An insulation batt is a pre-cut rectangular panel of flexible insulation designed to fit snugly between the studs, joists, and rafters in your walls, floors, and ceilings. It’s the most common and widely available type of insulation used in residential construction. Batts are typically made from fiberglass, though mineral wool, cotton, and sheep’s wool versions also exist.
How Batts Are Sized
Batts come in standardized widths that match typical framing layouts. If your studs are spaced 16 inches on center (the most common residential framing), you’ll use a batt that’s 15 inches wide. For 24-inch on-center framing, the batt is 23 inches wide. That slight undersizing is intentional: it lets you press the batt into the cavity where it expands to fill the space without buckling or leaving gaps.
Thickness varies by R-value, which measures how well the insulation resists heat flow. A standard 2×4 wall cavity (3.5 inches deep) holds an R-13 or R-15 batt. A 2×6 wall (5.5 inches deep) fits R-19 or R-21. Attic batts can be much thicker, with R-38 products running about 12 inches and R-49 reaching 14 inches.
Batts vs. Rolls
Batts and rolls are made from the same material. The only difference is packaging. Batts are pre-cut to specific lengths for fast installation in walls, where you’re fitting insulation between studs one cavity at a time. Rolls are continuous sheets you cut to length on site, which makes them better for long, unobstructed spaces like attic floors. If someone refers to “blanket insulation,” they’re talking about the broader category that includes both.
Faced vs. Unfaced Batts
Batts come in two styles: faced and unfaced. Faced batts have a layer of kraft paper or plastic attached to one side that acts as a vapor retarder, helping control moisture that could lead to mold and mildew inside your walls. Unfaced batts have no such layer.
Kraft-faced batts are the standard choice for exterior walls, exterior basement walls, and attic ceilings. You install them with the paper side facing toward the living space (the warm-in-winter side of the wall). Unfaced batts are used when a vapor retarder isn’t needed or when you’re adding a second layer of insulation over an existing one, since doubling up vapor retarders can actually trap moisture.
Common Batt Materials
Fiberglass dominates the market, and for straightforward reasons: it’s affordable, widely available, and carries a Class A fire rating, meaning it has the lowest possible flame spread and produces minimal smoke. At $0.30 to $1.50 per square foot, fiberglass batts are one of the cheapest insulation options you can buy.
Mineral wool batts (sometimes called rock wool or slag wool) cost more, typically $1.40 to $4.00 per square foot, but they offer advantages that matter in certain situations. Mineral wool is denser and more rigid, which makes it easier to cut precisely and hold its shape in a wall cavity. It also has a higher melting point than fiberglass, giving it an edge in fire resistance. Many builders use mineral wool batts specifically in walls where soundproofing matters.
Cotton and sheep’s wool batts exist as eco-friendly alternatives. They work on the same principle but represent a tiny fraction of the market.
Sound Reduction
Beyond temperature control, batts reduce noise transfer between rooms. Specialty products called sound attenuation batts are designed specifically for interior walls, shared walls in apartments or condos, and between floor levels. Adding these batts to a standard interior wall can improve its Sound Transmission Class (STC) rating by 2 to 11 points, which translates to a noticeable reduction in how much conversation, music, or TV sound passes through.
Why Proper Fit Matters
Batts rely on full, even contact with the cavity to perform at their labeled R-value. Compressing a batt into a space that’s too shallow reduces its effectiveness. An R-38 batt designed for 12 inches of depth will only deliver about R-37 when squeezed into an 11.25-inch 2×12 cavity. That’s a modest loss, but the penalty gets steeper with greater compression. An R-19 batt meant for 6.25 inches of space drops to around R-18 when compressed into a 5.5-inch 2×6 wall.
Gaps and voids are even worse than compression. If a batt is too narrow, too short, or poorly cut around electrical boxes and pipes, air moves freely through those openings and short-circuits the insulation entirely. The goal is a friction fit: snug enough to stay in place on its own, with no gaps at the edges, no compression in the middle, and no areas left uncovered.
Handling and Safety
Fiberglass batts cause skin, eye, and respiratory irritation on contact. The tiny glass fibers work their way into exposed skin and create an itchy, prickly sensation that’s hard to ignore. Long sleeves, long pants, gloves, and a head covering are the baseline for any installation work. Eye protection and a dust mask are smart additions, especially in enclosed spaces like attics where airborne fibers accumulate quickly.
Mineral wool produces similar irritation. If you’re installing batts yourself, dress for it: wear clothes you can wash immediately afterward, and avoid touching your face during the work. Showering in cool water afterward helps, since hot water opens your pores and can make the itching worse.
Where Batts Work Best
Batts are ideal for standard, evenly spaced framing with minimal obstructions. A row of clean wall cavities between studs is the perfect application. They also work well in floor joists and attic rafters, provided you can access the space to install them by hand.
They’re less suited to irregularly shaped cavities, walls crowded with plumbing and wiring, or hard-to-reach spaces. In those situations, blown-in insulation (loose fill pumped into cavities) or spray foam tends to fill gaps more completely. Many homes use a combination: batts in the straightforward wall and floor cavities, with other insulation types filling the trickier spots.