Introduction
Four insulation materials cover nearly all behind-drywall work — fiberglass batts, mineral wool batts, blown cellulose, and spray foam — plus rigid foam for special cases. They differ in R-value per inch, cost, sound performance, moisture behavior, and who installs them. This guide compares them head to head so you can match the material to the wall, then run the quantities through the Drywall Insulation Calculator with the right product in mind.
Fiberglass Batts: The Default
Fiberglass batts are the standard for a reason: pre-cut to stud-bay widths, inexpensive, and DIY-friendly. Typical R-value is ~3.2 per inch, sold as labeled assemblies — R-13 for 2×4 (3.5-in) cavities, R-19 or R-21 for 2×6 (5.5-in) cavities. Available faced (kraft vapor retarder) or unfaced. Material cost runs about $0.60–$0.90/ft² for R-13.
Strengths: lowest material cost, no special equipment, easy to cut and fit, widely stocked in every batt size. Weaknesses: performance depends heavily on installation quality — gaps and compression punish fiberglass more than denser materials. Fibers irritate skin and lungs, so protective gear is mandatory.
Best for: standard 2×4 and 2×6 walls where budget matters and installation will be careful. If you’re hiring out, fiberglass installed typically runs $1.50–$2.50/ft².
Mineral Wool Batts: The Upgrade
Mineral wool (stone wool) batts are denser and stiffer than fiberglass, at ~3.3 R per inch — R-15 for 2×4 cavities, R-23 for 2×6. The density is the point: batts hold their shape, friction-fit firmly without sagging, and damp sound better than fiberglass. Mineral wool is also noncombustible and water-repellent — it doesn’t absorb water and dries without losing shape.
Strengths: better sound damping, fire resistance, moisture tolerance, and easier to cut cleanly; holds its R-value in real-world installs because it resists slumping. Weaknesses: roughly 1.5–2× the material cost of fiberglass ($1.20–$1.80/ft²), heavier to handle, and fewer retail stockists.
Best for: sound-critical interior walls (bedrooms, baths, offices, party walls), basement walls where moisture tolerance matters, and anyone who wants the most forgiving batt to install. The DIY vs. pro guide notes where the premium pays back.
Blown Cellulose: For Attics and Dense-Pack Walls
Cellulose is shredded, treated recycled paper blown into place at ~3.5 R per inch. In attics it’s blown loose to depth (R-38 needs ~10.9 in / 277 mm; R-60 ~17.1 in / 434 mm). In walls it’s dense-packed behind netting or into closed cavities, filling every void around wires and pipes — which is its real advantage over batts in irregular framing.
Strengths: excellent void-filling, good R per inch, low material cost ($0.40–$0.70/ft² material), and the borate treatment resists pests and fire. Weaknesses: needs a blower (often free with bulk bag purchase), settles slightly over time (bag charts compensate), and must stay dry — wet cellulose loses performance and is slow to dry. Dense-pack walls are skilled work, not casual DIY.
Best for: attic floors before ceiling drywall, and wall cavities with heavy wiring/plumbing where batts would leave voids. Installed: roughly $1.00–$1.75/ft² for attics, $1.75–$3.00/ft² for dense-pack walls.
Spray Foam: Maximum R per Inch
Two chemistries, two profiles. Closed-cell foam delivers ~6.5 R per inch, air-seals as it expands, and adds racking strength — roughly double the R of a batt in the same cavity. Open-cell foam runs ~3.6–3.8 per inch, fills deeper cavities cheaply, and is better for sound, but needs greater depth for the same R and isn’t a vapor retarder.
Strengths: highest R per inch (closed-cell), built-in air sealing, and gap-free coverage. Weaknesses: cost — $4–$9/ft² installed for a 2×4 wall, several times batt pricing — plus professional-only application, off-ratio and overspray risks if the installer is careless, and code requires a thermal barrier (½-in drywall qualifies) over foam in living spaces.
Best for: shallow or irregular cavities where batts can’t reach target R, rim joists, and high-performance assemblies. Overkill for a standard 2×4 wall that batts already satisfy at code.
Rigid Foam: The Special Case
Rigid foam boards (polyiso, XPS, EPS) aren’t cavity insulation — they go outside the sheathing or inside as continuous layers. They matter to this calculator’s users in one scenario: when local energy code demands wall R-values beyond what the cavity alone provides, continuous exterior foam bridges the thermal short-circuit of the studs themselves. If your code target exceeds cavity capacity, price the continuous layer separately — it’s a different material, different labor, and different detailing at windows and doors.
Head-to-Head Comparison
| Material | R per inch | Material $/ft² | Installed $/ft² | Sound | Moisture | DIY? |
|---|---|---|---|---|---|---|
| Fiberglass batt | ~3.2 | $0.60–$0.90 | $1.50–$2.50 | Good | Dries slowly if wet | Yes |
| Mineral wool batt | ~3.3 | $1.20–$1.80 | $1.80–$3.00 | Better | Repels water | Yes |
| Blown cellulose | ~3.5 | $0.40–$0.70 | $1.00–$3.00 | Good | Must stay dry | Attics yes; walls pro |
| Closed-cell foam | ~6.5 | $4.00–$9.00 (installed) | Good | Vapor retarder | No | |
Costs are typical US planning ranges — the cost guide builds full project budgets from them.
Sound Insulation (STC) Notes
For interior walls, the material choice is about sound transmission class (STC), not R-value. Approximate performance for a standard 2×4 wall with ½-in drywall both sides:
- Empty cavity: ~STC 33 — normal speech clearly audible.
- Fiberglass batts: ~STC 39–42 — loud speech audible but muffled.
- Mineral wool batts: ~STC 43–45 — loud speech barely audible.
- Double ⅝-in drywall + mineral wool: ~STC 50+ — the practical ceiling for simple wood framing.
These are typical values, not guarantees — sealing, drywall mass, and resilient channels move the number as much as the insulation does. For party walls between units, check what the local code requires; STC 50 is a common multifamily threshold.
Faced vs. Unfaced: A Quick Decision
- Use faced batts when you want the built-in kraft vapor retarder — heating climates, facing toward the interior (warm side).
- Use unfaced batts when the assembly gets a separate vapor barrier (poly sheeting per code), for interior sound walls, and anywhere the facing would end up on the wrong side.
- Never sandwich a cavity between two vapor retarders (faced batts + poly on the other side). Moisture trapped between them has nowhere to go.
When in doubt, the code compliance guide explains what local inspectors expect to see.
Related Guides
- Drywall Insulation: The Complete Guide — the full overview behind the calculator.
- Cost Guide — material and installed price ranges.
- Pro Tips — installing each material for full labeled performance.
- DIY vs. Pro — which materials suit DIY and which need contractors.
- FAQ — quick answers on R-values, batts, and conversions.
Get Your Number
Pick the material for the wall, note its standard batt size and coverage, then run your walls through the Drywall Insulation Calculator — net area, batt count with waste, and R-value/RSI in one pass. The right product installed well beats the premium product installed carelessly, every time.
Disclaimer: Material data and costs are typical US planning figures that vary by region, supplier, and product line. R-values shown are nominal manufacturer ratings; installed performance depends on cavity fit and air sealing. Verify specifications with the product data sheet and your local building authority.
