A batt of insulation is a pre-cut, rectangular panel of insulating material designed to fit snugly between the studs, joists, or rafters in your walls, floors, and ceilings. Batts are the most common form of home insulation in the United States, sold in standard widths (typically 15 or 23 inches) that match standard framing spacing. Think of them as thick, flexible blankets that trap air to slow heat transfer through your home’s structure.
What Batts Are Made Of
Most insulation batts are made from one of two materials: fiberglass or mineral wool (sometimes called rockwool). Fiberglass batts are by far the more popular option. They’re lightweight, inexpensive, and come in both faced and unfaced versions. Mineral wool batts are denser and stiffer, which makes them easier to cut precisely and hold their shape in a wall cavity without sagging.
The thermal performance of the two materials is similar. Both deliver roughly 3.1 to 3.2 R-value per inch of thickness. Where they differ is in fire resistance and moisture handling. Mineral wool has a significantly higher melting point than fiberglass, making it the better choice in applications where fire resistance matters. Both unfaced versions are noncombustible. On the moisture side, fiberglass batts are available with a built-in paper or foil facing that acts as a vapor barrier, while mineral wool is sold unfaced only. If you need vapor control, fiberglass batts with kraft-paper facing are the simpler solution, since mineral wool requires you to install a separate vapor barrier.
R-Value and What the Numbers Mean
R-value measures how well insulation resists heat flow. Higher numbers mean better insulating performance. Batts are labeled with both their R-value and their thickness, and matching the right product to your wall or ceiling cavity is critical.
Common batt products include R-13 (3.5 inches thick, designed for 2×4 walls), R-15 (also 3.5 inches, available in mineral wool), R-19 (6.25 inches, for 2×6 walls), and R-30 or R-38 (for attic floors and cathedral ceilings). Your local building code specifies minimum R-values for walls, ceilings, and floors based on your climate zone.
Why Compression Matters
Batts work by trapping millions of tiny air pockets within the fibers. When you compress a batt into a space that’s too shallow, you squeeze out those air pockets and lose insulating power. For example, stuffing an R-19 batt (normally about 6.25 inches thick) into a 3.5-inch wall cavity drops its effective R-value to about R-15, a roughly 21% loss. Even modest compression costs you. An R-13 batt squeezed from 3.5 inches down to 2.5 inches drops to about R-10. The takeaway: always buy batts rated for the actual depth of your framing cavity rather than trying to cram thicker products into tight spaces.
Faced vs. Unfaced Batts
Kraft-faced batts have a paper layer bonded to one side that acts as a vapor retarder, helping prevent moisture from passing through the wall assembly and causing mold or mildew. Unfaced batts have no such layer.
The general rule is to use kraft-faced batts on exterior walls, exterior basement walls, and attic ceilings, with the paper side facing the warm-in-winter side of the wall (toward the interior of your home). When you’re adding a second layer of insulation on top of existing insulation, use unfaced batts so you don’t trap moisture between two vapor barriers. Local building codes vary on vapor retarder requirements, so it’s worth checking what your jurisdiction specifies before you buy.
Where Batts Work Best
Batts are ideal for open, accessible framing cavities with standard spacing and few obstructions. New construction walls, exposed attic floors between joists, and basement rim joists are all straightforward batt applications. They’re also a practical choice when you want easy future access to wiring, plumbing, or ductwork behind walls, since you can pull a section out and replace just that piece without disturbing the rest of the insulation.
Compared to blown-in insulation, batts are easier to install as a DIY project, more moisture-resistant (especially versus blown cellulose), and maintain their R-value better over the life of a building because they don’t settle. If you’ve had rodent problems in an attic or crawl space, batts make it easier to spot and replace damaged sections. They also keep attic spaces more usable for storage, since they sit between joists rather than burying the entire floor in loose fill.
Batts are less practical in irregularly shaped cavities, around lots of pipes and wiring, or in spaces you can’t physically reach. Tight attic corners, oddly framed walls, and enclosed cavities are better served by blown-in insulation, which flows around obstructions. For soundproofing between interior rooms, fiberglass batts actually outperform spray foam, making them a solid choice for home theaters, bedrooms, or offices.
What Batts Cost
Batt insulation is one of the most affordable insulation options. Materials alone for a standard fiberglass batt run roughly $1.80 to $2.40 per square foot, depending on the R-value and whether you choose faced or unfaced. Professional installation for ceiling batts (a common project) runs approximately $2.65 to $4.03 per square foot, including materials and labor. If a general contractor oversees the project, expect to add 13% to 22% on top of that. Wall insulation during new construction is typically cheaper per square foot because the cavities are wide open and accessible.
Because installation is straightforward, batts are one of the few insulation types where a confident DIYer can save meaningfully on labor costs. The main tools you need are a utility knife, a straightedge, and proper protective gear. Fiberglass fibers irritate skin, eyes, and lungs, so long sleeves, gloves, safety glasses, and a dust mask or respirator are essential when handling the material.
Getting a Good Installation
The performance gap between a well-installed batt and a poorly installed one is enormous. Gaps, voids, and compression are the enemies. A batt that’s cut too narrow leaves a strip of uninsulated wall. One that’s too wide bunches up and compresses in the center. Either scenario creates thermal weak spots where heat escapes freely.
For a proper fit, cut batts about half an inch wider than the cavity so they friction-fit snugly without buckling. Split batts around electrical wires rather than compressing the insulation over them. Around outlet boxes, cut the batt to fit behind and around the box rather than stuffing it in. Every gap you leave, even a small one, disproportionately reduces the wall’s overall thermal performance. Studies on real-world installations consistently show that a batt filling only 90% of a cavity delivers significantly less than 90% of its rated R-value, because heat finds the path of least resistance through the unfilled areas.