The One Rule That Decides Most Matches: Batts Are New Construction Only
Before comparing R-values, compare doorways. Batt insulation in attics is a new-construction product: rolls are unrolled across fully open joist bays before the drywall ceiling closes the assembly. You cannot push a 15-inch-wide, 39-foot batt through a 22×30-inch attic hatch and have it arrive flat, facing-up, and uncompressed. This single access constraint removes batts from consideration in roughly 85% of attic retrofit searches — if your house is already built and the ceiling is closed, the blown-in vs batt question is already answered.
The retrofit exceptions are narrow and expensive: a gut renovation with the ceiling opened anyway, an unfinished garage or bonus room, or knee-wall attics where short runs are reachable. In those cases batts become viable again — and competitive, since DIY batt cost per R is excellent. For everyone else, the decision moves down a level: which blown-in fiber, and at what depth. That is the head-to-head matrix below.
❌ The listing-photo trap: Retrofit guides showing neat batt rows in finished attics are shot during construction, before drywall. Copying the photo into a closed-ceiling attic produces folded, compressed batts with gaps at every joist — the worst of both worlds at batt prices.
Head-to-Head: Five Methods, Eleven Attributes, One Real Attic
Everything below assumes the same job: a 1200 sq ft Zone 4B attic, target R-49, settled values. Read the access row first — it eliminates two columns for most existing homes — then compare cost, thickness, and air sealing among the survivors.
| Attribute | Blown-In Loose Cellulose | Blown-In Loose Fiberglass | Batt Fiberglass Unfaced | Batt Mineral Wool | Closed-Cell Spray Foam |
|---|---|---|---|---|---|
| Achievable R-Value (1200 sq ft) | 49 | 49 | 49 | 49 | 49 |
| Required Settled Thickness | 14.0" | 19.6" | 14.4" | 14.8" | 7.5" |
| Settling | 12.5% — blow +15% | 7.5% — blow +10% | 0% | 0% | 0% |
| Access Required | Attic hatch OK — fills from above | Attic hatch OK | FULLY OPEN joist bays ONLY | FULLY OPEN joist bays ONLY | Rafter undersides exposed |
| DIY-Friendly | Yes — rent blower ~$150/day | Yes — same blower | NO — cannot push batts through hatches | NO — irritates eyes/skin | NO — $3,000+ rig + certified installer |
| Material + Install Cost (R-49) | DIY ~$2,300 · Pro $6,500 | DIY ~$1,200 · Pro $2,500 | DIY ~$2,400 · Pro $7,000 | DIY ~$3,500 · Pro $8,000 | Pro Only $7,000–$10,000 |
| Air Sealing Bonus | Yes — conforms to gaps/ducts | Moderate | No — leaves gaps | No | YES — 100% continuous seal |
| Moisture Performance | Good | Excellent — glass doesn't absorb | Good | Excellent | Perfect — impermeable |
| Fire Class Rating | C | C | C | A | B |
| Lifespan | 20–30 yr if dry | 25–35 yr if dry | 25–35 yr | 50+ yr | Lifetime |
| Key Limitation | Settling + moisture risk | Doesn't fill tiny ceiling gaps | NEW CONSTRUCTION ONLY | Coarse fibers irritate | Very expensive — 2–3× blown-in |
💡 The quiet winner: Blown cellulose’s air-sealing bonus is free insulation you don’t pay for — it conforms around duct boots and wiring that batts bridge across, closing the exact bypasses that drive ice dams.
⚠️ Thickness budget: R-49 loose fiberglass needs 19.6” settled — deeper than most joists, burying them entirely. Cellulose needs 14.0”. If headroom or walkways matter, thickness is the tiebreaker, not price.
💡 Price the whole job: Run your own square footage and zone through the attic insulation calculator — material, bags, and cost update together, which is how the scenario cards above were priced.
