to fit around wires. Batts must be cut to fit tightly around electrical boxes. The batts must be cut to the correct length so the bottoms are not compressed as they are stuffed into cavities. It may be necessary to run a putty knife between the batt and the framing to be sure it does not snag on rough spots on the wood and create voids.
There is some debate about whether faced batts or unfaced batts are better. Facing is used as a vapor barrier, but it is not necessary where another vapor barrier will be applied, as when the wall is painted with a paint with an adequate perm rating. Facing used as a vapor barrier must be fastened so no gaps exist. Stapling the tabs over the face of the framing is the best way to achieve this, but that method cannot be used when the gypsum board is to be glued to the framing. When tabs are stapled to the inside edge of the framing care must be taken to make sure the tabs fit tightly against the framing but do not compress the insulation or create voids.
Fiberglass insulation is not an air barrier. Air passes through it readily. It is necessary to air seal properly to prevent air and moisture infiltration into the insulation.