The Best Attic Insulation, Ranked on Cost per R-Point
Six materials, ordered by what each one costs to deliver a single point of R-value. That is the only comparison that matters once you know the target, and the target for most of North Carolina is R49.
The table
Six materials, by cost per R-point
Ordered cheapest first. The last column is the whole argument: dollars per square foot divided by R-value per inch.
The cheapest per R-point is the right answer on an attic floor. It is not the right answer for a crawl space or a cathedral ceiling, and the differences below are why.
1. Blown fiberglass, cheapest way to hit R49
$0.50 to $1.10 per sq ft installed, at R2.5 to R3.5 per inch. Works out to about 27 cents per square foot for each R-point.
Loose fiberglass blown over the attic floor to whatever depth the target R-value needs. It is the default for a reason: it fills around wiring and odd framing, it goes in fast, and it is the cheapest R-value per dollar you can buy.
The catch. It settles, so the depth you measure in year ten is less than the depth that went in. It also does nothing for air leaks underneath it.
2. Blown cellulose, best air-blocking of the loose fills
$0.60 to $1.80 per sq ft installed, at R3.2 to R3.7 per inch. Works out to about 35 cents per square foot for each R-point.
Recycled paper treated with borate, blown in the same way as fiberglass. It packs denser, so it resists air movement through it better and gets to a given R-value in fewer inches. Borate treatment handles fire rating and insects.
The catch. It is heavier, which matters on an old ceiling, and it holds water if the roof above it ever leaks. A wet cellulose attic has to come out.
3. Mineral wool batts, best where fire or sound matters
$1.40 to $2.10 per sq ft installed, at R3.0 to R4.3 per inch. Works out to about 48 cents per square foot for each R-point.
Spun rock fiber, denser and stiffer than fiberglass. It holds its shape, does not sag, is non-combustible without any added treatment, and cuts sound noticeably. Around a chimney chase or a flue it is the right material.
The catch. Two to three times the price of blown fiberglass for a similar R-value, and heavier to handle. Rarely worth it across a whole attic floor.
4. Fiberglass batts, best for a clean, square attic you want to do yourself
$0.75 to $2.50 per sq ft installed, at R2.9 to R3.8 per inch. Works out to about 49 cents per square foot for each R-point.
Pre-cut rolls laid between the joists. The one case where batts beat loose fill is an attic with regular joist spacing, no obstructions and a floor you want to keep walkable, because you can lay them and still find the joists.
The catch. Every gap, compression and badly cut end is a hole in the insulation. Batts installed to the standard most attics get perform well below their label.
5. Open-cell spray foam, best for sealing and insulating in one pass
$1.50 to $3.50 per sq ft installed, at R3.5 to R3.7 per inch. Works out to about 69 cents per square foot for each R-point.
Expanding foam sprayed on the underside of the roof deck, which seals the attic and insulates it at the same time. It is the only product here that stops air movement as part of doing its job, and it brings ductwork inside the conditioned envelope.
The catch. It changes the attic from vented to unvented, which is a roof assembly decision, not an insulation one. Done on a roof that still has soffit and ridge vents it traps moisture against the deck.
6. Closed-cell spray foam, highest R-value per inch
$3.00 to $6.00 per sq ft installed, at R5.0 to R7.0 per inch. Works out to about 75 cents per square foot for each R-point.
Denser foam at roughly twice the R-value per inch of anything else here, and it is a vapor retarder in its own right. Where you have very little depth to work with, this is the only way to reach a high R-value.
The catch. The most expensive option by a wide margin, and on an attic floor you are paying for a thinness you do not need. Its real use is crawl spaces, rim joists and cathedral ceilings.
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The Carolina part
Why ventilation decides whether this works
Every figure above assumes the insulation is doing one job: slowing heat through the ceiling. It cannot do anything about air moving through gaps, and it cannot do anything about an attic that has no way to breathe.
The failure we see most often is insulation blown right into the eaves. It looks like a thorough job and it closes off the soffit vents, so the attic stops moving air. A hot damp attic cooks the deck above it and takes years off the shingles.
Baffles at the eaves cost very little and keep the airway open. Get the ventilation right first, then insulate, and use our ventilation calculator to check the attic has the net free area it needs.
The other Carolina-specific point is air sealing. Recessed lights, plumbing stacks and the attic hatch leak conditioned air straight past any depth of insulation. Sealing them is the cheap half of the job and the half that gets skipped.
Questions
Attic insulation questions
What is the best attic insulation?
For an attic floor in North Carolina, blown fiberglass or blown cellulose. They are the cheapest route to the R49 Energy Star recommends here, they fill around obstructions that batts cannot, and they go in during a single visit. Spray foam is a better product for a different job.
How much does attic insulation cost?
Blown fiberglass runs $0.50 to $1.10 a square foot installed and blown cellulose $0.60 to $1.80. Spray foam runs $1.50 to $6.00 depending on whether it is open or closed cell. A 1,400 square foot attic brought up to R49 in blown fiberglass is usually a four figure job, not a five figure one.
Is spray foam worth it in an attic?
Sometimes, but it is a different decision from topping up insulation. Spraying the roof deck converts a vented attic to an unvented one, which means the vents have to come out and the assembly has to be designed for it. Done halfway it traps moisture against the deck.
What R-value do I need in North Carolina?
R49 for most of the state, which is climate zone 3, and R60 in the mountains, which is zone 4. R38 is the lower figure where there is already insulation up there. Our attic insulation calculator turns that into inches for the material you are using.
Can I just add more insulation on top?
Usually yes, and it is the normal approach. Use unfaced material, because a vapor retarder buried in the middle of the assembly traps moisture. Do not bury recessed lights that are not rated for contact, and do not block the soffit vents.
Does attic insulation affect my roof?
Only through ventilation. Insulation stuffed into the eaves stops the attic breathing and the deck runs hot, which costs you years off the shingles. Baffles at the eaves are cheap and they are what keeps the airway open.
Which insulation is best for noise?
Mineral wool, by a clear margin, because the density that makes it fire resistant also makes it absorb sound. It costs two to three times blown fiberglass, so most people use it in the one room that needs it rather than the whole attic.
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