Compression Destroys Batt R-Value — the 30–40% Penalty
Batts earn their labeled R only at labeled thickness. R-value comes from trapped air, and compressing the batt squeezes that air out. Squeeze an R-19 fiberglass batt (nominally 6.25”) into a 2×4 cavity that is 3.5” deep and you do not get R-19 in less space — you get roughly R-13.5, a 30–40% loss, plus a denser batt that leaves no room for the vapor-managing air gaps. The same physics applies to attic floors when stored boxes crush blown material or when rolled topping batts are walked on before they loft.
The fix is boring and absolute: buy the batt labeled for your exact cavity. 2×4 walls (3.5”) take R-13 or high-density R-15 batts. 2×6 walls (5.5”) take R-19 or high-density R-21. If a hardware aisle’s deeper discount is on the wrong thickness, the discount is the trap — the “over-cut risk” row in the roll table below exists because this is the most common DIY batt failure, and it is entirely invisible after the drywall closes.
R-19 squeezed to 3.5”
Delivers ~R-13.5 — you paid R-19 money for R-13 performance, 30–40% destroyed before the first winter.
R-13 at full 3.5”
Delivers labeled R-13 with proper loft. Cheaper than the compressed R-19 and performs as bought.
Need more in 3.5”?
Only dense-pack cellulose (R-13–15) or closed-cell foam (R-22) legitimately raise R in a fixed cavity — see the wall cavity guide.
Batt Roll Dimensions & 2025 Big-Box Prices — What Covers What
Every roll on the shelf answers three questions at once: width (16” or 24” framing), length (39 ft fiberglass, 31 ft mineral wool), and labeled R at full loft. The price column is 2025 big-box retail; the last row is the mistake the first six rows are designed to prevent.
| Batt Type | Facing | Width | Length/Roll | Coverage | R-Value | Price/Roll | Compression Risk |
|---|---|---|---|---|---|---|---|
| Fiberglass 2×4 Cavity | Unfaced | 15" — fits 16" OC | 39 ft | 48.75 sq ft | R-13 | $12–$18 | N/A — sized for cavity |
| Fiberglass 2×4 Cavity | Kraft-faced | 15" | 39 ft | 48.75 sq ft | R-13 | $15–$22 | N/A — sized for cavity |
| Fiberglass 2×6 Cavity | Unfaced | 23" — fits 24" OC | 39 ft | 74.75 sq ft | R-19 | $18–$28 | N/A — sized for cavity |
| Fiberglass 2×6 Cavity | Kraft-faced | 23" | 39 ft | 74.75 sq ft | R-19 | $22–$34 | N/A — sized for cavity |
| Mineral Wool 2×4 Cavity | Unfaced | 15" | 31 ft | 38.75 sq ft | R-15 | $25–$35 | N/A — sized for cavity |
| Mineral Wool 2×6 Cavity | Unfaced | 23" | 31 ft | 59.25 sq ft | R-23 | $35–$50 | N/A — sized for cavity |
| ⚠️ Over-Cut Risk | — | — | — | — | ~R-13 result | $0 premium | R-19 compressed into 3.5" loses 30–40% of R-value |
💡 Kraft or unfaced? In attics, unfaced — kraft facing is a vapor retarder and attic floors generally don’t need one. The facing decision belongs to walls in cold climates; our faced vs unfaced guide covers the zone rules.
⚠️ Buy 10% over: Cuts around crossing joists, wiring, and can-light boxes consume roughly a tenth of every bundle. The roll-count math in the FAQ already assumes clean bays — obstacles eat the margin.
