Fiberglass batts come in many sizes and thicknesses, here are the basics.
Walls
Exterior walls are typically 16″ o.c. (on centers) and interior non-bearing walls are 24″ o.c.
Wood walls require 15″ or 23″ wide batts and metal studs use 16″ or 24″.
2×4 walls can use R-11, R-13, and R-15 batts. R-11 is used for interior sound walls, R-13 is minimum code for exteriors, and R-15 is an upgrade but over 30% higher price.
2×6 walls can use R-19 or R-21. R-19 is standard and R-21 is the upgrade.
Ceiling joist and trusses are typically 24″ o.c. and use 24″ wide batts.
R-30 (9.25″ depth) batts are minimum code in most areas however check you local building code because some are requiring R-38 (12″ depth).
Kraft and Vapor barrier. Batts with paper backing are called Kraft faced, batts without paper are unfaced. For Phoenix and Tuscon markets batts do not need Kraft faced or vapor barrier because moisture is not an issue. Consult your local building codes for your area.
Cathedral batts are available for shallow joist depths. They are denoted R-30c and R-38c.
The Good and Bad about Fiberglass Batts
The Good
- Because batts are pieces of insulation, they can be used to fill wall cavities and in attics too tight for blown-in insulation.
- Batts come in standard R-values which reduces the likelihood of cheating. A R-30 batt is packaged and marked with R-30 right on it, there isn’t much wiggle room for a dishonest installer.
- All fiberglass manufactures are independently tested and maintain high quality standards.
The Bad
- Batts leave air gaps when installed in walls due to electrical and plumbing.
- Batts in attics will not achieve their labeled R-value if they do not have 5 sides of contact. This is difficult to achieve due to being compressed under electrical wire and plumbing. Batts are often bridged over truss bracing and cavities which doesn’t allow for direct contact with the drywall.
- Batts itch! Although batts are not as itchy as previous generations of fiberglass, they still do itch. This often results in electricians, plumbers, and the cable guy only wanting to touch the batt once to get it out of their way, it doesn’t always get put back.