Open bays — the only place rolls unroll flatPhoto: UnsplashInstallation Speed & Labor — How Each Method Actually Feels
Time and cleanup swing the DIY decision as much as dollars. A 1200 sq ft attic is the benchmark below; halve the hours for small caps, add a quarter for heavy obstruction.
| Method | Access Required | Tools | Labor (2-person) | Typical Cleanup |
|---|---|---|---|---|
| DIY Blown-In Cellulose | Attic hatch ≥22×30" | Rented blower + 100 ft hose + PPE | 4–6 hr for 1200 sq ft | 1–2 hr shop vac — lots of loose fibers |
| Pro Blown-In | Attic hatch | High-pressure blower + pro nozzle kit | 3–4 hr | <30 min — pro vac truck |
| DIY Batt | NEW CONSTRUCTION ONLY — open bays | Utility knife + straightedge + staple gun | 6–8 hr — meticulous around obstacles | 30 min sweep |
| Pro Batt | Open bays | Cutting table + stapler | 2–3 hr | 15 min |
| Pro Closed-Cell | Rafter undersides | 2-part rig + certified training + tyvek + face shield | 4–5 hr | <15 min — overspray scraped immediately |
⚠️ Blower reality check: Rental machines clog on damp or over-dense cellulose; budget a second day as insurance, and keep a broom handle for hopper agitation. If the DIY math stops working, the DIY vs professional breakdown prices both paths line by line.
Air Sealing and Lifespan — the Attributes the Price Tag Hides
Two rows of the matrix deserve their own section because they outlast the purchase. First, air sealing: blown cellulose partially seals as it fills, spray foam seals completely, and batts seal nothing — every batt job over an unsealed ceiling still leaks heated air into the attic through top plates, wire holes, and the hatch. That is why the professional sequence is always air-seal first, insulate second, regardless of material. Budget $200–$600 in caulk and foam for a 1200 sq ft ceiling; it is the highest-return money in the entire project.
Second, lifespan, which tracks moisture more than age. Dry attic fiberglass and cellulose hold R for decades; cellulose that has taken on moisture compacts and loses R permanently, and wet fiberglass recovers only if the source is fixed. Closed-cell foam, when correctly applied, is the only lifetime material — it is its own vapor barrier and cannot settle. Mineral wool’s 50+ year rating comes from inorganic fibers that neither absorb water nor interest pests.
If your existing attic insulation is 15+ years old and you are adding on top, it is worth knowing what the old layer is still delivering before you price the new one — the settling corrector calculator converts measured depth into real R-value in under a minute.
💡 Sequence that never changes: seal air leaks → install baffles at every eave → insulate. The baffle guide shows why the middle step protects both the insulation and the ventilation the code table assumes.
Frequently Asked Questions
What is the best type of insulation for an attic?▼
For an existing home, blown-in cellulose is the best attic insulation in most cases: it fills from above through the hatch, conforms around wiring and ducts to leave no gaps, costs about $2,300 DIY for a 1200 sq ft attic at R-49, and adds modest air sealing for free. Batt insulation wins only when joist bays are fully open (new construction or a gut rehab), and closed-cell spray foam wins only when you are converting to a conditioned attic. The access you have decides more than the material chart does.
Is blown-in or batt insulation better for existing homes?▼
Blown-in, decisively. An existing attic is reached through a hatch, and you cannot push a 15-inch-wide batt through a 22×30-inch hatch and expect it to unroll flat in a 14-inch-deep joist bay over existing wiring. Blown material fills from above, wraps every obstruction, and reaches the eaves where batts can never be fed. This access constraint eliminates batts from roughly 85% of attic retrofit decisions.
Why can't I use batt insulation in an existing attic?▼
Batts must feed through fully open rafter or joist bays — they are rolled out before the ceiling drywall goes up. Through a hatch, a batt would have to fold, and a folded batt is a compressed batt with destroyed R-value, plus the paper facing ends up wherever physics puts it. Retrofitting batts into an existing attic means opening the entire ceiling from below, which costs more than the insulation itself — that is why pros default to blown-in.
How much does batt insulation cost per square foot?▼
Material-only: R-13 fiberglass for 2×4 walls runs about $0.25–$0.37 per sq ft ($12–$18 per 48.75 sq ft roll), R-19 for 2×6 cavities about $0.24–$0.37 ($18–$28 per 74.75 sq ft roll), and mineral wool roughly double. Installed by a pro, batt insulation costs $1.20–$2.50 per sq ft in walls and $1.50–$2.00 per sq ft on attic floors. Compare blown-in attic retrofits at $0.90–$2.00 per sq ft installed.
How many rolls of insulation do I need?▼
Divide your area by the roll coverage: a standard 15-inch-wide × 39-foot fiberglass roll covers 48.75 sq ft (2×4 cavities, R-13), and the 23-inch × 39-foot roll covers 74.75 sq ft (2×6 cavities, R-19). A 1000 sq ft ceiling needs about 21 of the wide rolls or 41 of the narrow ones. Buy 10% extra for cuts around obstacles, and remember rolls only fit open bays — they cannot be retrofitted through a hatch.
How much does a roll of insulation cover?▼
The two big-box standard fiberglass formats cover 48.75 sq ft (15" × 39 ft) and 74.75 sq ft (23" × 39 ft). Mineral wool rolls are shorter: 38.75 sq ft (15" × 31 ft) and 59.25 sq ft (23" × 31 ft). Coverage assumes the batt sits at full labeled thickness — compressing it into a shallower cavity cuts both coverage efficiency and R-value.
How long is a roll of insulation, and what widths does it come in?▼
Fiberglass rolls are 39 feet long in the standard big-box packaging; mineral wool is 31 feet. Widths are 15 inches (friction-fits 16-inch-on-center framing) and 23 inches (24-inch-on-center). Those two widths map exactly to 2×4 and 2×6 cavity products — there is no universal roll, which is why the cavity-size table above matters before you load the cart.
What's the best blown-in insulation for an attic?▼
Cellulose for most retrofits: R-3.5 per inch settled means 14 inches for R-49 instead of 19.6 for fiberglass, it fills gaps better, and its recycled-newspaper content costs less per R. Loose fiberglass wins when budget is the only constraint (about half the material cost) and you can live with 24-inch total depth at R-60. Rockwool wins on fire rating but needs pro-grade blowers above R-30. Our material comparison page runs the full three-way numbers.
Can I put rolled insulation over blown-in insulation?▼
Yes — unfaced batts or rolls laid perpendicular across blown-in cellulose is an accepted topping method and adds R-3.2–3.3 per inch. Two rules: never add a kraft-faced layer over blown material (the second vapor barrier traps moisture), and lay the rolls perpendicular to the joists so they bridge the thermal bridge the joists themselves create. Blowing additional loose-fill on top is usually cheaper and easier.
Is batt insulation cheaper than blown-in installed?▼
On materials alone, sometimes: R-49 batt fiberglass for 1200 sq ft runs about $2,400 DIY versus $2,300 DIY for blown cellulose — effectively a tie. But in an existing home the batt number is theoretical because the bays are not open; opening ceilings to install them costs thousands more. In new construction, batts are usually the cheapest legitimate option per R at the framing stage.
Which is better for soundproofing, batts or blown-in?▼
Mineral wool batts lead for sound — their dense, coarse fiber structure absorbs mid and high frequencies far better than fiberglass of the same thickness, which is why they are the standard in shared walls and home theaters. Dense-pack cellulose is the best blown option for sound and also excels in walls. Both beat standard fiberglass batts; loose fiberglass is the weakest acoustic performer.
How long does each installation take for a 1200 sq ft attic?▼
DIY blown-in: 4–6 hours plus 1–2 hours of shop-vac cleanup. Professional blown-in: 3–4 hours with vac-truck cleanup under 30 minutes. DIY batts: 6–8 hours of meticulous fitting — and again, only possible with open bays. Pro batt crew: 2–3 hours. Closed-cell foam: 4–5 hours by a certified crew. For a one-weekend project, DIY blown-in is the only method that fits comfortably.